Shell Parameter Expansion

The most basic form of parameter expansion is ${parameter} where the value of the parameter is substituted in place.

There are operators that can be used with shell parameter expansion that offers certain features.

Expansion Based on Parameter Value

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

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${parameter:=word}

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.

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${parameter:?word}

If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.

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${parameter:+word}

This is the opposite of ${parameter:-word}. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

Substring Operations

${parameter:offset}
${parameter:offset:length}

These are substring operations:

• offset is a zero-based index. Outputs substring from offset index onwards (inclusive) up to the end of string.
• length is the desired length of substring. Outputs substring from offset index onwards up to the given length.

String Replace

The string replace operations do not mutate the parameter.

${parameter/pattern/string}

Replaces pattern (regex) in parameter with string. This replaces only the first occurrence of pattern.

${parameter//pattern/string}

Add a slash to pattern to replace all occurrences of pattern

Example:

${1//[-_*]/ }

Replaces all occurrences of either -, _ or * with a space

More can be found here

Indirect Variable Expansion

Useful when a variable stores the name of another variable, and we want to retrieve the value of the nested variable.

TOKEN_NAME='foobar'
MY_ENV=TOKEN_NAME

echo ${!MY_ENV}
# Outputs "foobar"

# If there is futher nesting, use temp variable
# to expand through multiple levels of nesting
ENV_NAME=MY_ENV
temp_env=${!ENV_NAME}

echo ${!temp_env}
# Outputs "foobar"