How to use parameter expansion in Zsh

Parameter expansion keeps small string changes inside a Zsh script when the script already has the value in a parameter. It fits default arguments, service labels, short prefixes, and path-derived filenames that should be built before the next command runs.

The common form is ${name...}, where braces separate the parameter name from the operation applied to its value. In ${empty:-fallback}, the colon makes Zsh use the fallback when the parameter is unset or empty, while modifiers such as :h, :t, :r, and :e work on path-like strings.

The sample script runs with a default service name and with an explicit service name. The output confirms which fields follow the argument, which fallback is used for an empty value, and how the (U) flag plus path modifiers build a rotated log filename.

1. Create and enter a temporary workspace for the example script.

   $ mkdir -p /tmp/zsh-parameter-demo
   $ cd /tmp/zsh-parameter-demo

2. Create a script with several Zsh parameter-expansion forms.

   expansion-demo.zsh

   #!/usr/bin/env zsh
   emulate -L zsh
   setopt err_exit no_unset
    
   service=${1:-api-worker}
   empty=""
   log_path="/srv/app/releases/${service}.log"
    
   print -r -- "service=${service}"
   print -r -- "fallback=${empty:-default-service}"
   print -r -- "upper=${(U)service}"
   print -r -- "prefix=${service:0:3}"
   print -r -- "file=${log_path:t}"
   print -r -- "dir=${log_path:h}"
   print -r -- "rotated=${log_path:r}.1.${log_path:e}"

   ${1:-api-worker} supplies a default argument, (U) uppercases the value, and the path modifiers split /srv/app/releases/api-worker.log without basename or dirname.

3. Check the script syntax.

   $ zsh -n expansion-demo.zsh

   No output means Zsh parsed the script without finding a syntax error.

4. Run the script with the default service name.

   $ zsh expansion-demo.zsh
   service=api-worker
   fallback=default-service
   upper=API-WORKER
   prefix=api
   file=api-worker.log
   dir=/srv/app/releases
   rotated=/srv/app/releases/api-worker.1.log

5. Run the script with an explicit service name.

   $ zsh expansion-demo.zsh frontend
   service=frontend
   fallback=default-service
   upper=FRONTEND
   prefix=fro
   file=frontend.log
   dir=/srv/app/releases
   rotated=/srv/app/releases/frontend.1.log

6. Quote expansion results when they become command arguments.

   $ log_path="/srv/app/releases/monthly report.log"
   $ print -r -- "${log_path:t}"
   monthly report.log

   Zsh does not split ordinary scalar parameter expansions on whitespace by default, but quotes still preserve empty values and protect scripts that later use arrays or option changes. Test dense forms with print -r -- before using them with commands such as rm, chmod, or mv.

7. Remove the sample workspace after testing.

   $ cd /tmp
   $ rm -rf /tmp/zsh-parameter-demo