Shell - Compound Commands

About

A compound command is one of the following:

Type

(list)

(list)

where

• () defines the execution environment as a subshell
  • Variable assignments and builtin commands that affect the shell’s environment do not remain in effect after the command completes.
  • The return status is the exit status of list.
• list is a list

{list}

{ list; }

where:

• {} defines a group command that is simply executed in the current shell environment.
• - Since '{}' do not cause a word break, '{}' must be separated from list by whitespace.
• list is a list
• ;: a semicolon is the terminator (or a newline)
• The return status is the exit status of list.

((expression))

((expression))

where:

• The expression is evaluated according to the rules described under ARITHMETIC EVALUATION.
• If the value of the expression is:
  • non-zero, the return status is 0;
  • otherwise the return status is 1.

This is exactly equivalent to/alias to let

let "expression"

[[ expression ]]

See Bash - Conditional Expression

Expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression )

Returns the value of expression. This may be used to override the normal precedence of operators.