1 SC2235

Vidar Holen edited this page 2018-05-22 23:10:37 -07:00

Table of Contents

• Use { ..; } instead of (..) to avoid subshell overhead.
• Problematic code:
• Correct code:
• Rationale:
• Exceptions:
• Related resources:

Use { ..; } instead of (..) to avoid subshell overhead.

Problematic code:

([ "$x" ] || [ "$y" ]) && [ "$z" ]

Correct code:

{ [ "$x" ] || [ "$y" ]; } && [ "$z" ]

Rationale:

You appear to be using (..) to group test commands. This creates a subshell, making it unnecessarily slow. Avoid this by using { ..; } to group.

Be careful to note that unlike (..), this requires both a space after the { and a semicolon before the }.

For example, (cmd), (cmd;) and ( cmd ) are all valid, but {cmd}, {cmd;} and { cmd } are all syntax errors because they lack either or both of the spaces and semicolon. The correct form is { cmd; }

Here's a small benchmark showing that the subshell version is more than 100x slower:

$ i=0; time for i in {1..10000}; do ([ "$x" ] || [ "$y" ]) && [ "$z" ]; done
real    0m7.122s
user    0m4.204s
sys     0m2.825s

$ i=0; time for i in {1..10000}; do { [ "$x" ] || [ "$y" ]; } && [ "$z" ]; done
real    0m0.055s
user    0m0.055s
sys     0m0.000s

Exceptions:

None.

Related resources:

• Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!

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