Bash: Command Substitution

Command substitution allows you to use the output of a command as a variable or as an argument for another command. This is a powerful feature for building dynamic scripts.

Syntax for Command Substitution

Using `$(...)`

current_date=$(date)
 echo "Today's date is $current_date"

The modern and preferred syntax is to enclose the command in `$()`. The shell will first execute the command inside the parentheses and then replace the entire expression with the command's standard output.

Using backticks `...`

current_date=`date`
 echo "Today's date is $current_date"

This is the older syntax for command substitution. It works similarly to `$(...)` but can be more difficult to read and less reliable in nested commands. The `$(...)` syntax is recommended.

Practical Examples

Creating a directory with the current date

mkdir "backup_$(date +%Y-%m-%d)"

This command will create a new directory with a name that includes the current year, month, and day. The `date +%Y-%m-%d` command provides a formatted string as its output.

Counting files in a directory

file_count=$(ls | wc -l)
 echo "There are $file_count files in this directory."

This example demonstrates piping within a command substitution. The output of `ls` is piped to `wc -l` (word count with lines), and the final result (the line count) is stored in the `file_count` variable.