Bash Command Substitution: Capture Output in Variables

The main issue with your script is that you’re using single quotes ' around the command substitution, which prevents variable expansion and command execution. To capture command output in Bash variables, you need to use either backticks ` or the $() syntax for command substitution.

Here’s how to fix your script:

#!/bin/bash

VAR1="$1"
MOREF=$(sudo run command against "$VAR1" | grep name | cut -c7-)

echo "$MOREF"

Contents

• Command Substitution Methods
• Variable Expansion Issues
• Proper Command Chaining
• Best Practices
• Debugging Tips
• Advanced Examples

Command Substitution Methods

Bash offers two primary methods for command substitution:

Backticks Method

The traditional method using backticks:

VAR1=`ls -la`
VAR2=`date +%Y-%m-%d`

$() Method (Recommended)

The modern method using $() syntax:

VAR1=$(ls -la)
VAR2=$(date +%Y-%m-%d)

Why $() is preferred:

• Easier to nest: $(command1 $(command2))
• Better readability with complex commands
• Doesn’t conflict with single quotes in scripts
• More consistent with other shell syntax

Variable Expansion Issues

When using command substitution, you need to be careful about variable expansion:

Correct Variable Expansion

VAR1="$1"
# Double quotes allow variable expansion
MOREF=$(sudo run command against "$VAR1" | grep name | cut -c7-)

Common Mistakes

• Single quotes prevent expansion: 'sudo run command against $VAR1' - $VAR1 won’t be expanded
• Missing quotes: sudo run command against $VAR1 - may fail with spaces in filenames
• Nested quotes: Be careful with quotes inside pipes and commands

Proper Command Chaining

For your specific case, here’s the correct approach:

#!/bin/bash

VAR1="$1"
# Use $() for command substitution
MOREF=$(sudo run command against "$VAR1" | grep name | cut -c7-)

# Always use quotes when echoing variables
echo "$MOREF"

Step-by-Step Breakdown:

1. Capture the variable: VAR1="$1" - captures the first argument
2. Command substitution: $() executes the command and captures its output
3. Command chaining: The | pipes work normally within $()
4. Safe variable access: "$VAR1" ensures proper handling of spaces and special characters

Best Practices

1. Always Use Quotes

# Good
echo "$variable"
# Bad
echo $variable  # May split on spaces and cause issues

2. Handle Errors

# Check if command succeeded
if command_result=$(some_command); then
    echo "Success: $command_result"
else
    echo "Command failed" >&2
    exit 1
fi

3. Use Temporary Files for Complex Operations

temp_file=$(mktemp)
your_command > "$temp_file"
result=$(cat "$temp_file")
rm "$temp_file"

4. Preserve Exit Status

# Command substitution doesn't preserve exit status
output=$(some_command || true)  # Always succeeds

# Better approach
if output=$(some_command 2>&1); then
    echo "Success: $output"
else
    echo "Error: $output" >&2
    exit $?
fi

Debugging Tips

1. Test Commands Individually

# Test each part of your pipe
sudo run command against "$VAR1"
sudo run command against "$VAR1" | grep name
sudo run command against "$VAR1" | grep name | cut -c7-

2. Use Set -x for Debugging

#!/bin/bash
set -x  # Enable debug mode

VAR1="$1"
MOREF=$(sudo run command against "$VAR1" | grep name | cut -c7-)

echo "$MOREF"

3. Check for Empty Output

#!/bin/bash

VAR1="$1"
MOREF=$(sudo run command against "$VAR1" | grep name | cut -c7-)

if [ -z "$MOREF" ]; then
    echo "Warning: No output captured" >&2
    # Debug individual commands
    echo "Debug:" >&2
    sudo run command against "$VAR1" | grep name | cut -c7- >&2
else
    echo "$MOREF"
fi

Advanced Examples

1. Multiple Command Substitutions

# Nested command substitution
timestamp=$(date +%Y%m%d)
backup_file=$(tar -czf "/backup_$timestamp.tar.gz" /important_data)
echo "Created backup: $backup_file"

2. Process Substitution for Complex Operations

# Process substitution with while loop
while read line; do
    echo "Processing: $line"
done < <(some_command | grep "pattern")

3. Arrays from Command Output

# Store command output in array
files=($(ls -1 *.txt))
echo "Found ${#files[@]} text files"
for file in "${files[@]}"; do
    echo "Processing $file"
done

4. Conditional Assignment

# Default value if command fails
result=$(some_command) || result="default_value"
echo "Result: $result"

Remember that the key difference between your original approach and the corrected version is the use of $() instead of single quotes around the command substitution. This allows the shell to execute the command and capture its output rather than treating it as a literal string.

Sources

1. Bash Guide for Beginners - Command Substitution
2. GNU Bash Manual - Command Substitution
3. Stack Overflow - Difference between backticks and $()
4. Bash Best Practices - Command Substitution
5. Advanced Bash Scripting Guide - Command Substitution

Conclusion

• Use $() syntax instead of backticks for modern, reliable command substitution
• Always quote variables to prevent word splitting and glob expansion
• Test commands individually when pipes aren’t working as expected
• Handle errors properly by checking command exit status
• Use debug mode (set -x) to trace what’s happening in your script
• Consider temporary files for complex multi-step operations

The main issue with your original script was the use of single quotes around the command substitution, which prevented the shell from executing the command. By changing 'command' to $(command), you enable proper command substitution and variable expansion.