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Command Substitution: Use Command Output

Command substitution runs commands and uses their output. Like asking a question and using the answer.

Here's the thing: Command substitution is common. Learn it. Use it.

Modern Syntax: $()​

current_date=$(date)
username=$(whoami)
files=$(ls)

echo "Date: $current_date"
echo "User: $username"

The $(): Runs command, captures output.

My take: Use $(). It's modern. It's preferred. It works.

Old Syntax: Backticks​

# Old syntax (avoid)
current_date=`date`
username=`whoami`

My take: Backticks work, but $() is better. Use $().

Common Uses​

Dynamic Values​

# Current date
backup_name="backup_$(date +%Y%m%d).tar.gz"

# Hostname
log_file="/var/log/$(hostname).log"

# User home
user_home=$(getent passwd $USER | cut -d: -f6)

My take: Use command substitution for dynamic values. Dates. Hostnames. Paths.

Calculations​

count=$(ls -1 | wc -l)
total=$(( $(du -s /path | cut -f1) / 1024 ))

My take: Combine with arithmetic. Calculate values.

Process Output​

# Get process count
nginx_count=$(ps aux | grep nginx | grep -v grep | wc -l)

# Get disk usage
disk_usage=$(df -h / | awk 'NR==2 {print $5}' | sed 's/%//')

My take: Process command output. Extract information.

Common Patterns​

Generate Filenames​

backup_file="backup_$(date +%Y%m%d_%H%M%S).tar.gz"

Get Configuration​

max_backups=$(grep "MAX_BACKUPS" config.txt | cut -d= -f2)

Check Status​

if [ "$(systemctl is-active nginx)" = "active" ]; then
    echo "Nginx is running"
fi

Common Mistakes (I've Made These)​

  1. Using backticks: Use $(). It's better. More readable.

  2. Not quoting: Quote command substitution. "$(command)" not $(command).

  3. Nested substitution: $($(command)) works, but be careful. It's complex.

  4. Command fails: If command fails, substitution is empty. Check exit codes.

  5. Too complex: Keep it simple. Complex substitutions are hard to read.

Real-World Examples​

Backup with Timestamp​

backup_file="backup_$(date +%Y%m%d_%H%M%S).tar.gz"
tar -czf "$backup_file" /data

Get System Info​

hostname=$(hostname)
uptime=$(uptime -p)
memory=$(free -h | awk '/^Mem:/ {print $2}')

Process Count​

nginx_count=$(pgrep -c nginx)
echo "Nginx processes: $nginx_count"

What's Next?​

Now that you can use command output, scripts become dynamic. Or learn about Parameter Expansion for more variable tricks.

Personal note: When I started, I'd run commands separately. Then I learned command substitution. Now I use it constantly. It makes scripts dynamic. Learn it. Use it.