[bash] bash assign default value

${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

I thought I could use this feature to write ${LONG_VARIABLE_NAME:=hello} instead of the longer LONG_VARIABLE_NAME=${LONG_VARIABLE_NAME:-hello}, but now bash also tries to execute 'hello' and that gives a command not found. Any way to avoid that? Or will I have to stick to the latter? Can someone give an example where the assign default is actually useful?

Please look at http://www.tldp.org/LDP/abs/html/parameter-substitution.html for examples

${parameter-default}, ${parameter:-default}

If parameter not set, use default. After the call, parameter is still not set.
Both forms are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null.

unset EGGS
echo 1 ${EGGS-spam}   # 1 spam
echo 2 ${EGGS:-spam}  # 2 spam

EGGS=
echo 3 ${EGGS-spam}   # 3
echo 4 ${EGGS:-spam}  # 4 spam

EGGS=cheese
echo 5 ${EGGS-spam}   # 5 cheese
echo 6 ${EGGS:-spam}  # 6 cheese

${parameter=default}, ${parameter:=default}

If parameter not set, set parameter value to default.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null

# sets variable without needing to reassign
# colons suppress attempting to run the string
unset EGGS
: ${EGGS=spam}
echo 1 $EGGS     # 1 spam
unset EGGS
: ${EGGS:=spam}
echo 2 $EGGS     # 2 spam

EGGS=
: ${EGGS=spam}
echo 3 $EGGS     # 3        (set, but blank -> leaves alone)
EGGS=
: ${EGGS:=spam}
echo 4 $EGGS     # 4 spam

EGGS=cheese
: ${EGGS:=spam}
echo 5 $EGGS     # 5 cheese
EGGS=cheese
: ${EGGS=spam}
echo 6 $EGGS     # 6 cheese

${parameter+alt_value}, ${parameter:+alt_value}

If parameter set, use alt_value, else use null string. After the call, parameter value not changed.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null

unset EGGS
echo 1 ${EGGS+spam}  # 1
echo 2 ${EGGS:+spam} # 2

EGGS=
echo 3 ${EGGS+spam}  # 3 spam
echo 4 ${EGGS:+spam} # 4

EGGS=cheese
echo 5 ${EGGS+spam}  # 5 spam
echo 6 ${EGGS:+spam} # 6 spam

You can also use := construct to assign and decide on action in one step. Consider following example:

# Example of setting default server and reporting it's status

server=$1
if [[ ${server:=localhost} =~ [a-z] ]]      # 'localhost' assigned here to $server
then    echo "server is localhost"          # echo is triggered since letters were found in $server
else
        echo "server was set" # numbers were passed
fi

If $1 is not empty, localhost will be assigned to server in the if condition field, trigger match and report match result. In this way you can assign on the fly and trigger appropriate action.

The default value parameter expansion is often useful in build scripts like the example one below. If the user just calls the script as-is, perl will not be built in. The user has to explicitly set WITH_PERL to a value other than "no" to have it built in.

$ cat defvar.sh
#!/bin/bash

WITH_PERL=${WITH_PERL:-no}

if [[ "$WITH_PERL" != no ]]; then
    echo "building with perl"
    # ./configure --enable=perl
else
    echo "not building with perl"
    # ./configure
fi

Build without Perl

$ ./defvar.sh
not building with perl

Build with Perl

$ WITH_PERL=yes ./defvar.sh
building with perl