Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
username2=
# username2 has been declared, but is set to null.
echo "username2 = ${username2:−`whoami`}"
# Will echo because of :− rather than just − in condition test.
exit 0
The default parameter construct finds use in providing "missing" command−line arguments in scripts.
DEFAULT_FILENAME=generic.data
filename=${1:−$DEFAULT_FILENAME}
# If not otherwise specified, the following command block operates
#+ on the file "generic.data".
#
# Commands follow.
See also Example 3−4, Example 29−2, and Example A−7.
Compare this method with using an and list to supply a default command−line argument.
${parameter=default}, ${parameter:=default}
If parameter not set, set it to default.
Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and
is null, [22] as above.
echo ${username=`whoami`}
# Variable "username" is now set to `whoami`.
${parameter+alt_value}, ${parameter:+alt_value}
If parameter set, use alt_value, else use null string.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is
null, see below.
echo "###### \${parameter+alt_value} ########"
echo
a=${param1+xyz}
echo "a = $a" # a =
param2=
a=${param2+xyz}
echo "a = $a"
param3=123
a=${param3+xyz}
echo "a = $a"
# a = xyz
# a = xyz
echo
echo "###### \${parameter:+alt_value} ########"
echo
a=${param4:+xyz}
echo "a = $a" # a =
param5=
Chapter 9. Variables Revisited 85