Linux Basic — Subshells & Grouping

Subshells ( ... ) & Grouping { ... ; }

Both forms run a block of commands together. The difference is whether they execute in a child process (subshell) or the current shell (group).

Form	New process?	Side-effects leak out?
( cmds )	yes — subshell	no
{ cmds ; }	no — same shell	yes

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Subshells: ( ... )

A subshell inherits everything (vars, functions, cwd) but changes inside don’t propagate back.

echo "before: $PWD"
( cd /tmp && pwd )       # runs in /tmp
echo "after:  $PWD"      # unchanged

Use cases

• Isolate cd so the rest of the script keeps its working directory
• Localise set -e / set -u for a risky block
• Run a function with a temporary umask or IFS
• Parallelism: ( long_task ) &

# isolate a strict-mode block inside a lax script
( set -euo pipefail
  do_critical_thing
  do_another_thing
)

Cost

A subshell is a real fork(). Inside tight loops it’s measurable — favour grouping when you don’t need isolation.

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Grouping: { ... ; }

Braces run commands in the current shell, with shared variables and cwd. The syntax is fiddly:

• Space after { is required
• Each command needs ; or newline before }

{ echo "line 1"; echo "line 2"; echo "line 3"; } > out.log

Use cases

• Apply one redirection to many commands
• Group commands for an if / while chain
• Combine output of a sequence into a pipe

# all three lines go into the pipeline
{
    echo "header"
    cat data.txt
    echo "footer"
} | gzip > report.gz

&& / || with grouping

test -d "$dir" || { echo "missing $dir" >&2; exit 1; }

This is the idiomatic one-liner to abort if a precondition fails.

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Side-by-Side: Variables

x=1
( x=99 )       # subshell
echo "$x"      # 1

x=1
{ x=99; }      # group
echo "$x"      # 99

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Pipelines Always Run in Subshells

The right-hand side of a pipe is a subshell regardless of grouping:

count=0
seq 5 | while read n; do ((count++)); done
echo "$count"   # 0 — `while` ran in a subshell!

Fix with process substitution or lastpipe:

# Fix 1: process substitution
while read n; do ((count++)); done < <(seq 5)
echo "$count"   # 5

# Fix 2: bash-only lastpipe option
shopt -s lastpipe
set +m
seq 5 | while read n; do ((count++)); done
echo "$count"   # 5

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When to Use Which

Need	Use
Don’t pollute parent shell state	( )
Group commands for one redirection / pipeline	{ ; }
Run something in parallel	( ) &
Test-then-fail one-liner	cmd \|\| { msg; exit 1; }
Loop accumulating to a variable	avoid pipe — use < <(cmd)

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Next → eval, coprocess & exec