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envsubst

envsubst is a Go package for expanding variables in a string using ${var} syntax. Includes support for bash string replacement functions.

Expression	Meaning
`${var}`	Value of `$var`
`${#var}`	String length of `$var`
`${var^}`	Uppercase first character of `$var`
`${var^^}`	Uppercase all characters in `$var`
`${var,}`	Lowercase first character of `$var`
`${var,,}`	Lowercase all characters in `$var`
`${var:n}`	Offset `$var` `n` characters from start
`${var:n:len}`	Offset `$var` `n` characters with max length of `len`
`${var#pattern}`	Strip shortest `pattern` match from start
`${var##pattern}`	Strip longest `pattern` match from start
`${var%pattern}`	Strip shortest `pattern` match from end
`${var%%pattern}`	Strip longest `pattern` match from end
`${var-default`	If `$var` is not set, evaluate expression as `$default`
`${var:-default`	If `$var` is not set or is empty, evaluate expression as `$default`
`${var=default`	If `$var` is not set, evaluate expression as `$default`
`${var:=default`	If `$var` is not set or is empty, evaluate expression as `$default`
`${var/pattern/replacement}`	Replace as few `pattern` matches as possible with `replacement`
`${var//pattern/replacement}`	Replace as many `pattern` matches as possible with `replacement`
`${var/#pattern/replacement}`	Replace `pattern` match with `replacement` from `$var` start
`${var/%pattern/replacement}`	Replace `pattern` match with `replacement` from `$var` end

For a deeper reference, see bash-hackers or gnu pattern matching.