bash

bash history

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Updated: 15 April 2023

History doesn’t get written to .bash_history until log off.

Show history

history

Clear session history

history -c

Delete item 234 from history

history -d 234

Writes all current session command history to the HISTFILE

history -w

Search history

Ctrl+R and start typing the previous command. Once a result appears, repeat Ctrl+R to see other matches. Enter to run command.

To re-run a command from history

!<item number>

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)

bash sort

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Updated: 01 January 2023

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: 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: 12 February 2023 

#!/bin/bash -xv

# run date command
date

# exit code is in $?
status=$?

[ $status -eq 0 ] && echo "success" || echo "failure"

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."