shell

bash process substitution

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Updated: 11 January 2023 

>(command_list)
<(command_list)

Process substitution feeds the output of a process (or processes) into the stdin of another process. It uses /dev/fd/<n> files to send the results of the process(es) within parentheses to another process. Effectively, process substitution turns a command into a temporary file, which is removed when the command completes.

cat <(echo hello world) # hello world
echo <(echo hello world) # /dev/fd/63

The first command converts the output of echo hello world into a file with contents hello world. The second command shows us the temp file in use.

The diff command requires files as arguments. So, comparing the contents of two directories can be achieved with diff and process substitution

diff <(ls /bin) <(ls /usr/bin)

Argbash

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Updated: 30 January 2025

https://argbash.dev/

Create an argbash template file

argbash-init --pos positional-arg --opt optional-arg minimal.m4

Use the template file to create a script

argbash minimal.m4 -o my-script.sh

Re-generate script after editing Argbash section

argbash my-script.sh -o my-script.sh

bash sort

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Updated: 04 March 2024

Sort by second column of tabulated data

$ printf "101\tc\n102\tb\n103\ta\n"
101	c
102	b
103	a

$ printf "101\tc\n102\tb\n103\ta\n" | sort -k2
103	a
102	b
101	c

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: 14 December 2025

until loop

Similar to a 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

for loop

my_array=("apple" "banana" "orange" "grape")

for element in "${my_array[@]}"; do
    echo "Current element: $element"
done

Alternatively

for run in {11..15}; do
  echo 'it works?'
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