Command Substitution

Use command output as part of another command.

Syntax

$(…) - Modern Syntax (Recommended)

result=$(command)
echo "Today is $(date)"

`…` - Backtick Syntax

result=`command`
echo "Today is `date`"

The $(...) syntax is preferred as it’s easier to nest and read.

Basic Usage

Capture Output

current_dir=$(pwd)
echo "You are in: $current_dir"

file_count=$(ls | wc -l)
echo "Files: $file_count"

In Commands

echo "Today is $(date)"
# Today is Mon Jan 1 12:00:00 UTC 2024

cd $(dirname "$0")
# Change to script's directory

In Variables

VERSION=$(cat VERSION)
HOSTNAME=$(hostname)
USER_HOME=$(eval echo ~$USER)

Common Patterns

Get Current Date/Time

NOW=$(date +%Y%m%d)
TIMESTAMP=$(date +%H%M%S)

Count Items

NUM_FILES=$(ls *.txt | wc -l)
NUM_LINES=$(wc -l < file.txt)

Find Commands

EDITOR=$(which vim)
GIT_ROOT=$(git rev-parse --show-toplevel)

Process Output

for file in $(find . -name "*.txt"); do
    echo "Processing: $file"
done

for user in $(cat /etc/passwd | cut -d: -f1); do
    echo "User: $user"
done

Nesting

The $(...) syntax allows easy nesting:

echo "Script dir: $(dirname $(realpath $0))"

# With backticks (harder to read)
echo "Script dir: `dirname \`realpath $0\``"

In Arithmetic

total=$(($(cat file1 | wc -l) + $(cat file2 | wc -l)))

With Quoting

# Preserve whitespace
files="$(ls -la)"
echo "$files"

# Word splitting (each line becomes argument)
for line in $(cat file.txt); do
    echo "$line"
done

Exit Codes

Command substitution captures output, not exit code:

result=$(false)
echo $?  # 1 (exit code available immediately after)

# Check both
if output=$(command); then
    echo "Success: $output"
else
    echo "Failed"
fi

Examples

Backup with Date

backup_file="backup_$(date +%Y%m%d).tar.gz"
tar -czf "$backup_file" /data

Dynamic Paths

cd "$(git rev-parse --show-toplevel)"
source "$(dirname $0)/config.sh"

Conditional on Output

if [ "$(whoami)" = "root" ]; then
    echo "Running as root"
fi

Build Version String

VERSION="$(cat VERSION)-$(git rev-parse --short HEAD)"

Limitations

• Trailing newlines are stripped
• Very large output may be slow
• Exit code only available immediately after