Bash-Beginners-Guide

10.3.3. Transformations of variables
Bash Guide for Beginners
10.3.3.1. Substitution
${VAR:-WORD}
If VAR is not defined or null, the expansion of WORD is substituted; otherwise the value of VAR is substituted:
[bob in ~] echo ${TEST:-test}
test
[bob in ~] echo $TEST
[bob in ~] export TEST=a_string
[bob in ~] echo ${TEST:-test}
a_string
[bob in ~] echo ${TEST2:-$TEST}
a_string
This form is often used in conditional tests, for instance in this one:
[ -z "${COLUMNS:-}" ] && COLUMNS=80
It is a shorter notation for
if [ -z "${COLUMNS:-}" ]; then
COLUMNS=80
fi
See Section 7.1.2.3 for more information about this type of condition testing.
If the hyphen (-) is replaced with the equal sign (=), the value is assigned to the parameter if it does not exist:
[bob in ~] echo $TEST2
[bob in ~] echo ${TEST2:=$TEST}
a_string
[bob in ~] echo $TEST2
a_string
The following syntax tests the existence of a variable. If it is not set, the expansion of WORD is printed to
standard out and non-interactive shells quit. A demonstration:
[bob in ~] cat vartest.sh
#!/bin/bash
# This script tests whether a variable is set. If not,
# it exits printing a message.
echo ${TESTVAR:?"There's so much I still wanted to do..."}
echo "TESTVAR is set, we can proceed."
[bob in testdir] ./vartest.sh
./vartest.sh: line 6: TESTVAR: There's so much I still wanted to do...
Chapter 10. More on variables 127