Miscellaneous useful Bash/shell commands

I've been cataloging various shell commands and scripts that have been quite useful for various Linux (and even Windows via cygwin/git-shell) activities. The ones I list here are either not found via Google search or required a bit of hackery to suit my purposes.

Add a copyright (or author) comment to a bunch of Java files.

find src -type f -exec sh -c "sed -i '1s/^/\/\/ Copyright 2013 Erik Reed (erik@somewhere.com)\n\n/' {}" \;

Result:

// Copyright 2013 Erik Reed (erik@somewhere.com)

package java.com.tons.of.nested.packages.for.no.reason.coolapp;

import java.io.Serializable;
import java.text.ParseException;

// ...

Convert many image formats in parallel (jpegs to png here). Using 64 processes in this case:

find . -type f -name '*.jpg' -print0 | xargs -0 -n1 -P 64 sh -c 'convert "$1" "${1%.jpg}.png"' sh

Make file extensions lowercase (a common issue I've had with mixing Windows/Linux).

find . -name '*.*' -exec sh -c '
  a=$(echo {} | sed -r "s/([^.]*)\$/\L\1/");
  [ "$a" != "{}" ] && mv "{}" "$a" ' \;

Rsync files to/from remote machine as root (e.g. to backup multiple users, root owned files, or a full disk backup).

rsync -avze ssh --rsync-path='sudo rsync' myUser@REMOTE_SERVER:/REMOTE_PATH/ LOCAL_PATH

Clear disk cache and swap:

# clear cache
sync; echo 3 > /proc/sys/vm/drop_caches

# clear swap
swapoff -a; swapon -a

Print the mean and standard deviation of a file/stdin (in one line):

awk '{ sum += $1; sumsq += $1*$1 } END { printf "Mean: %f, Std: %f\n", sum/NR, sqrt(sumsq/NR - (sum/NR)^2) }'