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.

Steps to use conditionals in Zsh:

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.

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.
Run the script without the config file to confirm the first guard stops execution.
```
$ zsh deploy-check.zsh
missing config: config/settings.conf
```
Create the config directory.
```
$ mkdir -p config
```

Write the required config file.

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

Run the script without an environment value to confirm the string guard stops execution.
```
$ zsh deploy-check.zsh
missing environment
```

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

Remove the sample files after testing.
```
$ rm -rf config deploy-check.zsh
```