$() (Command Substitution)

Command substitution (recommended).

$(command)

Backtick syntax (legacy, not recommended).

`command`

Assign to a variable.

variable=$(command)

Use as an argument to another command.

command1 $(command2)

Nesting ($() makes nesting easy).

$(command1 $(command2))

Pattern	Description
var=$(cmd)	Assigns the standard output of a command to a variable. Trailing newlines are automatically removed.
echo "Today: $(date)"	Embeds the result of a command inside a string.
for f in $(ls *.txt)	Uses the command output as a list to iterate over with a for loop.
$(cmd1 $(cmd2))	Nested command substitution (nesting is difficult with backticks).
files=$(find . -name "*.php")	Stores multi-line output in a single variable.

Assign the result of a command to a variable.

date_var.sh

today=$(date +%Y-%m-%d)
echo "Today's date: $today"

bash date_var.sh
Today's date: 2026-03-06

whoami_var.sh

current_user=$(whoami)
echo "Current user: $current_user"

bash whoami_var.sh
Current user: alice

Use a command's result directly, such as counting files.

count_php.sh

php_count=$(find . -name "*.php" | wc -l)
echo ".php file count: $php_count"

bash count_php.sh
.php file count: 42

Embed command results inside a string.

echo "Hostname: $(hostname), User: $(whoami)"
Hostname: myserver, User: alice

Use command output as a filename.

logname.sh

log_file="app_$(date +%Y%m%d_%H%M%S).log"
echo "Log file: $log_file"

bash logname.sh
Log file: app_20260306_143022.log

Nested command substitution. Unlike backticks, $() makes nesting straightforward.

nested.sh

largest=$(basename $(ls -S | head -1))
echo "Largest file: $largest"

bash nested.sh
Largest file: index.php

Combine with a for loop to process a list of files.

list_php.sh

echo "--- PHP file list ---"
for f in $(find . -maxdepth 1 -name "*.php" | sort); do
    size=$(wc -c < "$f")
    echo "$f: ${size} bytes"
done

bash list_php.sh
--- PHP file list ---
./ajax.php: 8340 bytes
./index.php: 5120 bytes

You can also enter a for loop directly in the terminal. After pressing Enter after do, a > prompt appears — this means the shell is waiting for more input. Enter done to execute.

for f in $(find . -maxdepth 1 -name "*.php" | sort); do
    size=$(wc -c < "$f")
    echo "$f: ${size} bytes"
done
./ajax.php: 8340 bytes
./index.php: 5120 bytes

Store multi-line output in a variable and process it.

count_dirs.sh

dirs=$(find . -maxdepth 1 -type d)
echo "Directory count: $(echo "$dirs" | wc -l)"

bash count_dirs.sh
Directory count: 8

The result of command substitution has trailing newlines automatically removed. If you store multi-line output in a variable, the lines are preserved, but expanding without double quotes ($variable) converts newlines to spaces. To process multi-line output line by line, quote the variable ("$variable") and pass it to a while read loop.

Backtick syntax (`cmd`) is hard to nest and difficult to read, so modern scripts should use $() instead.

To pass command output as arguments to another command, xargs is also useful.