Bash Variable Expansion

A compact reference for useful Bash parameter expansions: required variables, defaults, substrings, lengths, case conversion, and related forms.

This is one of those things that just doesn’t stick in my head, so I’m dropping this note to remind myself.

I often have trouble recalling cryptic variable names in bash, make, and perl. For my own sake, I thought I’d make a quick list of favorites and refer some sources.

Maybe you’ll find this useful too.

• Variable value, but exit if no such variable: ${var:?“error message”}
• Use default if variable isn’t set: ${var:=“default”}
• Get a substring: ${var: start: length}
• Length: ${#var}
• Uppercase: ${var@U}
• Lowercase: ${var@L}

The first one is quite helpful when you expect a particular set of parameters and don’t want to write if/then logic about each of them.

#/bin/env bash
directory=${1:? You must provide a directory to search}
pattern=${2:? You must provide a search term for filenames}
find ${directory} -name ${pattern}

See more at the gnu shell expansion page.