Subshells and Command Grouping

~8 min · subshell, grouping, scope

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Two ways to group commands

{ cmd1; cmd2; } — run in current shell. Variables persist after the group.
(cmd1; cmd2) — run in a subshell. Variable changes don't leak out. Slightly slower (fork).

When to use {}

Apply a single redirection to several commands:

{
  echo "=== build log ==="
  date
  ./build.sh
} > build.log 2>&1

The whole block writes to build.log. Saves three >> redirections.

When to use ()

Isolate side effects. Common: cd into a dir, do work, come back automatically:

(
  cd ~/projects/temp
  ./make.sh
)
# We're still in the original directory

Variables, cd, set options inside the parens don't affect the parent shell.

Capturing in $()

$(cmd) is also a subshell. That's why x=10; $(x=20) doesn't change x — the assignment lived in a fork.

Watch the syntax for {}

Need spaces inside the braces.
Need a trailing semicolon (or newline) before the closing brace.

{cmd} doesn't work; { cmd; } does. The shell's parser is picky here.

Code

Group + redirect·bash

{
  echo '== status =='
  date
  uname -a
  df -h /
} > /tmp/snapshot.txt 2>&1
cat /tmp/snapshot.txt

Subshell auto-reset·bash

pwd
(
  cd /tmp
  pwd          # /tmp
)
pwd            # back to original

External links

Bash command grouping

Exercise

Run (cd /tmp; pwd); pwd and confirm you stayed in your original dir. Then { echo a; echo b; } > /tmp/two.txt; cat /tmp/two.txt. Notice the difference.

Progress

Progress is local-only — sign in to sync across devices.

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