Shell by Example: Command Substitution

Shell by Example: Command Substitution POSIX

	`#!/bin/sh`
Command substitution lets you capture the output of a command and use it as a value. This is one of the most powerful features in shell scripting.
The modern syntax uses `$(command)`.	`current_date=$(date) echo "Current date: $current_date"`
You can also use backticks, but `$()` is preferred because it’s easier to nest and read.	current_user=`whoami` echo "Current user: $current_user"
Command substitution is commonly used to store command output in variables.	`file_count=$(ls \| wc -l) echo "Files in current directory: $file_count"`
You can use it directly in strings.	`echo "You are logged in as $(whoami) on $(hostname)"`
Command substitution can be nested with `$()` syntax.	`echo "Today is $(date +%A), week $(date +%V)"`
Capture the contents of a file.	`echo "Config: $(cat /etc/hostname)"`
Use it for conditional checks.	`kernel=$(uname -s) echo "Running on kernel: $kernel"`
Command substitution strips trailing newlines from the output. This is usually what you want.	`output=$(echo "Hello") echo "Output: '$output'"`
Store command exit in a variable too if needed.	`result=$(ls /nonexistent 2>&1) exit_code=$? echo "Exit code was: $exit_code"`
You can use command substitution in arithmetic.	`files=$(ls \| wc -l) echo "Double the files: $((files * 2))"`