A Better Bash Reference Manual - Parameter Expansion _ 091015 _

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Parameter Expansion

(Perhaps "Parameter evaluating" would be a better phrase, ed)
Especially useful within functions or scripts.

The simplist form is : ${parameter}. The value of parameter is substituted. Braces are optional when parameter is a single digit positional parameter.

> echo $1 +++ ${1} +++  
editing-mode +++ editing-mode +++

indirect expansion uses a ! prefix to parameter and the value of parameter is a variable used in the substitution.

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

With the : colon if unset OR null
without the colon only if unset.

`word` is expanded to produce a pattern as in filename expansion.
Provide default `${parameter[:]-default}`	If `parameter` is set, the value of `default` is substituted. If `parameter` is null or `parameter` is unset and followed by : : the expansion of `default` is substituted, function checkRange ,-no $ max=${3:-99} # max ~~is set to~~ $3~~, unless~~ $3 ~~is not provided or is null, then it is defaulted to~~ `99`. ^ ^ ~~no space~~ if [ $max -gt 1000 ]; then echo max to big; return 1;fi return 0}
assign `parameter` to `default` for the remainder of the script: `${parameter[:]=default}`	If `parameter` is unset or null, the expansion of `default` is assigned to `parameter` The value of `parameter` is then substituted. > echo ${LINES:=30} 30 > echo :$LINES: :30: > echo ${9-"ninth Parameter Is UNSET"} # ~~No : in expansion.~~ > echo ${LINES:-30} 30 > echo :$LINES: :: `parameter` ~~is unchanged.~~ ~~Positional parameters (for example `$2`,… and special parameters may not be assigned in this way.~~
Require a value: `{$parameter[:]?[error message]}`	If `parameter` is set, it is substituted. If `parameter` is null or `parameter` is unset and followed by : : the the script name, line number and an expansion of `error message` No quotes needed is written to standard error. Exits with a return code of 1 (unless it is not interactive). The default `error message` is `"parameter null or not set"` guess which one!. > cat 00.sh echo Size is ${SIZE:? must be set} echo positionalParameter1 is ${1? ":posPar1 is required"} echo +end of script+ > ./00.sh ./00.sh: line 1: size: : must be set echo $? 1 export SIZE=1234 > ./00.sh Size is 1234 ./00.sh: line 2: 1: :posPar1 is required > ./00.sh 88 Size is 1234 positionalParameter1 is 88 +end of script+
OVERRIDE a value: `${parameter:+word}`	If `parameter` is null or unset, nothing is substituted, otherwise the expansion of `word` is substituted.
use a substring of the value: `${parameter:i[:n]}`	substitutes the substring of `parameter`, starting with the `i`^th character for `n` characters. `i` and `n` are arithemtic expressions. Substring indexing is zero-based. `n` must evaluate to zero or greater. If `i` evaluates to a negative number, it is an offset from the end of the value of `parameter`. If `parameter` is an array name indexed by `@` or `*`, the result is the `n` members of the array beginning with `${parameter[i]}`. If `parameter` is `@`, the result is `n` positional parameters beginning at `i`, indexing starts at 1!
`${#parameter}`	The number of characters (of the expanded) value of `parameter` is substituted. echo $1 +++ ${#1} editing-mode +++ 12 123456789012 For `${}` or `${@}`, the value substituted is the number of positional parameters. If `parameter` is an array name subscripted by `` or `@`, the value substituted is the number of elements in the array.
`${parameter#word}` `${parameter##word}`	If that pattern matches the beginning of the expanded value of `parameter`, the result is the expanded value of `parameter` with the shortest ( `#` ) or longest ( `##` ) matching pattern deleted. For `${%` … or `${@%` …, the pattern removal is applied to each positional parameter and the result is the list. If `parameter` is an array variable subscripted with `@` or ``, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
`${parameter%word}` `${parameter%%word}`	If that pattern matches the ending …
`pattern` is expanded to produce a pattern as in filename expansion
`${parameter/[#\|%]pattern/string}` `${parameter//[#\|%]pattern/string}`	`Parameter` is expanded and the longest match of `pattern` against that value is replaced with `string`. Only the first (`/`) or all (`//`) match is replaced. For `#pattern` match beginning of `string`, for `%pattern` match end of `string`.
`${parameter/[#\|%]pattern}` `${parameter//[#\|%]pattern}`	If `string` is null, matches of `pattern` are deleted. The `/` following `pattern` is optional. For `${#` … or `${@#` …, `${%` … or `${@%` … the substitution is applied to positional parameters and the result is the list. If `parameter` is an array variable subscripted with `@` or `*`, the substitution is applied to each member of the array in turn. Result is that list.

Command Substitution