bash assign default value

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

Use a colon:

: ${A:=hello}

The colon is a null command that does nothing and ignores its arguments. It is built into bash so a new process is not created.

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