Command Substitution

Command substitution is a fundamental feature of Bash scripting that lets you capture the output of one command and embed it into another. This technique improves script readability and enables dynamic data handling in your shell scripts.

Recap: Variable Expansion

Use $ followed by a variable name to expand its value. Enclose the name in curly braces {} to avoid ambiguity, and wrap expansions in quotes to prevent word splitting and globbing.

name="John"
echo "${name}"
# Output: John

What Is Command Substitution?

Command substitution runs a command in a subshell and replaces the command with its standard output. There are two syntaxes:

Example: Counting Files on the Command Line

$ ls
DEV001  DEV002  DEV003  DEV004  MKT001  MKT002  MKT003  MKT004  command-substitution.sh
$ find . -type f | wc -l
9

Using $( ... )

Create command-substitution.sh:

#!/usr/bin/env bash
file_count=$(find . -type f | wc -l)
echo "Total files: ${file_count}"

Run it:

$ chmod +x command-substitution.sh
$ ./command-substitution.sh
Total files: 9

Using Backticks (Deprecated)

#!/usr/bin/env bash
file_count=`find . -type f | wc -l`
echo "Total files: ${file_count}"

Accepting a Directory Argument

Require the user to specify a target directory:

#!/usr/bin/env bash
if [[ -z "${1}" ]]; then
  echo "Usage: $0 <directory>"
  exit 1
fi

file_count=$(find "${1}" -type f | wc -l)
echo "Files in ${1}: ${file_count}"

$ ./command-substitution-v2.sh .
Files in .: 9

Timing Considerations

Command substitution runs once when assigned. If you modify files later, the stored value stays unchanged until you reassign it.

#!/usr/bin/env bash
if [[ -z "${1}" ]]; then
  echo "Usage: $0 <directory>"
  exit 1
fi

file_count=$(find "${1}" -type f | wc -l)
echo "Initial count: ${file_count}"

touch samplefile
echo "After touch: ${file_count}"

$ ./command-substitution-v3.sh .
Initial count: 9
After touch: 9

Re-running the script updates the count:

$ ./command-substitution-v3.sh .
Initial count: 10
After touch: 10

Subshell Scope

Command substitution executes in a subshell. Variables modified inside do not affect the parent shell:

#!/usr/bin/env bash
if [[ -z "${1}" ]]; then
  echo "Usage: $0 <directory>"
  exit 1
fi

dir=${1}
file_count=$(find "${dir}" -type f | wc -l)

sub_output=$(
  dir="/some/other/dir"
  echo "Dir in subshell: ${dir}"
)

echo "Dir in parent shell: ${dir}"
echo "${sub_output}"

$ ./command-substitution-v4.sh /usr/bin
Dir in parent shell: /usr/bin
Dir in subshell: /some/other/dir

A subshell inherits your environment but keeps its changes local.

Performance Considerations

Each command substitution spawns a new process. In most scripts this overhead is negligible, but in performance-critical loops, minimize unnecessary substitutions.

Honorable Mentions

echo "Errors: ${files}"
echo "Today's date: ${current_date}"

Links and References

• Bash Reference Manual
• Advanced Bash-Scripting Guide
• ShellCheck: Shell Script Analysis