How to use conditionals in Zsh

Conditionals let Zsh scripts choose a path after checking files, variables, or numeric state. They are the branch points behind setup checks, deployment guards, and cleanup scripts that should stop when a required input is missing.

The if, elif, and else structure works with [[ ... ]] for file and string tests. Arithmetic checks use (( ... )), which reads numeric expressions without extra bracket syntax.

The example below checks for a configuration file, requires an environment name, and confirms that retries remain before printing a ready state. The syntax check covers the whole script, and the command output shows each guard before the final success path.

1. Create a script with file, string, and arithmetic conditionals.

   deploy-check.zsh

   #!/usr/bin/env zsh
   set -e
    
   config_file="config/settings.conf"
   environment=${ENVIRONMENT:-}
   retries=3
    
   if [[ ! -f $config_file ]]; then
       print -r -- "missing config: $config_file"
       exit 1
   elif [[ -z $environment ]]; then
       print -r -- "missing environment"
       exit 1
   elif (( retries > 0 )); then
       print -r -- "environment=$environment"
       print -r -- "config=$config_file"
       print -r -- "status=ready"
   else
       print -r -- "no retries left"
       exit 1
   fi

   Use [[ ... ]] for file and string tests. Use (( ... )) for numeric expressions such as retries > 0.

2. Check the script syntax before running it.

   $ zsh -n deploy-check.zsh

   No output from zsh -n means Zsh parsed the conditional chain without finding a syntax error.

3. Run the script without the config file to confirm the first guard stops execution.

   $ zsh deploy-check.zsh
   missing config: config/settings.conf

4. Create the config directory.

   $ mkdir -p config

5. Write the required config file.

   $ printf 'region=us-east-1\n' > config/settings.conf

6. Run the script without an environment value to confirm the string guard stops execution.

   $ zsh deploy-check.zsh
   missing environment

7. Run the script with an environment value and confirm the ready state.

   $ ENVIRONMENT=production zsh deploy-check.zsh
   environment=production
   config=config/settings.conf
   status=ready

8. Remove the sample files after testing.

   $ rm -rf config deploy-check.zsh