onceupon/Bash-Oneliner

Bash-Oneliner

Handy Bash one-liners

Terminal Tricks

Using Ctrl keys

Ctrl + a : move to the beginning of line.
Ctrl + d : if you've type something, Ctrl + d deletes the character under the cursor, else, it escapes the current shell.
Ctrl + e : move to the end of line.
Ctrl + k : delete all text from the cursor to the end of line.
Ctrl + l : equivalent to clear.
Ctrl + n : same as Down arrow.
Ctrl + p : same as Up arrow.
Ctrl + q : to resume output to terminal after Ctrl + s.
Ctrl + r : begins a backward search through command history.(keep pressing Ctrl + r to move backward)
Ctrl + s : to stop output to terminal.
Ctrl + t : transpose the character before the cursor with the one under the cursor, press Esc + t to transposes the two words before the cursor.
Ctrl + u : cut the line before the cursor; then Ctrl + y paste it
Ctrl + w : cut the word before the cursor; then Ctrl + y paste it
Ctrl + x + backspace : delete all text from the beginning of line to the cursor.
Ctrl + x + Ctrl + e : launch editor defined by $EDITOR to input your command. Useful for multi-line commands.
Ctrl + z : stop current running process and keep it in background. You can use `fg` to continue the process in the foreground, or `bg` to continue the process in the background.
Ctrl + _ : undo typing.

Change case

Esc + u
# converts text from cursor to the end of the word to uppercase.
Esc + l
# converts text from cursor to the end of the word to lowercase.
Esc + c
# converts letter under the cursor to uppercase, rest of the word to lowercase.

Run history number (e.g. 53)

!53

Run last command

!!
# run the previous command using sudo
sudo !!

Run last command and change some parameter using caret substitution (e.g. last command: echo 'aaa' -> rerun as: echo 'bbb')

# last command: echo 'aaa'
^aaa^bbb

#echo 'bbb'
#bbb

# Notice that only the first aaa will be replaced, if you want to replace all 'aaa', use ':&' to repeat it:
^aaa^bbb^:&
# or
!!:gs/aaa/bbb/

Run past command that began with (e.g. cat filename)

!cat
# or
!c
# run cat filename again

Bash globbing

# '*' serves as a "wild card" for filename expansion.
/etc/pa*wd    #/etc/passwd

# '?' serves as a single-character "wild card" for filename expansion.
/b?n/?at      #/bin/cat

# '[]' serves to match the character from a range.
ls -l [a-z]*   #list all files with alphabet in its filename.

# '{}' can be used to match filenames with more than one patterns
ls *.{sh,py}   #list all .sh and .py files

Some handy environment variables

$0   :name of shell or shell script.
$1, $2, $3, ... :positional parameters.
$#   :number of positional parameters.
$?   :most recent foreground pipeline exit status.
$-   :current options set for the shell.
$$   :pid of the current shell (not subshell).
$!   :is the PID of the most recent background command.
$_   :last argument of the previously executed command, or the path of the bash script.

$DESKTOP_SESSION     current display manager
$EDITOR   preferred text editor.
$LANG   current language.
$PATH   list of directories to search for executable files (i.e. ready-to-run programs)
$PWD    current directory
$SHELL  current shell
$USER   current username
$HOSTNAME   current hostname

Using vi-mode in your shell

set -o vi
# change bash shell to vi mode
# then hit the Esc key to change to vi edit mode (when `set -o vi` is set)
k
# in vi edit mode - previous command
j
# in vi edit mode - next command
0
# in vi edit mode - beginning of the command
R
# in vi edit mode - replace current characters of command
2w
# in vi edit mode - next to 2nd word
b
# in vi edit mode - previous word
i
# in vi edit mode - go to insert mode
v
# in vi edit mode - edit current command in vi
man 3 readline
# man page for complete readline mapping

Terminal Multiplexers

Herdr for managing multiple coding agents

# Herdr (https://herdr.dev/) is a terminal multiplexer with mouse and coding-agent support, for running multiple agents.
# Install Herdr on macOS or Linux:
curl -fsSL https://herdr.dev/install.sh | sh

Manage multiple terminal sessions with tmux

# Create session and attach:
tmux

# Attach to session foo:
tmux attach -t foo

# Detached session foo:
^bd

# List sessions:
tmux ls

# Attach last session:
tmux attach

# Kill session foo:
tmux kill-session -t foo

# Create detached session foo:
tmux new -s foo -d

# Send command to all panes in tmux:
Ctrl-B
:setw synchronize-panes

# Some tmux pane control commands:
Ctrl-B
#   Panes (splits), Press Ctrl+B, then input the following symbol:
#   %  horizontal split
#   "  vertical split
#   o  swap panes
#   q  show pane numbers
#   x  kill pane
#   space - toggle between layouts

#   Distribute Vertically (rows):
select-layout even-vertical
#   or
Ctrl+b, Alt+2

# Distribute horizontally (columns):
select-layout even-horizontal
#   or
Ctrl+b, Alt+1

# Scroll
Ctrl-b then \[ then you can use your normal navigation keys to scroll around.
Press q to quit scroll mode.

Manage multiple terminal sessions with screen

# Create session and attach:
screen

# Create a screen and name it 'test'
screen -S test

# Create detached session foo:
screen -S foo -d -m

# Detached session foo:
screen: ^a^d

# List sessions:
screen -ls

# Attach last session:
screen -r

# Attach to session foo:
screen -r foo

# Kill session foo:
screen -r foo -X quit


# Scroll:
# Hit your screen prefix combination (C-a / control+A), then hit Escape.
# Move up/down with the arrow keys (↑ and ↓).  

# Redirect output of an already running process in Screen:
# (C-a / control+A), then hit 'H'  

# Store screen output for Screen:
# Ctrl+A, Shift+H  
# You will then find a screen.log file under current directory.  

Variable

Variable substitution within quotes

# foo=bar
echo $foo
# bar
echo "$foo"
# bar
# single quotes cause variables to not be expanded
echo '$foo'
# $foo
# single quotes within double quotes will not cancel expansion and will be part of the output
echo "'$foo'"
# 'bar'
# doubled single quotes act as if there are no quotes at all
echo ''$foo''
# bar

Get the length of variable

var="some string"
echo ${#var}
# 11

Get the first character of the variable

var=string
echo "${var:0:1}"
#s

# or
echo ${var%%"${var#?}"}

Remove the first or last string from variable

var="some string"
echo ${var:2}
#me string

Replacement (e.g. remove the first leading 0 )

var="0050"
echo ${var[@]#0}
#050

Replacement (e.g. replace 'a' with ',')

{var/a/,}

Replace all (e.g. replace all 'a' with ',')

{var//a/,}

Substitute environment variables in a file

export NAME="Alice"
envsubst < template.txt > output.txt
# template.txt:       Hello ${NAME}, your order is shipped.
# output.txt:         Hello Alice, your order is shipped.
# or
export USER="Bob" && echo 'Hi $USER, welcome!' | envsubst
# Hi Bob, welcome!

Grep lines with strings from a file (e.g. lines with 'stringA or 'stringB' or 'stringC')

# with grep
test="stringA stringB stringC"
grep ${test// /\\\|} file.txt
# turning the space into 'or' (\|) in grep

To change the case of the string stored in the variable to lowercase (Parameter Expansion)

var=HelloWorld
echo ${var,,}
helloworld

Expand and then execute variable/argument

cmd="bar=foo"
eval "$cmd"
echo "$bar" # foo

Record a terminal session

# https://github.com/asciinema/asciinema
asciinema rec demo.cast

Math

Arithmetic Expansion in Bash (Operators: +, -, *, /, %, etc)

echo $(( 10 + 5 ))  #15
x=1
echo $(( x++ )) #1 , notice that it is still 1, since it's post-increment
echo $(( x++ )) #2
echo $(( ++x )) #4 , notice that it is not 3 since it's pre-increment
echo $(( x-- )) #4
echo $(( x-- )) #3
echo $(( --x )) #1
x=2
y=3
echo $(( x ** y )) #8

Print out the prime factors of a number (e.g. 50)

factor 50
# 50: 2 5 5

Sum up input list (e.g. seq 10)

seq 10|paste -sd+|bc

Sum up a file (each line in file contains only one number)

awk '{s+=$1} END {print s}' filename

Column subtraction

cat file| awk -F '\t' 'BEGIN {SUM=0}{SUM+=$3-$2}END{print SUM}'

Simple math with expr

expr 10+20 #30
expr 10\*20 #600
expr 30 \> 20 #1 (true)

More math with bc

# Number of decimal digit/ significant figure
echo "scale=2;2/3" | bc

# .66

# Exponent operator
echo "10^2" | bc
# 100

# Using variables
echo "var=5;--var"| bc
# 4

Grep

Type of grep

grep = grep -G # Basic Regular Expression (BRE)
fgrep = grep -F # fixed text, ignoring meta-characters
egrep = grep -E # Extended Regular Expression (ERE)
rgrep = grep -r # recursive
grep -P # Perl Compatible Regular Expressions (PCRE)

Grep and count number of empty lines

grep -c "^$"

Grep and return only integer

grep -o '[0-9]*'
# or
grep -oP '\d*'

Grep integer with certain number of digits (e.g. 3)

grep '[0-9]\{3\}'

# or
grep -E '[0-9]{3}'

# or
grep -P '\d{3}'

Grep only IP address

grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'

# or
grep -Po '\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}'

Grep whole word (e.g. 'target')

grep -w 'target'

# or using RE
grep '\btarget\b'

Grep returning lines before and after match (e.g. 'bbo')

# return also 3 lines after match
grep -A 3 'bbo'

# return also 3 lines before match
grep -B 3 'bbo'

# return also 3 lines before and after match
grep -C 3 'bbo'

Grep string starting with (e.g. 'S')

grep -o 'S.*'

Extract text between words (e.g. w1,w2)

grep -o -P '(?<=w1).*(?=w2)'

Grep lines without word (e.g. 'bbo')

grep -v bbo filename

Grep lines not begin with string (e.g. #)

grep -v '^#' file.txt

Grep variables with space within it (e.g. myvar="some strings")

grep "$myvar" filename
# remember to quote the variable!

Grep only one/first match (e.g. 'bbo')

grep -m 1 bbo filename

Grep and return number of matching line(e.g. 'bbo')

grep -c bbo filename

Count occurrence (e.g. three times a line count three times)

grep -o bbo filename |wc -l

Case insensitive grep (e.g. 'bbo'/'BBO'/'Bbo')

grep -i "bbo" filename

COLOR the match (e.g. 'bbo')!

grep --color bbo filename

Grep search all files in a directory(e.g. 'bbo')

grep -R bbo /path/to/directory
# or
grep -r bbo /path/to/directory

Search all files in directory, do not ouput the filenames (e.g. 'bbo')

grep -rh bbo /path/to/directory

Search all files in directory, output ONLY the filenames with matches(e.g. 'bbo')

grep -rl bbo /path/to/directory

Grep OR (e.g. A or B or C or D)

grep 'A\|B\|C\|D'

Grep AND (e.g. A and B)

grep 'A.*B'

Regex any single character (e.g. ACB or AEB)

grep 'A.B'

Regex with or without a certain character (e.g. color or colour)

grep 'colou\?r'

Grep all content of a fileA from fileB

grep -f fileA fileB

Grep a tab

grep $'\t'

Grep variable from variable

$echo "$long_str"|grep -q "$short_str"
if [ $? -eq 0 ]; then echo 'found'; fi
# grep -q will output 0 if match found
# remember to add space between []!

Grep strings between a bracket()

grep -oP '\(\K[^\)]+'

Grep number of characters with known strings in between(e.g. AAEL000001-RA)

grep -o -w "\w\{10\}\-R\w\{1\}"
# \w word character [0-9a-zA-Z_] \W not word character

Skip directory (e.g. 'bbo')

grep -d skip 'bbo' /path/to/files/*

Sed

Remove the 1st line

sed 1d filename

Remove the first 100 lines (remove line 1-100)

sed 1,100d filename

Remove lines with string (e.g. 'bbo')

sed "/bbo/d" filename
# case insensitive:
sed "/bbo/Id" filename

Remove lines whose nth character not equal to a value (e.g. 5th character not equal to 2)

sed -E '/^.{5}[^2]/d'
#aaaa2aaa (you can stay)
#aaaa1aaa (delete!)

Edit infile (edit and save to file), (e.g. deleting the lines with 'bbo' and save to file)

sed -i "/bbo/d" filename

When using variable (e.g. $i), use double quotes " "

# e.g. add >$i to the first line (to make a bioinformatics FASTA file)
sed "1i >$i"
# notice the double quotes! in other examples, you can use a single quote, but here, no way!
# '1i' means insert to first line

Using environment variable and end-of-line pattern at the same time.

# Use backslash for end-of-line $ pattern, and double quotes for expressing the variable
sed -e "\$s/\$/\n+--$3-----+/"

Delete/remove empty lines

sed '/^\s*$/d'

# or

sed '/^$/d'

Delete/remove last line

sed '$d'

Delete/remove last character from end of file

sed -i '$ s/.$//' filename

Add string to beginning of file (e.g. "[")

sed -i '1s/^/[/' filename

Add string at certain line number (e.g. add 'something' to line 1 and line 3)

sed -e '1isomething' -e '3isomething'

Add string to end of file (e.g. "]")

sed '$s/$/]/' filename

Add newline to the end

sed '$a\'

Add string to beginning of every line (e.g. 'bbo')

sed -e 's/^/bbo/' filename

Add string to end of each line (e.g. "}")

sed -e 's/$/\}\]/' filename

Add \n every nth character (e.g. every 4th character)

sed 's/.\{4\}/&\n/g'

Add a line after the line that matches the pattern (e.g. add a new line with "world" after the line with "hello")

sed '/hello*/a world' filename
# hello
# world

Concatenate/combine/join files with a separator and next line (e.g separate by ",")

sed -s '$a,' *.json > all.json

Substitution (e.g. replace A by B)

sed 's/A/B/g' filename

Substitution with wildcard (e.g. replace a line start with aaa= by aaa=/my/new/path)

sed "s/aaa=.*/aaa=\/my\/new\/path/g"

Select lines start with string (e.g. 'bbo')

sed -n '/^@S/p'

Delete lines with string (e.g. 'bbo')

sed '/bbo/d' filename

Print/get/trim a range of line (e.g. line 500-5000)

sed -n 500,5000p filename

Print every nth lines

sed -n '0~3p' filename

# catch 0: start; 3: step

Print every odd # lines

sed -n '1~2p'

Print every third line including the first line

sed -n '1p;0~3p'

Remove leading whitespace and tabs

sed -e 's/^[ \t]*//'
# Notice a whitespace before '\t'!!

Remove only leading whitespace

sed 's/ *//'

# notice a whitespace before '*'!!

Remove ending commas

sed 's/,$//g'

Add a column to the end

sed "s/$/\t$i/"
# $i is the valuable you want to add

# To add the filename to every last column of the file
for i in $(ls); do sed -i "s/$/\t$i/" $i; done

Add extension of filename to last column

for i in T000086_1.02.n T000086_1.02.p; do sed "s/$/\t${i/*./}/" $i; done >T000086_1.02.np

Remove newline\ nextline

sed ':a;N;$!ba;s/\n//g'

Print a particular line (e.g. 123th line)

sed -n -e '123p'

Print a number of lines (e.g. line 10th to line 33 rd)

sed -n '10,33p' <filename

Change delimiter

sed 's=/=\\/=g'

Replace with wildcard (e.g A-1-e or A-2-e or A-3-e....)

sed 's/A-.*-e//g' filename

Remove last character of file

sed '$ s/.$//'

Insert character at specified position of file (e.g. AAAAAA --> AAA#AAA)

sed -r -e 's/^.{3}/&#/' filename

Awk

Set tab as field separator

awk -F $'\t'

Output as tab separated (also as field separator)

awk -v OFS='\t'

Pass variable

a=bbo;b=obb;
awk -v a="$a" -v b="$b" "$1==a && $10=b" filename

Print line number and number of characters on each line

awk '{print NR,length($0);}' filename

Find number of columns

awk '{print NF}'

Reverse column order

awk '{print $2, $1}'

Check if there is a comma in a column (e.g. column $1)

awk '$1~/,/ {print}'

Split and do for loop

awk '{split($2, a,",");for (i in a) print $1"\t"a[i]}' filename

Print all lines before nth occurrence of a string (e.g stop print lines when 'bbo' appears 7 times)

awk -v N=7 '{print}/bbo/&& --N<=0 {exit}'

Print filename and last line of all files in directory

ls|xargs -n1 -I file awk '{s=$0};END{print FILENAME,s}' filename

Add string to the beginning of a column (e.g add "chr" to column $3)

awk 'BEGIN{OFS="\t"}$3="chr"$3'

Remove lines with string (e.g. 'bbo')

awk '!/bbo/' filename

Remove last column

awk 'NF{NF-=1};1' filename

Usage and meaning of NR and FNR

# For example there are two files:
# fileA:
# a
# b
# c
# fileB:
# d
# e
awk 'print FILENAME, NR,FNR,$0}' fileA fileB
# fileA    1    1    a
# fileA    2    2    b
# fileA    3    3    c
# fileB    4    1    d
# fileB    5    2    e

AND gate

# For example there are two files:
# fileA:
# 1    0
# 2    1
# 3    1
# 4    0
# fileB:
# 1    0
# 2    1
# 3    0
# 4    1

awk -v OFS='\t' 'NR=FNR{a[$1]=$2;next} NF {print $1,((a[$1]=$2)? $2:"0")}' fileA fileB
# 1    0
# 2    1
# 3    0
# 4    0

Round all numbers of file (e.g. 2 significant figure)

awk '{while (match($0, /[0-9]+\[0-9]+/)){
    \printf "%s%.2f", substr($0,0,RSTART-1),substr($0,RSTART,RLENGTH)
    \$0=substr($0, RSTART+RLENGTH)
    \}
    \print
    \}'

Give number/index to every row

awk '{printf("%s\t%s\n",NR,$0)}'

Break combine column data into rows

# For example, separate the following content:
# David    cat,dog
# into
# David    cat
# David    dog

awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' filename

# Detail here:　http://stackoverflow.com/questions/33408762/bash-turning-single-comma-separated-column-into-multi-line-string

Average a file (each line in file contains only one number)

awk '{s+=$1}END{print s/NR}'

Print field start with string (e.g Linux)

awk '$1 ~ /^Linux/'

Sort a row (e.g. 1 40 35 12 23 --> 1 12 23 35 40)

awk ' {split( $0, a, "\t" ); asort( a ); for( i = 1; i <= length(a); i++ ) printf( "%s\t", a[i] ); printf( "\n" ); }'

Subtract previous row values (add column6 which equal to column4 minus last column5)

awk '{$6 = $4 - prev5; prev5 = $5; print;}'

Xargs

Set tab as delimiter (default:space)

xargs -d\t

Prompt commands before running commands

ls|xargs -L1 -p head

Display 3 items per line

echo 1 2 3 4 5 6| xargs -n 3
# 1 2 3
# 4 5 6

Prompt before execution

echo a b c |xargs -p -n 3

Print command along with output

xargs -t abcd
# bin/echo abcd
# abcd

With find and rm

find . -name "*.html"|xargs rm

# when using a backtick
rm `find . -name "*.html"`

Delete files with whitespace in filename (e.g. "hello 2001")

find . -name "*.c" -print0|xargs -0 rm -rf

Show limits on command-line length

xargs --show-limits
# Output from my Ubuntu:
# Your environment variables take up 3653 bytes
# POSIX upper limit on argument length (this system): 2091451
# POSIX smallest allowable upper limit on argument length (all systems): 4096
# Maximum length of command we could actually use: 2087798
# Size of command buffer we are actually using: 131072
# Maximum parallelism (--max-procs must be no greater): 2147483647

Move files to folder

find . -name "*.bak" -print 0|xargs -0 -I {} mv {} ~/old

# or
find . -name "*.bak" -print 0|xargs -0 -I file mv file ~/old

Move first 100th files to a directory (e.g. d1)

ls |head -100|xargs -I {} mv {} d1

Parallel

time echo {1..5} |xargs -n 1 -P 5 sleep

# a lot faster than:
time echo {1..5} |xargs -n1 sleep

Copy all files from A to B

find /dir/to/A -type f -name "*.py" -print 0| xargs -0 -r -I file cp -v -p file --target-directory=/path/to/B

# v: verbose|
# p: keep detail (e.g. owner)

With sed

ls |xargs -n1 -I file sed -i '/^Pos/d' filename

Add the file name to the first line of file

ls |sed 's/.txt//g'|xargs -n1 -I file sed -i -e '1 i\>file\' file.txt

Count all files

ls |xargs -n1 wc -l

Turn output into a single line

ls -l| xargs

Count files within directories

echo mso{1..8}|xargs -n1 bash -c 'echo -n "$1:"; ls -la "$1"| grep -w 74 |wc -l' --
# "--" signals the end of options and display further option processing

Count lines in all file, also count total lines

ls|xargs wc -l

Xargs and grep

cat grep_list |xargs -I{} grep {} filename

Xargs and sed (replace all old ip address with new ip address under /etc directory)

grep -rl '192.0.2.1' /etc | xargs sed -i 's/192.0.2.1/192.0.2.2/g'

Find

List all sub directory/file in the current directory

find .

List all files under the current directory

find . -type f

List all directories under the current directory

find . -type d

Edit all files under current directory (e.g. replace 'www' with 'ww')

find . -name '*.php' -exec sed -i 's/www/w/g' {} \;

# if there are no subdirectory
replace "www" "w" -- *
# a space before *

Find and output only filename (e.g. "mso")

find mso*/ -name M* -printf "%f\n"

Find large files in the system (e.g. >4G)

find / -type f -size +4G

Find and delete file with size less than (e.g. 74 byte)

find . -name "*.mso" -size -74c -delete

# M for MB, etc

Find empty (0 byte) files

find . -type f -empty
# to further delete all the empty files
find . -type f -empty -delete

Recursively count all the files in a directory

find . -type f | wc -l

Condition and loop

If statement

# if and else loop for string matching
if [[ "$c" == "read" ]]; then outputdir="seq"; else outputdir="write" ; fi

# Test if myfile contains the string 'test':
if grep -q hello myfile; then echo -e "file contains the string!" ; fi

# Test if mydir is a directory, change to it and do other stuff:
if cd mydir; then
  echo 'some content' >myfile
else
  echo >&2 "Fatal error. This script requires mydir."
fi

# Check if a variable is null
if [ -z "$var" ]; then echo "NULL"; else echo "Not NULL"; fi
# or 
[ -z "$var" ] && echo "NULL"

# Using test command (same as []), to test if the length of variable is nonzero
test -n "$myvariable" && echo myvariable is "$myvariable" || echo myvariable is not set

# Test if file exist
if [ -e 'filename' ]
then
  echo -e "file exists!"
fi

# Test if file exist but also including symbolic links:
if [ -e myfile ] || [ -L myfile ]
then
  echo -e "file exists!"
fi

# Test if the value of x is greater or equal than 5
if [ "$x" -ge 5 ]; then echo -e "greater or equal than 5!" ; fi

# Test if the value of x is greater or equal than 5, in bash/ksh/zsh:
if ((x >= 5)); then echo -e "greater or equal than 5!" ; fi

# Use (( )) for arithmetic operation
if ((j==u+2)); then echo -e "j==u+2!!" ; fi

# Use [[ ]] for comparison
if [[ $age -gt 21 ]]; then echo -e "forever 21!!" ; fi

For loop

# Echo the file name under the current directory
for i in $(ls); do echo file $i; done
# or
for i in *; do echo file $i; done

# Make directories listed in a file (e.g. myfile)
for dir in $(<myfile); do mkdir $dir; done

# Press any key to continue each loop
for i in $(cat tpc_stats_0925.log |grep failed|grep -o '\query\w\{1,2\}'); do cat ${i}.log; read -rsp $'Press any key to continue...\n' -n1 key; done

# Print a file line by line when a key is pressed,
oifs="$IFS"; IFS=$'\n'; for line in $(cat myfile); do ...; done
while read -r line; do ...; done <myfile

# If only one word a line, simply
for line in $(cat myfile); do echo $line; read -n1; done

# Loop through an array
for i in "${arrayName[@]}"; do echo $i; done

While loop,

# Column subtraction of a file (e.g. a 3 columns file)
while read a b c; do echo $(($c-$b)); done < <(head filename)
#there is a space between the two '<'s

# Sum up column subtraction
i=0; while read a b c; do ((i+=$c-$b)); echo $i; done < <(head filename)

# Keep checking a running process (e.g. perl) and start another new process (e.g. python) immediately after it. (BETTER use the wait command! Ctrl+F 'wait')
while [[ $(pidof perl) ]]; do echo f; sleep 10; done && python timetorunpython.py

switch (case in bash)

read type;
case $type in
  '0')
    echo 'how'
    ;;
  '1')
    echo 'are'
    ;;
  '2')
    echo 'you'
    ;;
esac

Time

Find out the time require for executing a command

time echo hi

Wait for some time (e.g 10s)

sleep 10

Print date with formatting

date +%F
# 2020-07-19

# or
date +'%d-%b-%Y-%H:%M:%S'
# 10-Apr-2020-21:54:40

# Returns the current time with nanoseconds.
date +"%T.%N"
# 11:42:18.664217000  

# Get the seconds since epoch (Jan 1 1970) for a given date (e.g Mar 16 2021)
date -d "Mar 16 2021" +%s
# 1615852800
# or
date -d "Tue Mar 16 00:00:00 UTC 2021"  +%s
# 1615852800  

# Convert the number of seconds since epoch back to date
date --date @1615852800
# Tue Mar 16 00:00:00 UTC 2021

Print current time point for N days ago or N days after

# print current date first (for the following example)
date +"%F %H:%M:%S"
# 2023-03-11 16:17:09

# print the time that is 1 day ago
date -d"1 day ago" +"%F %H:%M:%S"
# 2023-03-10 16:17:09

# print the time that is 7 days ago
date -d"7 days ago" +"%F %H:%M:%S"
# 2023-03-04 16:17:09

# print the time that is a week ago
date -d"1 week ago" +"%F %H:%M:%S"
# 2023-03-04 16:17:09

# add 1 day to date
date -d"-1 day ago" +"%F %H:%M:%S"
# 2023-03-12 16:17:09

wait for random duration (e.g. sleep 1-5 second, like adding a jitter)

sleep $[ ( $RANDOM % 5 ) + 1 ]

Log out your account after a certain period of time (e.g 10 seconds)

TMOUT=10
#once you set this variable, logout timer start running!

Set how long you want to run a command

# This will run the command 'sleep 10' for only 1 second.
timeout 1 sleep 10

Set when you want to run a command (e.g 1 min from now)

at now + 1min  #time-units can be minutes, hours, days, or weeks
warning: commands will be executed using /bin/sh
at> echo hihigithub >~/itworks
at> <EOT>   # press Ctrl + D to exit
job 1 at Wed Apr 18 11:16:00 2018

Download

Download the content of this README.md (the one your are viewing now)

curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc -f markdown -t man | man -l -

# or w3m (a text based web browser and pager)
curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc | w3m -T text/html

# or using emacs (in emac text editor)
emacs --eval '(org-mode)' --insert <(curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc -t org)

# or using emacs (on terminal, exit using Ctrl + x then Ctrl + c)
emacs -nw --eval '(org-mode)' --insert <(curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc -t org)

Download all from a page

wget -r -l1 -H -t1 -nd -N -np -A mp3 -e robots=off http://example.net

# -r: recursive and download all links on page
# -l1: only one level link
# -H: span host, visit other hosts
# -t1: numbers of retries
# -nd: don't make new directories, download to here
# -N: turn on timestamp
# -nd: no parent
# -A: type (separate by ,)
# -e robots=off: ignore the robots.txt file which stop wget from crashing the site, sorry example.net

Upload a file to web and download (https://transfer.sh/)

#  Upload a file (e.g. filename.txt):
curl --upload-file ./filename.txt https://transfer.sh/filename.txt
# the above command will return a URL, e.g: https://transfer.sh/tG8rM/filename.txt

# Next you can download it by:
curl https://transfer.sh/tG8rM/filename.txt -o filename.txt

Download file if necessary

data=file.txt
url=http://www.example.net/$data
if [ ! -s $data ];then
    echo "downloading test data..."
    wget $url
fi

Wget to a filename (when a long name)

wget -O filename "http://example.net"

Wget files to a folder

wget -P /path/to/directory "http://example.net"

Instruct curl to follow any redirect until it reaches the final destination:

curl -L example.net

Random

Random generate password (e.g. generate 5 password each of length 13)

sudo apt install pwgen
pwgen 13 5
#sahcahS9dah4a xieXaiJaey7xa UuMeo0ma7eic9 Ahpah9see3zai acerae7Huigh7

Random pick 100 lines from a file

shuf -n 100 filename

Random order (lucky draw)

for i in a b c d e; do echo $i; done | shuf

Echo series of random numbers between a range (e.g. shuffle numbers from 0-100, then pick 15 of them randomly)

shuf -i 0-100 -n 15

Echo a random number

echo $RANDOM

Random from 0-9

echo $((RANDOM % 10))

Random from 1-10

echo $(((RANDOM %10)+1))

Xwindow

Enable X11 forwarding,in order to use graphical application on servers

ssh -X user_name@ip_address

# or setting through xhost
# --> Install the following for Centos:
# xorg-x11-xauth
# xorg-x11-fonts-*
# xorg-x11-utils

Little xwindow tools

xclock
xeyes
xcowsay

Open pictures/images from ssh server

1. ssh -X user_name@ip_address
2. apt-get install eog
3. eog picture.png

Watch videos on server

1. ssh -X user_name@ip_address
2. sudo apt install mpv
3. mpv myvideo.mp4

Use gedit on server (GUI editor)

1. ssh -X user_name@ip_address
2. apt-get install gedit
3. gedit filename.txt

Open PDF file from ssh server

1. ssh -X user_name@ip_address
2. apt-get install evince
3. evince filename.pdf

Use google-chrome browser from ssh server (Spyware, so don't)

1. ssh -X user_name@ip_address
2. apt-get install libxss1 libappindicator1 libindicator7
3. wget https://dl.example.net/linux/direct/google-chrome-stable_current_amd64.deb
4. sudo apt-get install -f
5. dpkg -i google-chrome*.deb
6. google-chrome

System

Work with yum history

# List yum history (e.g install, update)
sudo yum history
# Example output:
# Loaded plugins: extras_suggestions, langpacks, priorities, update-motd
# ID     | Login user               | Date and time    | Action(s)      | Altered
# -------------------------------------------------------------------------------
#     11 |  ... <myuser>       | 2020-04-10 10:57 | Install        |    1 P<
#     10 |  ... <myuser>       | 2020-03-27 05:21 | Install        |    1 >P
#      9 |  ... <myuser>       | 2020-03-05 11:57 | I, U           |   56 *<
# ...

# Show more details of a yum history (e.g. history #11)
sudo yum history info 11

# Undo a yum history (e.g. history #11, this will uninstall some packages)
sudo yum history undo 11

Audit files to see who made changes to a file [RedHat based system only]

# To audit a directory recursively for changes (e.g. myproject)
auditctl -w /path/to/myproject/ -p wa

# If you delete a file name "VIPfile", the deletion is recorded in /var/log/audit/audit.log
sudo grep VIPfile /var/log/audit/audit.log
#type=PATH msg=audit(1581417313.678:113): item=1 name="VIPfile" inode=300115 dev=ca:01 mode=0100664 ouid=1000 ogid=1000 rdev=00:00 nametype=DELETE cap_fp=0000000000000000 cap_fi=0000000000000000 cap_fe=0 cap_fver=0

Check out whether SELinux is enabled

sestatus
# SELinux status:                 enabled
# SELinuxfs mount:                /sys/fs/selinux
# SELinux root directory:         /etc/selinux
# Loaded policy name:             targeted
# Current mode:                   enforcing
# Mode from config file:          enforcing
# Policy MLS status:              enabled
# Policy deny_unknown status:     allowed
# Max kernel policy version:      31

Generate public key from private key

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

Copy your default public key to remote user

ssh-copy-id <user_name>@<server_IP>
# then you need to enter the password
# and next time you won't need to enter password when ssh to that user

Copy default public key to remote user using the required private key (e.g. use your mykey.pem key to copy your id_rsa.pub to the remote user)

# before you need to use mykey.pem to ssh to remote user.
ssh-copy-id -i ~/.ssh/id_rsa.pub -o "IdentityFile ~/Downloads/mykey.pem" <user_name>@<server_IP>
# now you don't need to use key to ssh to that user.

SSH Agent Forwarding

# To bring your key with you when ssh to serverA, then ssh to serverB from serverA using the key.
ssh-agent
ssh-add /path/to/mykey.pem
ssh -A <username>@<IP_of_serverA>
# Next you can ssh to serverB
ssh <username>@<IP_of_serverB>

Set the default user and key for a host when using SSH

# add the following to ~/.ssh/config
Host myserver
  User myuser
  IdentityFile ~/path/to/mykey.pem

# Next, you could run "ssh myserver" instead of "ssh -i ~/path/to/mykey.pem myuser@myserver"

Follow the most recent logs from service

journalctl -u <service_name> -f

Eliminate the zombie

# A zombie is already dead, so you cannot kill it. You can eliminate the zombie by killing its parent.
# First, find PID of the zombie
ps aux| grep 'Z'
# Next find the PID of zombie's parent
pstree -p -s <zombie_PID>
# Then you can kill its parent and you will notice the zombie is gone.
sudo kill 9 <parent_PID>

Show memory usage

free -c 10 -mhs 1
# print 10 times, at 1 second interval

Display CPU and IO statistics for devices and partitions.

# refresh every second
iostat -x -t 1

Display bandwidth usage on an network interface (e.g. enp175s0f0)

iftop -i enp175s0f0

Tell how long the system has been running and number of users

uptime

Check if it's root running

if [ "$EUID" -ne 0 ]; then
        echo "Please run this as root"
        exit 1
fi

Change shell of a user (e.g. bonnie)

chsh -s /bin/sh bonnie
# /etc/shells: valid login shells

Change root / fake root / jail (e.g. change root to newroot)

chroot /home/newroot /bin/bash

# To exit chroot
exit

Display file status (size; access, modify and change time, etc) of a file (e.g. filename.txt)

stat filename.txt

Snapshot of the current processes

ps aux

List processes by top memory usage

ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head

Display a tree of processes

pstree

# or
ps aux --forest

Find maximum number of processes

cat /proc/sys/kernel/pid_max

Print or control the kernel ring buffer

dmesg

Show IP address

$ip add show

# or
ifconfig

Print previous and current SysV runlevel

runlevel

# or
who -r

Change SysV runlevel (e.g. 5)

init 5
# or
telinit 5

Display all available services in all runlevels,

chkconfig --list
# update-rc.d equivalent to chkconfig in ubuntu

Check system version

cat /etc/*-release

Linux Programmer's Manuel: hier- description of the filesystem hierarchy

man hier

Control the systemd system and service manager

# e.g. check the status of cron service
systemctl status cron.service

# e.g. stop cron service
systemctl stop cron.service

List job

jobs -l

Run a program with modified priority (e.g. ./test.sh)

# nice value is adjustable from -20 (most favorable) to +19
# the nicer the application, the lower the priority
# Default niceness: 10; default priority: 80

nice -10 ./test.sh

Export PATH

export PATH=$PATH:~/path/you/want

Make file executable

chmod +x filename
# you can now ./filename to execute it

Print system information

uname -a

# Check system hardware-platform (x86-64)
uname -i

Surf the net

links www.example.net

Add user, set passwd

useradd username
passwd username

Edit PS1 variable for bash (e.g. displaying the whole path)

1. vi ~/.bash_profile
2. export PS1='\u@\h:\w\$'
# $PS1 is a variable that defines the makeup and style of the command prompt
# You could use emojis and add timestamp to every prompt using the following value:
# export PS1="\t@🦁:\w\$ "
3. source ~/.bash_profile

Edit environment setting (e.g. alias)

1. vi ~/.bash_profile
2. alias pd="pwd" //no more need to type that 'w'!
3. source ~/.bash_profile

Print all alias

alias -p

Unalias (e.g. after alias ls='ls --color=auto')

unalias ls

Set and unset shell options

# print all shell options
shopt

# to unset (or stop) alias
shopt -u expand_aliases

# to set (or start) alias
shopt -s expand_aliases

List environment variables (e.g. PATH)

echo $PATH
# list of directories separated by a colon

List all environment variables for current user

env

Unset environment variable (e.g. unset variable 'MYVAR')

unset MYVAR

Show partition format

lsblk

Inform the OS of partition table changes

partprobe

Soft link program to bin

ln -s /path/to/program /home/usr/bin
# must be the whole path to the program

Show hexadecimal view of data

hexdump -C filename.class

Jump to different node

rsh node_name

Check port (active internet connection)

netstat -tulpn

Print resolved symbolic links or canonical file names

readlink filename

Find out the type of command and where it link to (e.g. python)

type python
# python is /usr/bin/python
# There are 5 different types, check using the 'type -f' flag
# 1. alias    (shell alias)
# 2. function (shell function, type will also print the function body)
# 3. builtin  (shell builtin)
# 4. file     (disk file)
# 5. keyword  (shell reserved word)

# You can also use `which`
which python
# /usr/bin/python

List all functions names

declare -F

List total size of a directory

du -hs .

# or
du -sb

Copy directory with permission setting

cp -rp /path/to/directory

Store current directory

pushd .

# then pop
popd

# or use dirs to display the list of currently remembered directories.
dirs -l

Show disk usage

df -h

# or
du -h

# or
du -sk /var/log/* |sort -rn |head -10

check the Inode utilization

df -i
# Filesystem      Inodes  IUsed   IFree IUse% Mounted on
# devtmpfs        492652    304  492348    1% /dev
# tmpfs           497233      2  497231    1% /dev/shm
# tmpfs           497233    439  496794    1% /run
# tmpfs           497233     16  497217    1% /sys/fs/cgroup
# /dev/nvme0n1p1 5037976 370882 4667094    8% /
# tmpfs           497233      1  497232    1% /run/user/1000

Show all file system type

df -TH

Show current runlevel

runlevel

Switch runlevel

init 3

# or
telinit 3

Permanently modify runlevel

1. edit /etc/init/rc-sysinit.conf
2. env DEFAULT_RUNLEVEL=2

Become root

su

Become somebody

su somebody

Report user quotes on device

repquota -auvs

Get entries in a number of important databases

getent database_name

# (e.g. the 'passwd' database)
getent passwd
# list all user account (all local and LDAP)

# (e.g. fetch list of grop accounts)
getent group
# store in database 'group'

Change owner of file

chown user_name filename
chown -R user_name /path/to/directory/
# chown user:group filename

Mount and unmount

# e.g. Mount /dev/sdb to /home/test
mount /dev/sdb /home/test

# e.g. Unmount /home/test
umount /home/test

List current mount detail

mount
# or
df

List current usernames and user-numbers

cat /etc/passwd

Get all username

getent passwd| awk '{FS="[:]"; print $1}'

Show all users

compgen -u

Show all groups

compgen -g

Show group of user

group username

Show uid, gid, group of user

id username

# variable for UID
echo $UID

Check if it's root

if [ $(id -u) -ne 0 ];then
    echo "You are not root!"
    exit;
fi
# 'id -u' output 0 if it's not root

Find out CPU information

more /proc/cpuinfo

# or
lscpu

Set quota for user (e.g. disk soft limit: 120586240; hard limit: 125829120)

setquota username 120586240 125829120 0 0 /home

Show quota for user

quota -v username

Display current libraries from the cache

ldconfig -p

Print shared library dependencies (e.g. for 'ls')

ldd /bin/ls

Check the most recent login of all users

lastlog

Check last reboot history

last reboot

Edit path for all users

joe /etc/environment
# edit this file

Show and set user limit

ulimit -u

Print out number of cores/ processors

nproc --all

Check status of each core

1. top
2. press '1'

Show jobs and PID

jobs -l

List all running services

service --status-all

Schedule shutdown server

shutdown -r +5 "Server will restart in 5 minutes. Please save your work."

Cancel scheduled shutdown

shutdown -c

Broadcast to all users

wall -n hihi

Kill all process of a user

pkill -U user_name

Kill all process of a program

kill -9 $(ps aux | grep 'program_name' | awk '{print $2}')

Set gedit preference on server

# You might have to install the following:

apt-get install libglib2.0-bin;
# or
yum install dconf dconf-editor;
yum install dbus dbus-x11;

# Check list
gsettings list-recursively

# Change some settings
gsettings set org.gnome.gedit.preferences.editor highlight-current-line true
gsettings set org.gnome.gedit.preferences.editor scheme 'cobalt'
gsettings set org.gnome.gedit.preferences.editor use-default-font false
gsettings set org.gnome.gedit.preferences.editor editor-font 'Cantarell Regular 12'

Add user to a group (e.g add user 'nice' to the group 'docker', so that he can run docker without sudo)

sudo gpasswd -a nice docker

Pip install python package without root

1. pip install --user package_name
2. You might need to export ~/.local/bin/ to PATH: export PATH=$PATH:~/.local/bin/

Removing old linux kernels (when /boot almost full...)

1. uname -a  #check current kernel, which should NOT be removed
2. sudo apt-get purge linux-image-X.X.X-X-generic  #replace old version

Change hostname

sudo hostname your-new-name

# if not working, do also:
hostnamectl set-hostname your-new-hostname
# then check with:
hostnamectl
# Or check /etc/hostname

# If still not working..., edit:
/etc/sysconfig/network
/etc/sysconfig/network-scripts/ifcfg-ensxxx
#add HOSTNAME="your-new-hostname"

List installed packages

apt list --installed

# or on Red Hat:
yum list installed

Check for package update

apt list --upgradeable

# or
sudo yum check-update

Run yum update excluding a package (e.g. do not update php packages)

sudo yum update --exclude=php*

Check which file make the device busy on umount

lsof /mnt/dir

When sound not working

killall pulseaudio
# then press Alt-F2 and type in pulseaudio

List information about SCSI devices

lsscsi

Tutorial for setting up your own DNS server

Tutorial for creating a simple daemon

Tutorial for using your gmail to send email

Using telnet to test open ports, test if you can connect to a port (e.g 53) of a server (e.g 192.168.2.106)

telnet 192.168.2.106 53

Change network maximum transmission unit (mtu) (e.g. change to 9000)

ifconfig eth0 mtu 9000

Get pid of a running process (e.g python)

pidof python

# or
ps aux|grep python

Check status of a process using PID

ps -p <PID>

# or
cat /proc/<PID>/status
cat /proc/<PID>/stack
cat /proc/<PID>/stat

NTP

# Start ntp:
ntpd

# Check ntp:
ntpq -p

Remove unnecessary files to clean your server

sudo apt-get autoremove
sudo apt-get clean
sudo rm -rf ~/.cache/thumbnails/*

# Remove old kernal:
sudo dpkg --list 'linux-image*'
sudo apt-get remove linux-image-OLDER_VERSION

Increase/ resize root partition (root partition is an LVM logical volume)

pvscan
lvextend -L +130G /dev/rhel/root -r
# Adding -r will grow filesystem after resizing the volume.

Create a UEFI Bootable USB drive (e.g. /dev/sdc1)

sudo dd if=~/path/to/isofile.iso of=/dev/sdc1 oflag=direct bs=1048576

Locate and remove a package

sudo dpkg -l | grep <package_name>
sudo dpkg --purge <package_name>

Create a ssh tunnel

ssh -f -L 9000:targetservername:8088 root@192.168.14.72 -N
#-f: run in background; -L: Listen; -N: do nothing
# the 9000 of your computer is now connected to the 8088 port of the targetservername through 192.168.14.72
# so that you can see the content of targetservername:8088 by entering localhost:9000 from your browser.

Get process ID of a process (e.g. sublime_text)

# pidof
pidof sublime_text

# pgrep, you don't have to type the whole program name
pgrep sublim

# pgrep, echo 1 if process found, echo 0 if no such process
pgrep -q sublime_text && echo 1 || echo 0

# top, takes longer time
top|grep sublime_text

Some benchmarking tools for your server

Performance monitoring tool - sar

# installation
# It collects the data every 10 minutes and generate its report daily. crontab file (/etc/cron.d/sysstat) is responsible for collecting and generating reports.
yum install sysstat
systemctl start sysstat
systemctl enable sysstat

# show CPU utilization 5 times every 2 seconds.
sar 2 5

# show memory  utilization 5 times every 2 seconds.
sar -r 2 5

# show paging statistics 5 times every 2 seconds.
sar -B 2 5

# To generate all network statistic:
sar -n ALL

# reading SAR log file using -f
sar -f /var/log/sa/sa31|tail

Reading from journal file

journalctl --file ./log/journal/a90c18f62af546ccba02fa3734f00a04/system.journal  --since "2020-02-11 00:00:00"

Show a listing of last logged in users

last

Show a listing of unsuccessful (bad) login attempts

lastb

Show a listing of current logged in users, print information of them

who

Show who is logged on and what they are doing

w

Print the user names of users currently logged in to the current host.

users

Stop tailing a file on program terminate

tail -f --pid=<PID> filename.txt
# replace <PID> with the process ID of the program.

List all enabled services

systemctl list-unit-files|grep enabled

Hardware

Collect and summarize all hardware info of your machine

lshw -json >report.json
# Other options are: [ -html ]  [ -short ]  [ -xml ]  [ -json ]  [ -businfo ]  [ -sanitize ] ,etc

Finding Out memory device detail

sudo dmidecode -t memory

Print detail of CPU hardware

dmidecode -t 4
#          Type   Information
#          0   BIOS
#          1   System
#          2   Base Board
#          3   Chassis
#          4   Processor
#          5   Memory Controller
#          6   Memory Module
#          7   Cache
#          8   Port Connector
#          9   System Slots
#         11   OEM Strings
#         13   BIOS Language
#         15   System Event Log
#         16   Physical Memory Array
#         17   Memory Device
#         18   32-bit Memory Error
#         19   Memory Array Mapped Address
#         20   Memory Device Mapped Address
#         21   Built-in Pointing Device
#         22   Portable Battery
#         23   System Reset
#         24   Hardware Security
#         25   System Power Controls
#         26   Voltage Probe
#         27   Cooling Device
#         28   Temperature Probe
#         29   Electrical Current Probe
#         30   Out-of-band Remote Access
#         31   Boot Integrity Services
#         32   System Boot
#         34   Management Device
#         35   Management Device Component
#         36   Management Device Threshold Data
#         37   Memory Channel
#         38   IPMI Device
#         39   Power Supply

Count the number of Segate hard disks

lsscsi|grep SEAGATE|wc -l
# or
sg_map -i -x|grep SEAGATE|wc -l

Get UUID of a disk (e.g. sdb)

lsblk -f /dev/sdb

# or
sudo blkid /dev/sdb

Generate an UUID

uuidgen

Print detail of all hard disks

lsblk -io KNAME,TYPE,MODEL,VENDOR,SIZE,ROTA
#where ROTA means rotational device / spinning hard disks (1 if true, 0 if false)

List all PCI (Peripheral Component Interconnect) devices

lspci
# List information about NIC
lspci | egrep -i --color 'network|ethernet'

List all USB devices

lsusb

Linux modules

# Show the status of modules in the Linux Kernel
lsmod

# Add and remove modules from the Linux Kernel
modprobe

# or
# Remove a module
rmmod

# Insert a module
insmod

Controlling IPMI-enabled devices (e.g. BMC)

# Remotely finding out power status of the server
ipmitool -U <bmc_username> -P <bmc_password> -I lanplus -H <bmc_ip_address> power status

# Remotely switching on server
ipmitool -U <bmc_username> -P <bmc_password> -I lanplus -H <bmc_ip_address> power on

# Turn on panel identify light (default 15s)
ipmitool chassis identify 255

# Found out server sensor temperature
ipmitool sensors |grep -i Temp

# Reset BMC
ipmitool bmc reset cold

# Prnt BMC network
ipmitool lan print 1

# Setting BMC network
ipmitool -I bmc lan set 1 ipaddr 192.168.0.55
ipmitool -I bmc lan set 1 netmask 255.255.255.0
ipmitool -I bmc lan set 1 defgw ipaddr 192.168.0.1

Networking

Resolve a domain to IP address(es)

dig +short www.example.net

# or
host www.example.net

Check public IP address

curl http://checkip.amazonaws.com

Get DNS TXT record a of domain

dig -t txt www.example.net

# or
host -t txt www.example.net

Send a ping with a limited TTL to 10 (TTL: Time-To-Live, which is the maximum number of hops that a packet can travel across the Internet before it gets discarded.)

ping 192.0.2.1 -t 10

Print the route packets trace to network host

traceroute example.net

Check connection to host (e.g. check connection to port 80 and 22 of example.net)

nc -vw5 example.net 80
# Connection to example.net 80 port [tcp/http] succeeded!

nc -vw5 example.net 22
# nc: connect to example.net port 22 (tcp) timed out: Operation now in progress
# nc: connect to example.net port 22 (tcp) failed: Network is unreachable

Nc as a chat tool!

# From server A:
$ sudo nc -l 80
# then you can connect to the 80 port from another server (e.g. server B):
# e.g. telnet <server A IP address> 80
# then type something in server B
# and you will see the result in server A!

Check which ports are listening for TCP connections from the network

# note that some companies might not like you using nmap
nmap -sT -O localhost

# check port 0-65535
nmap  -p0-65535 localhost

Check if a host is up and scan for open ports, also skip host discovery.

# skips checking if the host is alive which may sometimes cause a false positive and stop the scan.
$ nmap example.net -Pn

# Example output:
# Starting Nmap 7.01 ( https://nmap.org ) at 2020-07-18 22:59 CST
# Nmap scan report for example.net (172.217.24.14)
# Host is up (0.013s latency).
# Other addresses for example.net (not scanned): 2404:6800:4008:802::200e
# rDNS record for 172.217.24.14: tsa01s07-in-f14.1e100.net
# Not shown: 998 filtered ports
# PORT    STATE SERVICE
# 80/tcp  open  http
# 443/tcp open  https
#
# Nmap done: 1 IP address (1 host up) scanned in 3.99 seconds

Scan for open ports and OS and version detection (e.g. scan the domain "scanme.nmap.org")

$ nmap -A -T4 scanme.nmap.org
# -A to enable OS and version detection, script scanning, and traceroute; -T4 for faster execution

Look up website information (e.g. name server), searches for an object in a RFC 3912 database

whois example.net

Show the SSL certificate of a domain

openssl s_client -showcerts -connect www.example.net:443

Display network interfaces and their associated IP addresses

ip a

Display route table

ip r

Display ARP cache (ARP cache displays the MAC addresses of device in the same network that you have connected to)

ip n

Add transient IP addres (reset after reboot) (e.g. add 192.168.140.3/24 to device eno16777736)

ip address add 192.168.140.3/24 dev eno16777736

Persisting network configuration changes

sudo vi /etc/sysconfig/network-scripts/ifcfg-enoxxx
# then edit the fields: BOOTPROT, DEVICE, IPADDR, NETMASK, GATEWAY, DNS1 etc

Refresh NetworkManager

sudo nmcli c reload

Restart all interfaces

sudo systemctl restart network.service

To view hostname, OS, kernal, architecture at the same time!

hostnamectl

Set hostname (set all transient, static, pretty hostname at once)

hostnamectl set-hostname "mynode"

Find out the web server (e.g Nginx or Apache) of a website

curl -I http://example.net/
# HTTP/1.1 200 OK
# Server: nginx
# Date: Thu, 02 Jan 2020 07:01:07 GMT
# Content-Type: text/html
# Content-Length: 1119
# Connection: keep-alive
# Vary: Accept-Encoding
# Last-Modified: Mon, 09 Sep 2019 10:37:49 GMT
# ETag: "xxxxxx"
# Accept-Ranges: bytes
# Vary: Accept-Encoding

Find out the time spent between request and response

curl -v -o /dev/null -s -w 'Total: %{time_total}s\n' google.com

Find out the http status code of a URL

curl -s -o /dev/null -w "%{http_code}" https://www.example.net

Unshorten a shortended URL

curl -s -o /dev/null -w "%{redirect_url}" https://bit.ly/34EFwWC

Perform network throughput tests

# server side:
$ sudo iperf -s -p 80

# client side:
iperf -c <server IP address> --parallel 2 -i 1 -t 2 -p 80

To block port 80 (HTTP server) using iptables.

sudo iptables -A INPUT -p tcp --dport 80 -j DROP

# only block connection from an IP address
sudo iptables –A INPUT –s <IP> -p tcp –dport 80 –j DROP

Data wrangling

Print some words that start with a particular string (e.g. words start with 'phy')

# If file is not specified, the file /usr/share/dict/words is used.
look phy|head -n 10
# phycic
# Phyciodes
# phycite
# Phycitidae
# phycitol
# phyco-
# phycochrom
# phycochromaceae
# phycochromaceous
# phycochrome

Repeat printing string n times (e.g. print 'hello world' five times)

printf 'hello world\n%.0s' {1..5}

Do not echo the trailing newline

username=`echo -n "bashoneliner"`

Copy a file to multiple files (e.g copy fileA to file(B-D))

tee <fileA fileB fileC fileD >/dev/null

Delete all non-printing characters

tr -dc '[:print:]' < filename

Remove newline / nextline

tr --delete '\n' <input.txt >output.txt

Replace newline

tr '\n' ' ' <filename

To uppercase/lowercase

tr /a-z/ /A-Z/

Translate a range of characters (e.g. substitute a-z into a)

echo 'something' |tr a-z a
# aaaaaaaaa

Compare two files (e.g. fileA, fileB)

diff fileA fileB
# a: added; d:delete; c:changed

# or
sdiff fileA fileB
# side-to-side merge of file differences

Compare two files, strip trailing carriage return/ nextline (e.g. fileA, fileB)

diff fileA fileB --strip-trailing-cr

Find common/differing lines

# having two sorted and uniqed files (for example after running `$ sort -uo fileA fileA` and same for fileB):
# ------
# fileA:
# ------
# joey
# kitten
# piglet
# puppy
# ------
# fileB:
# ------
# calf
# chick
# joey
# puppy
#
# Find lines in both files
comm -12 fileA fileB
# joey
# puppy
#
# Find lines in fileB that are NOT in fileA
comm -13 fileA fileB
# calf
# chick
#
# Find lines in fileA that are NOT in fileB
comm -23 fileA fileB
# kitten
# piglet

Number a file (e.g. fileA)

nl fileA

# or
nl -nrz fileA
# add leading zeros

# or
nl -w1 -s ' '
# making it simple, blank separate

Join two files field by field with tab (default join by the first column of both file, and default separator is space)

# fileA and fileB should have the same ordering of lines.
join -t '\t' fileA fileB

# Join using specified field (e.g. column 3 of fileA and column 5 of fileB)
join -1 3 -2 5 fileA fileB

Combine/ paste two or more files into columns (e.g. fileA, fileB, fileC)

paste fileA fileB fileC
# default tab separate

Group/combine rows into one row

# e.g.
# AAAA
# BBBB
# CCCC
# DDDD
cat filename|paste - -
# AAAABBBB
# CCCCDDDD
cat filename|paste - - - -
# AAAABBBBCCCCDDDD

Fastq to fasta (fastq and fasta are common file formats for bioinformatics sequence data)

cat file.fastq | paste - - - - | sed 's/^@/>/g'| cut -f1-2 | tr '\t' '\n' >file.fa

Reverse string

echo 12345| rev

Generate sequence 1-10

seq 10

Find average of input list/file of integers

i=`wc -l filename|cut -d ' ' -f1`; cat filename| echo "scale=2;(`paste -sd+`)/"$i|bc

Generate all combination (e.g. 1,2)

echo {1,2}{1,2}
# 1 1, 1 2, 2 1, 2 2

Generate all combination (e.g. A,T,C,G)

set = {A,T,C,G}
group= 5
for ((i=0; i<$group; i++)); do
    repetition=$set$repetition; done
    bash -c "echo "$repetition""

Read file content to variable

foo=$(<test1)

Echo size of variable

echo ${#foo}

Echo a tab

echo -e ' \t '

Split file into smaller file

# Split by line (e.g. 1000 lines/smallfile)
split -d -l 1000 largefile.txt

# Split by byte without breaking lines across files
split -C 10 largefile.txt

Create a large amount of dummy files (e.g 100000 files, 10 bytes each):

#1. Create a big file
dd if=/dev/zero of=bigfile bs=1 count=1000000

#2. Split the big file to 100000 10-bytes files
 split -b 10 -a 10 bigfile

Rename all files (e.g. remove ABC from all .gz files)

rename 's/ABC//' *.gz

Remove file extension (e.g remove .gz from filename.gz)

basename filename.gz .gz

zcat filename.gz> $(basename filename.gz .gz).unpacked

Add file extension to all file(e.g add .txt)

rename s/$/.txt/ *
# You can use rename -n s/$/.txt/ * to check the result first, it will only print sth like this:
# rename(a, a.txt)
# rename(b, b.txt)
# rename(c, c.txt)

Squeeze repeat patterns (e.g. /t/t --> /t)

tr -s "/t" < filename

Do not print nextline with echo

echo -e 'text here \c'

View first 50 characters of file

head -c 50 filename

Cut and get last column of a file

cat file|rev | cut -d/ -f1 | rev

Add one to variable/increment/ i++ a numeric variable (e.g. $var)

((var++))
# or
var=$((var+1))

Cut the last column

cat filename|rev|cut -f1|rev

Create or replace a file with contents

cat >myfile
let me add sth here
# exit with ctrl+d

# or using tee
tee myfile
let me add sth else here
# exit with ctrl+d

Append to a file with contents

cat >>myfile
let me add sth here
# exit with ctrl+d

# or
cat << EoF >> filename
> add something here
> EoF 

# or using tee
tee -a myfile
let me add sth else here
# exit with ctrl+d

Clear the contents of a file (e.g. filename)

>filename

Append to file (e.g. hihi)

echo 'hihi' >>filename

Working with json data

# Install the useful jq package
# sudo apt-get install jq
# e.g. to get all the values of the 'url' key, simply pipe the json to the following jq command(you can use .[]. to select inner json, i.e jq '.[].url')
cat file.json | jq '.url'

Decimal to Binary (e.g get binary of 5)

D2B=({0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1})
echo -e ${D2B[5]}
#00000101
echo -e ${D2B[255]}
#11111111

Wrap each input line to fit in specified width (e.g 4 integers per line)

echo "00110010101110001101" | fold -w4
# 0011
# 0010
# 1011
# 1000
# 1101

Sort a file by column and keep the original order

sort -k3,3 -s

Right align a column (right align the 2nd column)

cat file.txt|rev|column -t|rev

To both view and store the output

echo 'hihihihi' | tee outputfile.txt
# use '-a' with tee to append to file.

Show non-printing (Ctrl) characters with cat

cat -v filename

Convert tab to space

expand filename

Convert space to tab

unexpand filename

Display file in octal ( you can also use od to display hexadecimal, decimal, etc)

od filename

Reverse cat a file

tac filename

Reverse the result from uniq -c

while read a b; do yes $b |head -n $a ; done <test.txt

Others

Describe the format and characteristics of image files

identify myimage.png
#myimage.png PNG 1049x747 1049x747+0+0 8-bit sRGB 1.006MB 0.000u 0:00.000

Bash auto-complete (e.g. show options "now tomorrow never" when you press'tab' after typing "dothis")

complete -W "now tomorrow never" dothis
# ~$ dothis  
# never     now       tomorrow
# press 'tab' again to auto-complete after typing 'n' or 't'

Displays a calendar

# print the current month, today will be highlighted.
cal
# October 2019      
# Su Mo Tu We Th Fr Sa  
#    1  2  3  4  5  
# 6  7  8  9 10 11 12  
# 13 14 15 16 17 18 19  
# 20 21 22 23 24 25 26  
# 27 28 29 30 31  

# only display November
cal -m 11

Convert the hexadecimal MD5 checksum value into its base64-encoded format.

openssl md5 -binary /path/to/file| base64
# NWbeOpeQbtuY0ATWuUeumw==

Forces applications to use the default language for output

export LC_ALL=C

# to revert:
unset LC_ALL

Encode strings as Base64 strings

echo test|base64
#dGVzdAo=

Get parent directory of current directory

dirname `pwd`

Read .gz file without extracting

zmore filename

# or
zless filename

Run command in background, output error file

some_commands  &>log &

# or
some_commands 2>log &

# or
some_commands 2>&1| tee logfile

# or
some_commands |& tee logfile

# or
some_commands 2>&1 >>outfile
#0: standard input; 1: standard output; 2: standard error

Run multiple commands in background

# run sequentially
(sleep 2; sleep 3) &

# run parallelly
sleep 2 & sleep 3 &

Run process even when logout (immune to hangups, with output to a non-tty)

# e.g. Run myscript.sh even when log out.
nohup bash myscript.sh

Send mail

echo 'heres the content'| mail -a /path/to/attach_file.txt -s 'mail.subject' me@gmail.com
# use -a flag to set send from (-a "From: some@mail.tld")

Convert .xls to csv

xls2csv filename

Make BEEP sound

speaker-test -t sine -f 1000 -l1

Set beep duration

(speaker-test -t sine -f 1000) & pid=$!;sleep 0.1s;kill -9 $pid

Editing your history

history -w
vi ~/.bash_history
history -r

# or
history -d [line_number]

Interacting with history

# list 5 previous command (similar to `history |tail -n 5` but wont print the history command itself)
fc -l -5

Delete current bash command

Ctrl+U

# or
Ctrl+C

# or
Alt+Shift+#
# to make it to history

Add something to history (e.g. "addmetohistory")

# addmetodistory
# just add a "#" before~~

Get last history/record filename

head !$

Clean screen

clear
# or simply Ctrl+l

Backup with rsync

rsync -av filename filename.bak
rsync -av directory directory.bak
rsync -av --ignore_existing directory/ directory.bak
rsync -av --update directory directory.bak

rsync -av directory user@ip_address:/path/to/directory.bak
# skip files that are newer on receiver (i prefer this one!)

Create a temporary directory and cd into it

cd $(mktemp -d)
# for example, this will create a temporary directory "/tmp/tmp.TivmPLUXFT"

Make all directories at one time!

mkdir -p project/{lib/ext,bin,src,doc/{html,info,pdf},demo/stat}
# -p: make parent directory
# this will create:
# project/
# project/bin/
# project/demo/
# project/demo/stat/
# project/doc/
# project/doc/html/
# project/doc/info/
# project/doc/pdf/
# project/lib/
# project/lib/ext/
# project/src/
#
# project/
# ├── bin
# ├── demo
# │   └── stat
# ├── doc
# │   ├── html
# │   ├── info
# │   └── pdf
# ├── lib
# │   └── ext
# └── src

Run command only if another command returns zero exit status (well done)

cd tmp/ && tar xvf ~/a.tar

Run command only if another command returns non-zero exit status (not finish)

cd tmp/a/b/c ||mkdir -p tmp/a/b/c

Use backslash "" to break long command

cd tmp/a/b/c \
> || \
>mkdir -p tmp/a/b/c

List file type of file (e.g. /tmp/)

file /tmp/
# tmp/: directory

Writing Bash script ('#!'' is called shebang )

#!/bin/bash
file=${1#*.}
# remove string before a "."

Python simple HTTP Server

python -m SimpleHTTPServer
# or when using python3:
python3 -m http.server

Read user input

read input
echo $input

Array

declare -a array=()

# or
declare array=()

# or associative array
declare -A array=()

Send a directory

scp -r directoryname user@ip:/path/to/send

Fork bomb

# Don't try this at home!
# It is a function that calls itself twice every call until you run out of system resources.
# A '# ' is added in front for safety reason, remove it when seriously you are testing it.
# :(){:|:&};:

Trigger kernel crash

# Don't try this at home! 
echo c > /proc/sysrq-trigger

Use the last argument

!$

Check last exit code

echo $?

Extract .xz

unxz filename.tar.xz
# then
tar -xf filename.tar

Unzip tar.bz2 file (e.g. file.tar.bz2)

tar xvfj file.tar.bz2

Unzip tar.xz file (e.g. file.tar.xz)

unxz file.tar.xz
tar xopf file.tar

Extract to a path

tar xvf -C /path/to/directory filename.gz

Zip the content of a directory without including the directory itself

# First cd to the directory, they run:
zip -r -D ../myzipfile .
# you will see the myzipfile.zip in the parent directory (cd ..)

Output a y/n repeatedly until killed

# 'y':
yes

# or 'n':
yes n

# or 'anything':
yes anything

# pipe yes to other command
yes | rm -r large_directory

Create large dummy file of certain size instantly (e.g. 10GiB)

fallocate -l 10G 10Gigfile

Create dummy file of certain size (e.g. 200mb)

dd if=/dev/zero of=/dev/shm/200m bs=1024k count=200
# or
dd if=/dev/zero of=/dev/shm/200m bs=1M count=200

# Standard output:
# 200+0 records in
# 200+0 records out
# 209715200 bytes (210 MB) copied, 0.0955679 s, 2.2 GB/s

Keep /repeatedly executing the same command (e.g Repeat 'wc -l filename' every 1 second)

watch -n 1 wc -l filename

Use Bash Strict Mode

# These options can make your code safer but, depending on how your pipeline is written, it might be too aggressive 
# or it might not catch the errors that you are interested in

# for reference see https://gist.github.com/mohanpedala/1e2ff5661761d3abd0385e8223e16425
#               and https://mywiki.wooledge.org/BashPitfalls#set_-euo_pipefail

set -o errexit      # exit immediately if a pipeline returns a non-zero status
set -o errtrace     # trap ERR from shell functions, command substitutions, and commands from subshell
set -o nounset      # treat unset variables as an error
set -o pipefail     # pipe will exit with last non-zero status, if applicable
set -Eue -o pipefail  # shorthand for above (pipefail has no short option)

Print commands and their arguments when execute (e.g. echo expr 10 + 20 )

set -x; echo `expr 10 + 20 `
# or
set -o xtrace; echo `expr 10 + 20 `

# to turn it off..
set +x

Print some meaningful sentences to you (install fortune first)

fortune

Colorful (and useful) version of top (install htop first)

htop

Press any key to continue

read -rsp $'Press any key to continue...\n' -n1 key

Run sql-like command on files from terminal

# download:
# https://github.com/harelba/q
# example:
q -d "," "select c3,c4,c5 from /path/to/file.txt where c3='foo' and c5='boo'"

Pass password to ssh

sshpass -p mypassword ssh root@10.102.14.88 "df -h"

Wait for a pid (job) to complete

wait %1
# or
wait $PID
wait ${!}
#wait ${!} to wait till the last background process ($! is the PID of the last background process)

Convert pdf to txt

sudo apt-get install poppler-utils
pdftotext example.pdf example.txt

List only directory

ls -d */

List one file per line.

ls -1
# or list all, do not ignore entries starting with .
ls -1a

Capture/record/save terminal output (capture everything you type and output)

script output.txt
# start using terminal
# to logout the screen session (stop saving the contents), type exit.

List contents of directories in a tree-like format.

tree
# go to the directory you want to list, and type tree (sudo apt-get install tree)
# output:
# home/
# └── project
#     ├── 1
#     ├── 2
#     ├── 3
#     ├── 4
#     └── 5
#

# set level directories deep (e.g. level 1)
tree -L 1
# home/
# └── project

Set up virtualenv(sandbox) for python

# 1. install virtualenv.
sudo apt-get install virtualenv
# 2. Create a directory (name it .venv or whatever name your want) for your new shiny isolated environment.
virtualenv .venv
# 3. source virtual bin
source .venv/bin/activate
# 4. you can check check if you are now inside a sandbox.
type pip
# 5. Now you can install your pip package, here requirements.txt is simply a txt file containing all the packages you want. (e.g tornado==4.5.3).
pip install -r requirements.txt
# 6. Exit virtual environment
deactivate