Conditional OR List

A conditional OR list is a sequence of one or more pipelines separated by the || control operator. It implements short-circuit evaluation, executing a subsequent command in the sequence strictly if the preceding AND/OR list evaluates to a non-zero (failure) exit status.

command1 || command2 [|| command3 ...]

Execution Mechanics

The shell evaluates a conditional OR list from left to right.

1. command1 is executed, and the shell captures its exit status.
2. If command1 returns an exit status of 0 (success), the shell short-circuits the evaluation. command2 and any subsequent commands in the contiguous OR list are skipped and not executed.
3. If command1 returns a non-zero exit status (failure), the shell proceeds to execute command2.
4. This evaluation model propagates down the chain until a command returns 0 or the end of the list is reached.

Exit Status

The overall exit status of a conditional OR list is determined by the exit status of the last command executed within that list.

• If any command in the sequence evaluates to 0, the list terminates immediately, and the exit status of the entire list is 0.
• If all commands in the sequence fail, the list evaluates to the non-zero exit status of the final command in the chain.

Precedence and Associativity

The || control operator shares equal precedence with the conditional AND operator (&&). Both operators are strictly left-associative.

When || and && are mixed in a single un-grouped command string, the shell parses and evaluates them sequentially from left to right. Because of left-associativity, the execution of a subsequent command depends on the exit status of the entire preceding AND/OR list evaluated so far, not just the single pipeline immediately preceding the operator.

# command3 executes if the evaluated result of 'command1 || command2' is 0
command1 || command2 && command3

In the structure above, if command1 succeeds (returns 0), command2 is skipped. However, the exit status of the preceding list (command1 || command2) is 0. Consequently, the && operator sees a successful preceding list, causing command3 to execute.

To override default left-associativity and enforce specific evaluation precedence, commands must be explicitly grouped using compound commands, such as subshells ( ... ) or brace groups { ...; }.

# Grouping alters the evaluation hierarchy
command1 || { command2 && command3; }

Master Bash with Deep Grasping Methodology!

Learn More