Parameter Expansion Advanced

The expansion forms you'll use forever

Bash and zsh share a rich parameter-expansion language. Once these patterns are in your fingers, you stop reaching for sed for half of the tiny manipulations.

Defaults

${var:-default} — value of var, or default if unset/empty.
${var:=default} — same, but also assign the default.
${var:+alt} — alt if var is set, else empty. Inverse.
${var:?error} — exit with error if unset/empty. Great for required env vars.

Length

${#var}                    # string length
${#arr[@]}                 # array length

Substring

${var:5}                  # from char 5 to end
${var:5:3}                # 3 chars starting at 5
${var: -4}                # last 4 chars (note the space!)

Trim from front

${var#prefix} — strip shortest match of prefix glob.
${var##prefix} — strip longest.

path=/usr/local/bin
echo ${path#*/}            # usr/local/bin
echo ${path##*/}           # bin             (basename!)

Trim from back

${var%suffix} — strip shortest match of suffix.
${var%%suffix} — strip longest.

file=foo.tar.gz
echo ${file%.gz}           # foo.tar
echo ${file%%.*}           # foo

Substitution

${var/from/to} — first match.
${var//from/to} — every match.
${var/#from/to} — only at the start.
${var/%from/to} — only at the end.

Case

${var^^}                  # uppercase (bash 4+)
${var,,}                  # lowercase
${var^}                   # capitalize first letter

Why memorize

These run inside the shell with no fork — orders of magnitude faster than spawning sed/awk for the same job. Most one-line text manipulation in shell can be done with parameter expansion alone.

file=/path/to/document.tar.gz echo "${file##*/}" # document.tar.gz (basename) echo "${file%/*}" # /path/to (dirname) echo "${file%.gz}" # /path/to/document.tar echo "${file%%.*}" # /path/to/document name=pippa echo "${name^^}" # PIPPA