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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 ...]

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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.

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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.

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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; }
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