Would anyone mind doing me a favor by explaining xargs in more detail? I've tried learning it a couple times but I always seem to forget the primary situations in which it's useful. Thank you in advance!
Xargs takes a newline separated list and maps the list to a command.
find ./ -name '*.log' | xargs rm
Finds all log files and map them to 'rm' commands. e.g. if it finds system.log and rails.log it will run the command `rm system.log rails.log`.
xargs will automatically do things like break up very long lists into multiple command calls so that it doesn't exceed the maximum number of arguments a command can have.
Other useful things about xargs are '-P <NUM>' which lets you run the same command in parallel. I use this with curl to do ghetto benchmarks.
The next major flag is `-n <NUM>` which changes the number of arguments per command call. e.g. `-n 1` will run the command per argument passed to xargs.
And the last flag I commonly use is `-I {}`. This sets `{}` as a variable which contains the arguments. (This also forces `-n 1`). This is useful for things like:
find ./ -name '*.log' | xargs -I{} sh -c "if [ -f {}.gz ]; then rm {}; fi"
Only do that if you know exactly what '*.log' will expand to (i.e. don't use it in scripts and avoid using it on the command line). This is because the delimiter for xargs is a newline character, but filenames can have a newline character in them. This can lead to unexpected results.
Almost everywhere I see xargs used, find ...-exec {} ; will work as well and find ...-exec {} + may work even better.
Fixes that issue and xargs is far more efficient, it doesn't launch a new process for each line like exec does, but far more importantly, xargs is generalizable to all commands so you only have to learn it once; exec is just an ugly hack on find, you can't generalize it across all commands; xargs is much more unixy.
True, but the issue isn't removing files, the issue is generalizing a command for mapping output of a command to another command. rm was just a simple example. xargs is far more useful than simply deleting files.
Removing files is a common need coupled with find yet many readers don't know of -delete; I was pointing it out. That doesn't weaken xargs's valid uses. You seem a little defensive? :-)
Note that xargs actually takes a whitespace-delimited list. This often leads to problems when a filename has a space in it. To fix it, you should either use:
Maybe you'll call me a pedant, but you should be aware that `find .` is not equivalent to `ls .* *`. The find command starts at the indicated directories (. in this case) and lists each file and directory within it, recursing into subdirectories. You can use things like -type, -[i]name, and -mtime to filter the results, as well as -mindepth and -maxdepth to constrain the traversal.
Note also that "-print" is the default command for find, so you can leave it off. Other commands include -print0 (NUL-delimited instead of newline-delimited) and -exec.
