Assigning Default Values to Shell Variables with a Single Command in Bash

For command line arguments:

VARIABLE="${1:-$DEFAULTVALUE}"

which assigns to VARIABLE the value of the 1st argument passed to the script or the value of DEFAULTVALUE if no such argument was passed. Quoting prevents globbing and word splitting.

If the variable is same, then

: "${VARIABLE:=DEFAULT_VALUE}"

assigns DEFAULT_VALUE to VARIABLE if not defined.

• The colon builtin (:) ensures the variable result is not executed
• The double quotes (") prevent globbing and word splitting.

Also see Section 3.5.3, Shell Parameter Expansion, in the Bash manual.

To answer your question and on all variable substitutions

echo "${var}"
echo "Substitute the value of var."
    

echo "${var:-word}"
echo "If var is null or unset, word is substituted for var. The value of var does not change."
    

echo "${var:=word}"
echo "If var is null or unset, var is set to the value of word."
    

echo "${var:?message}"
echo "If var is null or unset, message is printed to standard error. This checks that variables are set correctly."
    

echo "${var:+word}"
echo "If var is set, word is substituted for var. The value of var does not change."

You can escape the whole expression by putting a \ between the dollar sign and the rest of the expression.

echo "$\{var}"

Even you can use like default value the value of another variable

having a file defvalue.sh

#!/bin/bash
variable1=$1
variable2=${2:-$variable1}

echo $variable1
echo $variable2

run ./defvalue.sh first-value second-value output

first-value
second-value

and run ./defvalue.sh first-value output

first-value
first-value

see here under 3.5.3(shell parameter expansion)

so in your case

${VARIABLE:-default}

FWIW, you can provide an error message like so:

USERNAME=${1:?"Specify a username"}

This displays a message like this and exits with code 1:

./myscript.sh
./myscript.sh: line 2: 1: Specify a username

A more complete example of everything:

#!/bin/bash
ACTION=${1:?"Specify 'action' as argv[1]"}
DIRNAME=${2:-$PWD}
OUTPUT_DIR=${3:-${HOMEDIR:-"/tmp"}}

echo "$ACTION"
echo "$DIRNAME"
echo "$OUTPUT_DIR"

Output:

$ ./script.sh foo
foo
/path/to/pwd
/tmp

$ export HOMEDIR=/home/myuser
$ ./script.sh foo
foo
/path/to/pwd
/home/myuser

• $ACTION takes the value of the first argument, and exits if empty
• $DIRNAME is the 2nd argument, and defaults to the current directory
• $OUTPUT_DIR is the 3rd argument, or $HOMEDIR (if defined), else, /tmp. This works on OS X, but I'm not positive that it's portable.

Then there's the way of expressing your 'if' construct more tersely:

FOO='default'
[ -n "${VARIABLE}" ] && FOO=${VARIABLE}

Here is an example

#!/bin/bash

default='default_value'
value=${1:-$default}

echo "value: [$value]"

save this as script.sh and make it executable. run it without params

./script.sh
> value: [default_value]

run it with param

./script.sh my_value
> value: [my_value]