How do I set a variable to the output of a command in Bash?

To set a variable to the output of a command in Bash, use command substitution with the $(...) syntax. This captures the standard output (stdout) of the command and assigns it to the variable. Here’s how to do it:

Basic Syntax

variable_name=$(command_here)

or for older shells (not recommended):

variable_name=`command_here`  # Backticks (legacy syntax)

Examples

1. Capture the output of date:

current_date=$(date +"%Y-%m-%d")
echo "Today is $current_date"  # Output: Today is 2023-10-05

2. Capture the list of files in a directory:

files=$(ls)
echo "Files: $files"  # Lists all files in the current directory

3. Capture multi-line output (e.g., git status):

git_status=$(git status)
echo "$git_status"  # Preserves newlines and formatting

Key Notes

1. Quoting:

• Always quote the variable when using it ("$variable_name") to preserve whitespace, newlines, and special characters:

output=$(echo -e "Hello\nWorld") 
echo "$output" # Outputs two lines: "Hello" and "World"

• Without quotes, Bash splits the output into “words” (based on spaces/tabs/newlines):

echo $output # Outputs: Hello World (collapsed into one line)

1. Error Handling:

• By default, only stdout is captured. To include stderr, redirect it:

output=$(command 2>&1) # Captures both stdout and stderr

1. Trailing Newlines:

• Command substitution removes trailing newlines. To preserve them, append a dummy character and strip it:

output=$(printf "Hello\n"; printf x) # Add 'x' to preserve trailing newline 
output="${output%x}" # Remove the 'x'

1. Exit Status:

• The exit status of the command is not stored in the variable. Use $? to check success/failure:

output=$(ls) 
if [ $? -ne 0 ]; 
then 
  echo "Command failed" 

fi

Advanced Use Cases

1. Nested Command Substitution:

combined_output=$(echo "Today is $(date +'%A')")
echo "$combined_output"  # Output: Today is Thursday

2. Capture Command Output Line-by-Line:

while IFS= read -r line; do
  echo "Line: $line"
done < <(command_here)

3. Store Output in an Array:

files_array=($(ls))  # Splits output into array elements (caution: spaces in filenames break this)
echo "First file: ${files_array[0]}"

Common Pitfalls

• Spaces in Filenames/Output: Use mapfile or readarray to safely store multi-word output in arrays:

  mapfile -t files < <(ls)  # Stores each line as an array element
  echo "First file: ${files[0]}"

• Binary Data: Avoid command substitution for binary data (use pipes or redirection instead).

Summary

• Use var=$(command) to capture stdout.
• Quote variables ("$var") to preserve formatting.
• Redirect stderr with 2>&1 if needed.
• Prefer $(...) over backticks for readability and nesting.