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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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linux – Bash: Command grouping (&&, ||, …) – Stack Overflow

Posted by jpluimers on 2020/02/18

Excellent answer at [WayBack] linux – Bash: Command grouping (&&, ||, …) – Stack Overflow by Charles Duffy:

Operator precedence for && and || is strictly left-to-right.

Thus:

pwd; (ls) || { cd .. && ls student/; }  && cd student || cd / && cd ;

…is equivalent to…

pwd; { { { (ls) || { cd .. && ls student/; }; } && cd student; } || cd /; } && cd ; }

…breaking that down graphically:

pwd; {                                      # 1
       {                                    # 2
         { (ls) ||                          # 3
                   { cd .. &&               # 4
                              ls student/;  # 5
                   };                       # 6
         } && cd student;                   # 7
       } || cd /;                           # 8
     } && cd ;                              # 9
  1. pwd happens unconditionally
  2. (Grouping only)
  3. ls happens (in a subshell) unconditionally.
  4. cd .. happens if (3) failed.
  5. ls student/ happens if (3) failed and (4) succeeded
  6. (Grouping only)
  7. cd student happens if either (3) succeeded or both (4) and (5) succeeded.
  8. cd / happens if either [both (3) and one of (4) or (5) failed], or [(7) failed].
  9. cd happens if (7) occurred and succeeded, or (7) occurred and succeeded.

Using explicit grouping operators is wise to avoid confusing yourself. Avoiding writing code as hard to read as this is even wiser.

–jeroen

This entry was posted on 2020/02/18 at 18:00 and is filed under *nix, *nix-tools, bash, bash, Development, Power User, Scripting, Software Development. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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