Parameter Substitution

Manipulating and/or expanding variables

${parameter}

    Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works.

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

    If parameter not set, use default.

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

    If parameter not set, set it to default.

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

    If parameter set, use alt_value, else use null string.

${parameter?err_msg}, ${parameter:?err_msg}

    If parameter set, use it, else print err_msg and abort the script with an exit status of 1.

Variable length / Substring removal

${#var}

    String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array.

${var#Pattern}, ${var##Pattern}

    ${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end of $var.

    ${var##Pattern} Remove from $var the longest part of $Pattern that matches the front end of $var.

${var%Pattern}, ${var%%Pattern}

    ${var%Pattern} Remove from $var the shortest part of $Pattern that matches the back end of $var.

    ${var%%Pattern} Remove from $var the longest part of $Pattern that matches the back end of $var.

Variable expansion / Substring replacement

    These constructs have been adopted from ksh.
${var:pos}

    Variable var expanded, starting from offset pos.
${var:pos:len}

    Expansion to a max of len characters of variable var, from offset pos. See Example A-13 for an example of the creative use of this operator.
${var/Pattern/Replacement}

    First match of Pattern, within var replaced with Replacement.

    If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted.
${var//Pattern/Replacement}

    Global replacement. All matches of Pattern, within var replaced with Replacement.

    As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted.

${var/#Pattern/Replacement}

    If prefix of var matches Pattern, then substitute Replacement for Pattern.
${var/%Pattern/Replacement}

    If suffix of var matches Pattern, then substitute Replacement for Pattern.

${!varprefix*}, ${!varprefix@}

    Matches names of all previously declared variables beginning with varprefix.