pv: Progress Bar for md5sum et al.

Tools that know they will take a long time often come with a built-in progress indicator, but there are other utilities on Linux that often leave the user frustratedly tapping their fingers, wondering how much longer they will have to wait.

Luckily, there is a nifty little tool called pv that will donate a progress bar to any program that can read from standard input or a pipe. pv probably stands for pipe viewer.

1. Simple example: figure out how long an md5sum will take:

pv eternal.avi |md5sum

will display something like

96.5MB 0:00:05 [25.3MB/s] [=======>                                    ]  9% ETA 0:00:48

Notes:

• pv reads from file and prints to stdout.
• md5sum reads from stdin.
• pv outputs the progress bar to stderr so as not to interfere with the piped data. See the man page for ways to customize pv’s output.
• since the bottleneck of such an operation is the media you’re reading from, not the CPU, there will be no noticeable overhead.

2. Complex example: add a progress bar to tar/bzip2 compression/decompression:

tar cf - mydir | pv -n -s $(du -sb mydir | awk ‘{print $1}’) | bzip2 >mydir.tar.bz2

Notes:

• this example is adapted from the pv man page.
• the -n switch makes pv output only percentage values.
• no file is passed to pv, so it reads from stdin (piped to the output of tar).
• on a system with good cache and enough memory, doing the extra du -s mydir shouldn’t hurt much, since tar will go through the entire directory anyway.

Now let’s decompress it:

pv mydir.tar.bz2 |tar xjf -

By now you realize how awesome this is.

3. Fun example: measure /dev/null throughput:

pv /dev/zero >/dev/null

is close to 3.3GB/s on my 3-year-old system.

Notes:

• this is not a benchmark ™.
• pv can’t know the size of its input in this case (infinity), so it obviously can’t display an ETA.

pv is a brilliant example of the UNIX philosophy: simple puzzle pieces combining to create useful results. A couple of last-word remarks:

• There is apparently a very similar tool called cpipe.
• There are, unfortunately, programs for which you will not be able to use pv. One example is dpkg, which apparently tries seeking in its input, thus failing to work with pipes.
• Thanks to my boss for pointing me to this awesome tool.