Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
a=${param5:+xyz}
echo "a = $a" # a =
# Different result from a=${param5+xyz}
param6=123
a=${param6+xyz}
echo "a = $a" # a = xyz
${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is
null, as above.
Example 9−13. Using parameter substitution and error messages
#!/bin/bash
# Check some of the system's environmental variables.
# If, for example, $USER, the name of the person at the console, is not set,
#+ the machine will not recognize you.
: ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
echo
echo "Name of the machine is $HOSTNAME."
echo "You are $USER."
echo "Your home directory is $HOME."
echo "Your mail INBOX is located in $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# The ${variablename?} construction can also check
#+ for variables set within the script.
ThisVariable=Value−of−ThisVariable
# Note, by the way, that string variables may be set
#+ to characters disallowed in their names.
: ${ThisVariable?}
echo "Value of ThisVariable is $ThisVariable".
echo
echo
: ${ZZXy23AB?"ZZXy23AB has not been set."}
# If ZZXy23AB has not been set,
#+ then the script terminates with an error message.
# You can specify the error message.
# : ${ZZXy23AB?"ZZXy23AB has not been set."}
# Same result with: dummy_variable=${ZZXy23AB?}
# dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
#
# echo ${ZZXy23AB?} >/dev/null
Chapter 9. Variables Revisited 86