Basics

~ Current user home directory
~john User john home directory
~- $OLDPWD
~+ $PWD
xsel -b file.txt Copy to buffer
set -o vi Set the editor of the command line to vi

Change file extension

mv file.{txt,csv}

Rewrite all extensions

for f in * ; do mv "$f" "${f/.csv/.txt}" ; done

<file while read line ; do echo $line ; done
cat <<<wut
cat <(echo wut)
diff foo <(sort foo)

Get the extension

echo "${foo##*.}"
echo "$foo" | rev | cut -f 1 -d '.' | rev

Omit the extension

echo "${foo%.*}"

Write ones

tr '\0' '\377' < /dev/zero
tr '\0' '\377' < /dev/zero | dd of=ones

Error handling

#!/usr/bin/env bash

set -euo pipefail

e: Exit immediately on non-zero return
u: Treat empty variables as errors
o:
pipefail: Piping stops when something errors out
b: Notify of job termination immediately.
v: Print shell input lines as they are read.
x: Print commands and their arguments as they are executed.
C: Prevent existing files from being overwritten by the shell's '>' redirection operator
n: Read commands but does not execute them

All options

man set

Traps

man trap
trap 'read foo; echo "-$foo-"' 0 # Executed when exit code is 0
trap 'eval " $cmd"' 0 # Executed when exit code is 0
trap 'error_exit ${LINENO}' ERR # Executed on error
trap 'error_exit ${BASH_COMMAND}' ERR # Executed on error
trap 'echo "removing $TMPFILE"; rm -f $TMPFILE' INT TERM EXIT q

Restart a program if it crashes

while ! discord ; do : ; done

Grab the first line

head -n 1 longfile
cat longfile | head -n 1
cat longfile 2>&1 | head -n 1
awk 'FNR <= 1' longfile

Grab the Nth line

awk 'FNR == 74' longfile

Grab string between X and Y

echo "This is Xthe stringY I want" | cut -d "X" -f2 | cut -d "Y" -f1
echo "This is Xthe stringY I want" | awk -F'[XY]' '{print $2}'
echo "This is Xthe stringY I want" | sed 's/.*\X\([^]]*\)\Y.*/\1/g'

#!/usr/bin/env sh

items="1 item number 0
2 item number 1
3 item number 2"

echo "$items"

read -p "Selection? " answer

case "$answer" in
    1 ) echo 1 ;;
    2 ) echo 2 ;;
    3 ) echo 3 ;;
    * ) echo other ;;
esac

Arguments

#!/usr/bin/env sh

while [ $# -gt 0 ]; do
    case "$1" in
        -h | --help | h | [hH]elp | [hH]* )
            echo "help"
            exit ;; ## Stop the execution
        -a | --aa )
            echo "Option a"
            shift ;; ## Next argument
        -b | --bb )
            echo "Option b" ; shift ;; ## One line
        -d | --dir )
            shift
            DIR="$1" ## Save the value in a variable
            echo "Directory: $DIR"
            shift ;;
        * ) ## Anything else
            echo "Unknown option $1"
            shift ;;
    esac
done

CSV file

IFS denotes the character used to separate fields in the csv file.

#!/usr/bin/env sh

while IFS=';' read -r time date var3 location ; do
    echo "The time is $time"
    echo "The date is $date"
    echo "The third filed in the csv file is $var3"
    echo "Finally $location"
done < file.csv

Function to add text to a varible

Instead of doing echo "Text" >> output.txt you can create a variable to hold the text and write it at the end of the script, instead of multiple times during the execution of the script.

#!/usr/bin/env bash
add() {
    document="${document}
${1}"
}

add "text to add to the variable"

echo "$document" > output.txt

While read from the output of another command

cat file.txt | while IFS='' read -r line ; do
    echo "Line: $line"
done

cat file.txt | while IFS=';' read -r name id position ; do
    echo "$name is at $position with ID $id"
done

while IFS='' read -r line ; do
    echo "Line: $line"
done < "$( cat file.txt )"

While read from file

while IFS="" read line ; do 
    echo "line: ${line}"
done < file.txt

Change directory in a script

In bash when you execute a script it executes in your current path. To make it run in the path the script is contained add the following at the beginning of the script.

cd "$( dirname "$0" )" || exit 1

Logical operators

true && echo here
---
here

true || echo here
---

false && echo here
---

false || echo here
---
here

true && false || true

echo 1 && (echo 2 ; false) || echo 3
---
1
2
3

true || false && true

echo 1 || (echo 2 ; false) && echo 3
---
1
3

false && true || true

( echo 1 ; false ) && echo 2 || echo 3
---
1
3

false || true && true

( echo 1 ; false ) || echo 2 && echo 3
---
1
2
3

( false || true ) && true

( ( echo 1 ; false ) || echo 2 ) && echo 3
---
1
2
3

...

Other

|& Pipe stdout and stderr