Bash

Handling unset or empty variables

Syntax

• + -- if the variable is set, use the substitution
• - -- if the variable is not set, use the substitution
• : -- handle empty string as though it were unset

Table

	unset VAR	export VAR=''	export VAR='x'
echo ">${VAR+y}<"	><	>y<	>y<
echo ">${VAR:+y}<"	><	><	>y<
echo ">${VAR-y}<"	>y<	><	>x<
echo ">${VAR:-y}<"	>y<	>y<	>x<

When the variable is not set

$ unset VAR
$ echo ">${VAR+y}<"
><
$ echo ">${VAR:+y}<"
><
$ echo ">${VAR-y}<"
>y<
$ echo ">${VAR:-y}<"
>y<

When the variable is set to empty string

$ export VAR=''
$ echo ">${VAR+y}<"
>y<
$ echo ">${VAR:+y}<"
><
$ echo ">${VAR-y}<"
><
$ echo ">${VAR:-y}<"
>y<

When the variable is set to a non-empty string

$ export VAR='x'
$ echo ">${VAR+y}<"
>y<
$ echo ">${VAR:+y}<"
>y<
$ echo ">${VAR-y}<"
>x<
$ echo ">${VAR:-y}<"
>x<

(bash:unofficial-strict-mode)=

Unofficial "Strict Mode"

TL;DR

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

Explanation

• set -e option - cease execution on first error
• set -u option - treat undefined variables as an error
• set -o pipefail option - treat an error in any sub-command within a pipeline as a failure of the whole pipeline

Colors Cheat Sheet

Regular Colors

Value	Color
\e[0;30m	Black
\e[0;31m	Red
\e[0;32m	Green
\e[0;33m	Yellow
\e[0;34m	Blue
\e[0;35m	Purple
\e[0;36m	Cyan
\e[0;37m	White

Bold

Value	Color
\e[1;30m	Black
\e[1;31m	Red
\e[1;32m	Green
\e[1;33m	Yellow
\e[1;34m	Blue
\e[1;35m	Purple
\e[1;36m	Cyan
\e[1;37m	White

Underline

Value	Color
\e[4;30m	Black
\e[4;31m	Red
\e[4;32m	Green
\e[4;33m	Yellow
\e[4;34m	Blue
\e[4;35m	Purple
\e[4;36m	Cyan
\e[4;37m	White

Background

Value	Color
\e[40m	Black
\e[41m	Red
\e[42m	Green
\e[43m	Yellow
\e[44m	Blue
\e[45m	Purple
\e[46m	Cyan
\e[47m	White

High Intensity

Value	Color
\e[0;90m	Black
\e[0;91m	Red
\e[0;92m	Green
\e[0;93m	Yellow
\e[0;94m	Blue
\e[0;95m	Purple
\e[0;96m	Cyan
\e[0;97m	White

Bold High Intensity

Value	Color
\e[1;90m	Black
\e[1;91m	Red
\e[1;92m	Green
\e[1;93m	Yellow
\e[1;94m	Blue
\e[1;95m	Purple
\e[1;96m	Cyan
\e[1;97m	White
===

High Intensity backgrounds

Value	Color
\e[0;100m	Black
\e[0;101m	Red
\e[0;102m	Green
\e[0;103m	Yellow
\e[0;104m	Blue
\e[0;105m	Purple
\e[0;106m	Cyan
\e[0;107m	White

Reset

Value	Color
\e[0m	Reset

References

• https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
• http://redsymbol.net/articles/unofficial-bash-strict-mode/