Bash Command Substitution

Command substitution allows you to capture the output of a command and use it as part of another command or assignment.

Basic Syntax

There are two syntaxes for command substitution in Bash:


# Original backtick syntax (legacy)
`command`

# Modern POSIX syntax (preferred)
$(command)

Simple Examples


# Store command output in a variable
files=$(ls)
count=$(wc -l < file.txt)

# Use command output directly
echo "Today is $(date)"
echo "You are running $(uname -s) version $(uname -r)"

Nested Command Substitution

You can nest command substitutions when needed:


# Get the name of the largest file in directory
largest=$(ls -S $(pwd) | head -1)
echo "Largest file is: $largest"

Command Substitution in Loops


# Process each line from a command's output
while read -r line; do
    echo "Processing: $line"
done < <(grep "pattern" *.txt)

# Loop through files found by find command
for file in $(find . -name "*.txt"); do
    echo "Found text file: $file"
done

Command Substitution vs. Pipes

Command substitution captures output for later use, while pipes connect commands directly:


# Using command substitution
contents=$(cat file.txt)
echo "$contents" | grep "pattern"

# Using pipes directly
cat file.txt | grep "pattern"

Process Substitution

A related concept that treats command output as a file:


# Compare sorted versions of two files
diff <(sort file1.txt) <(sort file2.txt)

# Merge sorted output from two commands
sort -m <(command1) <(command2) > merged.txt

Best Practices

Prefer $(...) over backticks for better readability and nesting
Always quote command substitutions to preserve whitespace: "$(command)"
For complex commands, consider using temporary variables for clarity
Be aware that command substitution removes trailing newlines
For multiline output, use double quotes when echoing: echo "$var"

Advanced Examples


# Generate a dynamic filename
backup_file="backup-$(date +%Y%m%d).tar.gz"
tar -czf "$backup_file" /path/to/files

# Create a menu from command output
select option in $(ls *.txt); do
    echo "You selected $option"
    break
done

# Count occurrences in multiple files
total=$(grep -c "error" $(find /var/log -name "*.log") | awk '{sum += $1} END {print sum}')
echo "Total errors found: $total"

Common Pitfalls

Forgetting to quote command substitutions, leading to word splitting
Using command substitution when a pipe would be more efficient
Assuming command substitution works the same in all shells (some features are Bash-specific)
Using command substitution in performance-critical sections (it creates subshells)

Next: Arrays in Bash