How to use command substitution in Zsh

Command substitution lets a Zsh script turn command output into a value before the next command runs. It fits short captures such as line counts, current directory names, generated filenames, or command-derived labels that need to be reused later.

The standard form is $(command). Zsh runs the command in a subshell, captures standard output, removes trailing newline characters, and places the remaining text where the substitution appears.

Quote a command substitution when its output becomes another command's argument. Zsh can still split raw command-substitution output into words, so values containing spaces should be protected before they are passed to commands such as mv, mkdir, or printf.

Steps to use command substitution in Zsh:

Create and enter a small workspace for the command-substitution example.

$ mkdir -p use-command-substitution/reports
$ cd use-command-substitution
$ printf 'alpha\nbeta\ngamma\n' > reports/input.txt

Create the script that captures command output in Zsh parameters.

capture-output.zsh

#!/usr/bin/env zsh
emulate -L zsh
setopt err_exit
 
report=${1:-reports/input.txt}
line_count=$(wc -l < "$report")
report_name=$(basename "$report")
run_dir=$(basename "$PWD")
 
print -r -- "report=$report_name"
print -r -- "lines=$line_count"
print -r -- "directory=$run_dir"

The redirection in wc -l < "$report" keeps the filename out of the captured output, so line_count receives only the count.

Check the script syntax.
```
$ zsh -n capture-output.zsh
```
No output means Zsh parsed the command substitutions and the rest of the script without a syntax error.

Run the script and verify the captured values.

$ zsh capture-output.zsh
report=input.txt
lines=3
directory=use-command-substitution

Quote command substitutions when the result may contain spaces.
```
$ report_path="reports/monthly report.txt"
$ print -r -- "archive/$(basename "$report_path")"
archive/monthly report.txt
```
The inner quotes protect $report_path before basename runs. The outer quotes protect the substituted filename before print receives it.
Use the $(...) form when command substitutions are nested or placed inside larger quoted strings.
```
run_dir=$(basename "$PWD")
print -r -- "archive/$(basename "$report_path")"
```
The older backtick form is harder to read when nested and can break quickly once quoting is added.
Remove the sample workspace after testing.
```
$ cd ..
$ rm -rf use-command-substitution
```