Subshells

Bash · Reference cheat sheet

📋 Overview

A subshell is a child shell process. Changes to variables, cd, and shell options inside it do not affect the parent. Subshells appear with (…), pipelines, command substitution $(…), and process substitution contexts.

🔧 Core concepts

Use subshells to isolate cd, temporary umask, or option changes. Use \{ …; \} when you need side effects in the current shell.

💡 Examples

Isolate directory change:

pwd
( cd /tmp && ls )
pwd   # unchanged

Group vs subshell:

{ echo a; echo b; } > both.txt     # current shell
( echo a; echo b ) > both.txt      # subshell (usually fine for I/O)

Capture without polluting:

version="$(
  cd "$repo" || exit 1
  git describe --tags
)"

Avoid pipeline subshell for vars:

# bad: count lost
# printf '%s\n' a b | while read -r x; do ((count++)); done

count=0
while read -r x; do ((count++)) || true; done < <(printf '%s\n' a b)
echo "$count"   # 2

⚠️ Pitfalls

• Variable assignments in pipelines often don’t persist—use process substitution or lastpipe.
• (exit 1) only exits the subshell; parent continues unless you check status.
• Nested subshells add process overhead; don’t overuse in tight loops.
• cd in scripts: prefer subshell or pushd/popd over relying on later cd -.
• Functions defined in a subshell vanish when it ends.
• export inside subshell does not affect parent environment.

🔗 Related

• process_substitution.md
• pipes_redirection.md
• variables.md
• loops.md
• functions.md