Combining Tests

Scope: joining tests into one expression — side-by-side AND, -o for OR, -not and ! — and the precedence that decides what an expression actually says, including the trap where -print joins only half of it.

AND: just stand them together

Two tests side by side both have to pass. There is no keyword; adjacency is AND (formally -a, almost never written):

$ find tree -type f -name '*.log'
tree/archive/recent.log
tree/archive/old.log
tree/archive/ancient.log
tree/big.log

Files, and named *.log. Most real find commands are a chain of these: a type, a name, an age, all narrowing one walk.

OR: -o

-o joins alternatives — either side may pass:

$ find tree -name '*.jpg' -o -name '*.md'
tree/photos/img1.jpg
tree/report.md

One of the tree's quiet jokes is in that listing: img2.JPG is missing, because as chapter 2 established, -name '*.jpg' is case-sensitive and nobody said -iname. -o widens the net; it doesn't mend the individual tests.

NOT: -not and !

-not (or a bare !, escaped from the shell as \! where history expansion is watching) inverts the test after it:

$ find tree/photos -not -type d
tree/photos/raw/img3.CR2
tree/photos/img1.jpg
tree/photos/img2.JPG

Everything in photos that isn't a directory. Note the raw file sails through a -not whose job was only about types — NOT applies to the one test it precedes, nothing wider, unless you group, which is next.

Precedence, and the -print trap

AND binds tighter than OR: a -o b c reads as a, or (b and c). Usually that's what you meant. But actions are part of the expression too, evaluated in the same left-to- right flow — and here is the capture this chapter exists for. It looks like "print the text files and the markdown file":

$ find tree -name '*.txt' -o -name '*.md' -print
tree/report.md

Only the markdown. Here is why. The expression is

-name '*.txt'   -o   ( -name '*.md' -a -print )

-print stands on the right of the -o, so it is part of the right-hand alternative: entries matching *.md are printed, entries matching *.txt merely pass — and pass to nothing. Chapter 1 said find adds a -print of its own when an expression has no action; this expression has an action, so no automatic one is added, and the fifteen text files were selected and silently discarded. How many were lost, for the record:

$ find tree -type f -name '*.txt' | wc -l
15

The fix is to group with parentheses, so the action stands outside the choice. Parentheses are shell metacharacters, so they are escaped with backslashes (or the whole expression piece quoted):

$ find tree \( -name '*.txt' -o -name '*.md' \) -print
tree/subdir/nested.txt
tree/times/fresh.txt
tree/times/halfhour.txt
tree/times/marker.txt
tree/times/yesterday.txt
tree/times/twodays.txt
tree/perms/world-writable.txt
tree/perms/secret.txt
tree/perms/normal.txt
tree/perms/not-mine.txt
tree/notes.txt
tree/report.md
tree/.hidden.txt
tree/empty.txt
tree/meeting notes.txt
tree/o'brien.txt

Sixteen lines: fifteen text files and report.md, the answer the author of the unparenthesised version believed they were getting. Grouping also composes with NOT — ! \( a -o b \) is "neither" — and with longer chains, and the rule for reading any expression you didn't write this morning is simple: find where the actions stand, draw the parentheses the author didn't, and only then decide what the command does.

This trap is placed here, before the actions chapter, on purpose. Nothing in the broken command was exotic — two ordinary tests and the most ordinary action — and its failure was silent, complete, and plausible-looking. Remember its shape when chapter 8 gives actions teeth.