$() (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))

Assign the result of a command to a variable.

sample_date_var.sh

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

Run the following command:

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

sample_whoami_var.sh

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

Run the following command:

bash whoami_var.sh
Current user: alice

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

sample_count_php.sh

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

Run the following command:

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.

sample_logname.sh

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

Run the following command:

bash logname.sh
Log file: app_20260306_143022.log

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

sample_nested.sh

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

Run the following command:

bash nested.sh
Largest file: index.php

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

sample_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

Run the following command:

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.

sample_count_dirs.sh

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

The following example demonstrates this:

bash count_dirs.sh
Directory count: 8

Common Mistake 1: Expanding without quotes splits on word boundaries

Without double quotes, the result of command substitution is subject to word splitting and glob expansion.

sample_no_quote.sh

files=$(ls *.php)
for f in $files; do
    echo "$f"
done

If filenames contain spaces, each space-separated word becomes a separate item. Always quote the result.

sample_quoted.sh

files=$(ls *.php)
for f in "$files"; do
    echo "$f"
done

Common Mistake 2: Trailing newlines are removed from the output

Command substitution strips trailing newlines. If you need to preserve them, workarounds are required.

result=$(printf "line1\nline2\n")
echo "$result" | wc -l
2

The two trailing newlines from printf are stripped but the internal newline between lines is kept. If you need the exact output, redirect to a file instead of using command substitution.

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.