grouping predicates in find

Question

This part " ( -name *txt -o -name *html ) " confuses me in the code:

find $HOME \( -name \*txt -o -name \*html \) -print0 | xargs -0 grep -li vpn

Can someone explain the the brackets and "-o"? Is "-o" a command or a parameter? I know the brackets are escaped by "\" , but why are they for?

Answer 1

By default, the conditions in the find argument list are 'and'ed together. The -o option means 'or'.

If you wrote:

find $HOME -name \*txt -o -name \*html -print0

then there is no output action associated with the file names end with 'txt', so they would not be printed. By grouping the name options with parentheses, you get both the 'html' and 'txt' files.

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Consider the example:

mkdir test-find
cd test-find
cp /dev/null file.txt
cp /dev/null file.html

The comments below have an interesting side-light on this. If the command was:

find . -name '*.txt' -o -name '*.html'

then, since no explicit action is specified for either alternative, the default -print (not -print0!) action is used for both alternatives and both files are listed. With a -print or other explicit action after one of the alternatives (but not the other), then only the alternative with the action takes effect.

find . -name '*.txt' -print -o -name '*.html'

This also suggests that you could have different actions for the different alternatives. You could also apply other conditions, such as a modification time:

find . \( -name '*.txt' -o -name '*.html' \) -mtime +5 -print0

find . \( -name '*.txt'  -mtime +5 -o -name '*.html' \) -print0

The first prints txt or html files older than 5 days (so it prints nothing for the example directory - the files are a few seconds old); the second prints txt files older than 5 days or html files of any age (so just file.html). And so on...

Thanks to DevSolar for his comments leading to this addition.

Answer 2

The "-o" means OR. I.e., name must end in "txt" or "html". The brackets just group the two conditions together.