Default/Fallback Argument Handling

Providing a Fallback with :-

${var:-default} expands to var's value if it is set and non-empty, or to default otherwise, without modifying var itself. This is the most common way to give an argument a sensible default.

Example: Providing a Fallback with :-

#!/bin/bash
set --
name=${1:-"anonymous"}
echo "Hello, $name"

Assigning the Default Back with :=

${var:=default} behaves like :- but also assigns default into var, so subsequent uses of var see the default too. This only works on ordinary variables, not positional parameters like $1.

Note: The leading : (a no-op command) is a common trick to apply := for its side effect without needing to use the expanded value anywhere.

Example: Assigning the Default Back with :=

#!/bin/bash
unset timeout
: "${timeout:=30}"
echo "timeout is now set to: $timeout"

The Inverse Case with :+

${var:+alt} is the mirror image of :-: it expands to alt only when var is set and non-empty, and to nothing otherwise, which is useful for conditionally including a flag only when a variable is present.

Example: The Inverse Case with :+

#!/bin/bash
verbose="1"
flag=${verbose:+"--verbose"}
echo "flag is: $flag"

unset verbose
flag=${verbose:+"--verbose"}
echo "flag is now: [${flag}]"

Combining Defaults with Validation

Defaults and required-value checks can be combined: give optional settings a fallback with :-, while still using :? for values that truly must be provided by the caller.

Example: Combining Defaults with Validation

#!/bin/bash
set -- "myapp"
app_name=${1:?app name is required}
log_level=${2:-"info"}
echo "starting $app_name with log level $log_level"