Shell snippets

Argument parser

A simple argument parser for bash.

#!/bin/bash

# Settings variables go here
FOO="foo"

show_help() {
cat <<EOF
Usage: foo <<options>> <bar>

Blubber blah blah ...
EOF
}

while [[ "$#" -gt 0 ]]; do
	case "$1" in
		--foo) FOO="$2"; shift 2;;
		
		--help) show_help; exit 0;;
		*) printf "Unknown option: %s\n" "$1"; exit 1;;
	esac
done

# Logic goes here
echo "$FOO"

Hook helper

This little function calls its argument as an ad-hoc shellscript if it is non-empty, if it reurns a nonzero, one should call the default action.

# on fail do default action
run_hook() {
	[ -n "$1" ] && bash -c "$1"
}

export VARIABLE_WHICH_IS_USEFUL_FOR_HOOK="something"
run_hook "$HOOK_COMMAND" || default_action

An example of this being used can be found in my dotfiles with the filedialog. (Fun fact: It was created for this script)

Escape Sequences (ANSI)

Ansi Escape squences can be achieved using printf or echo -e in shell scripts (I personally prefer printf).

I'll focus on Select Graphic Rendition (SGR) codes here, Wikipedia has a more complete list.

Setting Text Color and Effects

printf '\x1b[01;31m_FOOBAR_\x1b[00m\n'
# With %s placeholder
printf '\x1b[01;31m%s\x1b[00m\n' "_FOOBAR_"

As you can see in the example, multiple escape-codes can be chained in one sequence by delimiting them with a ;.

The \x1b part is an ASCII escape caracter it can also be written as \e, the [ tells the terminal that this is an ANSI Control Sequence Initiator (CSI) after that can come one or more codes. The sequence is finished by an m to tell the terminal that everything between the CSI and it are SGR codes.

If you are using escape sequences in your scripts: Please make sure that no information gets lost when those seqences are stripped, People with processing pipelines and screenreaders will thank (or at least not curse at) you.

Note: The leading zeros are not needed.

Colors

You may have noticed that the 3x, 4x, 9x, and 10x family of codes all use the last digit in the range from 0 to 7 as a color parameter.