shell

bash expand

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Updated: 01 January 2023

shell-expand-line

Check if shell-expand-line is bound to any keys

$ bind -P | grep shell-expand-line

If not add this to .inputrc

"\e\C-e": shell-expand-line

Test it

$ $HOME # Ctr+Alt+e, should print full path.

bash quoting

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Updated: 31 December 2022

Single quotes

Single quoting is strong quoting, it preserves the literal value of each character

chris@ctd:~$ echo 'My home dir is $HOME'
My home dir is $HOME

bash loops

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Updated: 31 December 2022

until loop

Very similar to the while loop, except that the loop executes until the test-command executes successfully. As long as this command fails, the loop continues.

until test-command; do
    consequent-commands;
done

bash exit code

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Updated: 07 December 2024

Simple exit status example

#!/bin/bash

date # run date command
status=$? # exit code is in $?
[ $status -eq 0 ] && echo "success" || echo "failure"

set -e Exit immediately if a pipeline, which may consist of a single simple command, a list, or a compound command returns a non-zero status

#!/bin/bash -e

rm does-not-exist.cs
ls # this command never runs

bash debugging

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Updated: 03 May 2023

see https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_03.html

Start subshell with -x option, which will run the entire script in debug mode. Traces of each command plus its arguments are printed to standard output after the commands have been expanded but before they are executed.

bash -x script1.sh

Debugging can be activated for a portion of the script

set -x			# activate debugging from here
call_my_func
set +x			# stop debugging from here

Other useful bash options

Short notation Long notation Result
set -v set -o verbose Prints shell input lines as they are read.
set -x set -o xtrace Print command traces before executing command.

Alternatively, these modes can be specified in the script itself, by adding the desired options to the first line shell declaration. Options can be combined, as is usually the case with UNIX commands

#!/bin/bash -xv

Examples of using echo for debugging

echo "debug message: now attempting to start w command"; w

echo "Variable VARNAME is now set to $VARNAME."

bash export

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Updated: 28 December 2022

See https://www.digitalocean.com/community/tutorials/export-command-linux

Export is a built-in command of the Bash shell. It is used to mark variables and functions to be passed to child processes.

Display all exported variables

export

View all exported variables on the current shell

export -p

Declare and export a variable

export STUDENT=Divya

Display a variable

printenv STUDENT

Remove an environment variable

unset STUDENT

bash

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Updated: 21 July 2023

Documentation

Bash guide for beginners
Bash reference manual
Bash reference manual

Miscellaneous

Shorten prompt in current terminal
PS1='\u:\W\$ '

Exit status of last command
echo $?

Repeat a command n times
for run in {1..100}; do command; sleep 5; done

Run A and then B, regardless of success of A
A; B

Run B if and only if A succeeded
A && B

Run B if and only if A failed
A || B

Run A in background.
A &

stderr and stdout

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Updated: 04 May 2023

0 – stdin the standard input stream.
1 – stdout the standard output stream.
2 – stderr the standard error stream.

Redirecting output
ls -la > file.out

is equivalent to
ls -la 1> file.out

To redirect stderr
command 2> file.err

Sending both stderr and stdout to separate files
command 2> error.txt 1> output.txt

To suppress error messages in the terminal redirect to /dev/null
command 2> /dev/null

Redirect stderr to the current location of stdout and have error messages sent to the same file as standard output
command > file 2>&1