Command line tips

The intention of this post is to collect a list of my favorite command line tips. Feel free to leave yours in the comment section.

• Instead of using clear you can just press ctrl-r.
• The command units can be used for any unit conversion you can imagine. See man page.
• You can use multitail to monitor multiple log files on a terminal.

Sources:

Give me that one command you wish you knew years ago

Mark Stosberg's bash tips
TechBar: Linux shell tips and tricks

Extract decimals from text using grep

Let's say you have a log file that looks like this:

I/Profiling: TimeInterval1:  3271.72 ms
I/Profiling: TimeInterval1:  3379.15 ms
I/Profiling: TimeInterval1:  3645.32 ms
...

and you want to extract all decimal numbers so that you can get the average, or plot them. Here's how you would use grep to do this:

grep -o ' [0-9]\+\.[0-9]\+' mylogfile.log

The -o option is for just printing the matching text and not the whole line.

Elapsed Time for Shell Scripts

A lot of times you want to calculate the time it takes for a part of a shell script to run. Here is one of the ways you can do this:

T1=$(date +%s)
# Do something here
T2=$(date +%s)
diffsec="$(expr $T2 - $T1)"
echo | awk -v D=$diffsec '{printf "Elapsed time: %02d:%02d:%02d\n",D/(60*60),D%(60*60)/60,D%60}'

This will tell you the elapsed time between setting T1 and setting T2. The output is formatted as HH:MM:SS and printed to the console. You can easily redirect to a file for logging.

Using FFMPEG

FFMPEG is a complete, cross-platform solution to record, convert and stream audio and video. It includes libavcodec; the leading audio/video codec library. FFMPEG is a free software project and it is published under the GNU Lesser General Public License 2.1+ or GNU General Public License 2+ (depending on which options are enabled).

To install FFMPEG in Ubuntu just type:

sudo apt-get install ffmpeg

This post contains a collection of useful examples of using FFMPEG from the command line.

Video Information
Get information on the video (codec, frame rate, etc.).

ffmpeg -i myvideo.avi

Video Encoding
To encode the video using a different codec we specify it using the -vcodec option.

ffmpeg -i myvideo.avi -vcodec mpg4 video_out.avi

Now let's say we want a better quality compression and a different frame rate from the original. We specify 1Mb/s bitrate using the -b option (default is 200kb/s) and we change the frame rate to 12 fps using the -r option.

ffmpeg -i myvideo.avi -vcodec mpg4 -b 1M -r 12 video_out.avi

Other values for the -vcodec option include:

copy:	Copy raw codec data as is (do not reencode)
rawvideo:	Raw video (uncompressed)
xvid:	XviD (MPEG-4 Part2)
wmv2:	Windows Video Media

Extracting Video Frames
The following example extracts the frames from the video and saves them as frame_001.png, frame_002.png,...

ffmpeg -i myvideo.avi -f image2 frame_%03d.png

Cropping a video to a specific time interval
The following example extracts a scene from the input video that start at 1 minute 45 seconds and has a duration of 30 seconds.

ffmpeg -i myvideo.avi -ss 00:01:45 -t 00:00:45 myscene.avi

Slow down or speed up a video
Let's say we need to slow down a video now. Unfortunately, decreasing the frame rate with the -r option will not do, since ffmpeg will drop frames to keep the duration of the video the same. We need to use mjpegtools to do that. To install mjpegtools type:

sudo apt-get install mjpegtools

For this example the original video has a frame rate of 30fps and we want to make it play at 1/3 of the speed. So we have to trick ffmpeg to believe that the original video had a 10fps frame rate. The yuvfps tool does exactly that. Make sure that the input frame rate (-s option) and the output frame rate (-r option) are the same.

ffmpeg -i myVideo.avi -f yuv4mpegpipe - | yuvfps -s 10:1 -r 10:1 | ffmpeg -f yuv4mpegpipe -i - -vcodec mpeg4 -b 12M  mySlowVideo.avi

Note that this will not work unless your video has one of the yuv pixel formats. If your pixel format is not compatible (rgba for example) you would have to convert it like this:

ffmpeg -i myVideo.avi -pix_fmt yuv420p myVideo2.avi

Command Line Shortcuts

I was reading this post today on how to become a command line ninja and here is some interesting stuff I learned:

• Ctrl+U: This clears the entire line so you can type in a completely new command.
• Ctrl+K: This deletes the line from the position of the cursor to the end of the line.
• Ctrl+W: This deletes the word before the cursor only.
• Ctrl+R: This lets you search your command history for something specific. For example, if you wanted to search for the recent commands that included nano, you would hit Ctrl+R and type nano. It would show your most recent  command, and you could use the up and down arrows to cycle through your history of commands using nano in them.