find -exec and xargs

Finding files is half the job; usually you then want to delete them, copy them, count their lines or change their permissions. find can run any command on each result through the -exec action, and the separate tool xargs turns any list of names, from find or from anywhere else, into arguments for a command. Between them they cover every "do X to all files that Y" task.

-exec CMD {} \;

After -exec you write a command as you would type it, with {} where the file name should go, and end it with \;. The semicolon tells find where the command stops; the backslash (or quotes, ';') stops the shell from treating it as its own command separator. find build -name '*.tmp' -exec rm {} \; runs rm once per file found. The BusyBox find here has no -delete or -ok, so -exec rm is the way to delete what you find.

~% cd /root/lab/l37
l37% find build -name '*.tmp' -exec echo rm {} \;
rm build/a.tmp
rm build/obj/b.tmp
l37% find build -name '*.tmp' -exec rm {} \;
l37% find srv -name '*.log' -exec cp {} collected/ \;
l37% ls collected
a.log  b.log  c.log
l37% find srv -name '*.log' -exec wc -l {} +
        3 srv/a.log
        1 srv/web/b.log
        2 srv/db/c.log
        6 total

Notice the first line: putting echo in front of the real command is a dry run that shows exactly what would execute. Ending with + instead of \; makes find pass many names to a single run of the command, like wc -l a b c, which is faster and gives commands such as wc the chance to print a total.

xargs

xargs reads words from stdin and appends them as arguments to a command: find build -name '*.o' | xargs rm becomes rm build/x.o build/y.o. It works with any producer, not just find: cat list.txt | xargs touch creates every file named in a list. -n 1 runs the command once per item, -I {} places each item wherever {} appears (xargs -I {} cp {} {}.bak), -r skips running when the input is empty and -t prints each command before running it.

The space problem

xargs splits its input on spaces and newlines, so a file called my notes.bak arrives as two arguments, my and notes.bak, and both fail. The fix is to separate names with the NUL character, which can never appear in a file name: find ... -print0 | xargs -0 rm. -exec never has this problem, because find hands each name over intact.