Milk, Cookies & SegFaults…

Category: One-liners

  • Recursive chmod distinguishing files from folders

    Version 3

    An even better method is:

    find "$target" -type f -exec chmod -c "$mode_files" {} \; \
     -or -type d -exec chmod -c "$mode_dir" {} \;

    A true one-liner! ๐Ÿ˜€

    Version 2

    A better method is this:

    find "$target" -type f -exec chmod -c "$mode_files" {} \;
find "$target" -type d -exec chmod -c "$mode_dir" {} \;

    This one can also be used from the command line.

    Version 1

    Many times I needed to apply certain permissions recursively on a given path but with different permissions on files than on directories (i.e. I want 0644 for files and 0744 for directories). This behaviour is not provided by the chmod tool so here is a simple and effective bash function to do just that:

    # Recursively apply chmod to path.
# If mode_files is missing then apply mode_dir to files too.
# Params: target mode_dir [mode_files]
function deep_chmod() {
    function _walk() {
        local F
        for F in `find "$1"`; do
            local M="$3"; [[ `file -b "$F"` == "directory" ]] && M="$2"
            chmod -c "$M" "$F" > /dev/null
        done
    }
    if [[ $# > 2 ]]; then
        _walk "$1" "$2" "$3"
    else
        chmod -Rc "$2" "$1"
    fi
}

    I’m looking for a way to improve on this since it is quite costly for large directories: for each file or directory at least two programs are executed (file and chmod) which is not very efficient! For now, it gets the job done.

    Enjoy! ๐Ÿ™‚

  • Recursive file/directory change-detection

    Disclaimer

    This article explores a way in which an approximate “fingerprint” of a file tree can be created! If all you want is to detect file changes a much more appropriate method would be to use inotify/incron.

    Version 2 (update)

    Another, much faster method would be to use ls -lR to browse over the filesystem. On a newly installed Debian virtual machine (on Xen) hashing the entire filesystem (the root directory) took approximately 1.7 seconds. So, here it is:

    ls -lR "$D" | sha1sum | sed 's/[ -]//g'

    This method is sensitive to file name, size and modification size; usually that would be enough but if you need more control use…

    Version 1

    Detect when the contents of a file or directory ($D) changes:

    find "$D" | while read f; do stat -t "$f"; done | sha1sum | sed 's/[ -]//g'

    This yields a hash of the current state of the file or directory which is extremely sensitive to even the most subtle changes (even a simple touch to any file/directory somewhere inside "$D" changes the generated hash).

    (more…)

  • Detect number of CPUs on a machine

    UPDATE: Steven pointed out (very nicely) that there’s no need for cat in this picture, grep would do just fine on its own. So, thanks Steven!

    Detect how many CPU cores are present on the running machine:

    grep -c processor /proc/cpuinfo

    This can be very useful when writing multi-threaded programs to properly match the number of threads with the number of CPU cores.

    (more…)