bash readline vi editing mode default keyboard shortcut cheat sheet Bash provides two modes for command line editing - emacs and vi. Emacs editing mode is the default and I already wrote an article and created a cheat sheet for this mode.

This time I am going to introduce you to bash's vi editing mode and give out a detailed cheat sheet with the default keyboard mappings for this mode.

The difference between the two modes is what command each key combination (or key) gets bound to. You may inspect your current keyboard mappings with bash's built in bind command:

$ bind -P

abort can be found on "\C-g", "\C-x\C-g", "\M-\C-g".
accept-line can be found on "\C-j", "\C-m".
alias-expand-line is not bound to any keys

To get into the vi editing mode type

$ set -o vi

in your bash shell (to switch back to emacs editing mode, type set -o emacs).

If you are used to a vi text editor you will feel yourself at home.

The editing happens in two modes - command mode and insert mode. In insert mode everything you type gets output to the terminal, but in the command mode the keys are used for various commands.

Here are a few examples with screenshots to illustrate the vi editing mode.

Let '[i]' be the position of cursor in insert mode in all the examples and '[c]' be the position of cursor in command mode.


Once you have changed the readline editing mode to vi (by typing set -o vi), you will be working in insert mode.

The example will be performed on this command:

$ echo arg1 arg2 arg3 arg4[i]

Example 1:

Suppose you have typed a command with a few arguments and want to insert another argument before an argument which is three words backward.

$ echo arg1 (want to insert arg5 here) arg2 arg3 arg4[i]

Hit 'ESC' to switch to command mode and press '3' followed by 'B':

$ echo arg1 [c]arg2 arg3 arg4

Alternatively you could have hit 'B' three times: 'BBB'.

Now, enter insert mode by hitting 'i' and type 'arg5 '

$ echo arg1 arg5 [i]arg2 arg3 arg4

Example 2:

Suppose you wanted to change arg2 to arg5:

$ echo arg1 [c]arg2 arg3 arg4

To do this, you can type 'cw' which means 'change word' and just type out 'arg5':

$ echo arg1 arg5[c] arg3 arg4

Or even quicker, you can type 'f2r5', where 'f2' moves the cursor right to next occurrence of character '2' and 'r5' replaces the character under the cursor with character '5'.

Example 3:

Suppose you typed a longer command and you noticed that you had made several mistakes, and wanted to do the correction in the vi editor itself. You can type 'v' to edit the command in the editor and not on the command line!

Example 4:

Suppose you typed a long command and remembered that you had to execute another one before it. No need to erase the current command! You can switch to command mode by hitting ESC and then type '#' which will send the current command as a comment in the command history. After you type the command you had forgotten, you may go two commands back in history by typing 'kk' (or '2k'), erase the '#' character which was appended as a comment and execute the command, this makes the whole command look like 'ESC 2k0x ENTER'.

These are really basic examples, and it doesn't get much more complex than this. You should check out the cheat sheet for other tips and examples, and try them out!

To create the cheat sheet, I downloaded bash-2.05b source code and scanned through lib/readline/vi_keymap.c source code file and lib/readline/vi_mode.c to find all the default key bindings.

It turned out that the commands documented in vi_keymap.c were all documented in man 3 readline and I didn't find anything new.

After that I checked bashline.c source file function initialize_readline to find how the default keyboard shortcuts were changed. I found that 'CTRL-e' (which switched from vi mode to emacs) got undefined, 'v' got defined which opens the existing command in the editor, and '@' which replaces a macro key (char) with the corresponding string.

The cheat sheet includes:

  • Commands for entering input mode,
  • Basic movement commands,
  • Character finding commands,
  • Character finding commands,
  • Deletion commands,
  • Undo, redo and copy/paste commands,
  • Commands for history manipulation,
  • Completion commands,
  • A few misc. commands, and
  • Tips and examples

Download Vi Editing Mode Cheat Sheet

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Download link: bash vi editing mode cheat sheet (.pdf)
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ASCII .txt format:
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LaTeX format (.tex):
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This cheat sheet is released under GNU Free Document License.

Do you want to have a broader discussion on this topic?
Discuss it on catonmat forums!

charles darvin - natural selectionI have started working on my bachelor's thesis in physics. It's about using genetic algorithms for finding optimal solutions to physics problems. One of the problems I will solving is simulating equilibrium configurations of two dimensional systems of particles which can attract and repel (dipole systems). This problem is NP hard, which means there is no effective algorithm to find the exact solution in an adequate time. Several other algorithms can be used to approximate the solution and find near-equilibrium configurations, one of them being genetic algorithm technique.

The easiest situation is when the particles are bounded in a circle which border they can't trespass. The goal is to find how these particles will position themselves inside this circle.

This case has already been tackled using the simulated annealing method and the near-optimum solutions for hundreds of particles have been found. This method uses different principles than genetic algorithm method which I will describe shortly. The main idea of simulated annealing method is that the system is given an initial temperature which basically controls how much the particles will fluctuate inside the system. The greater the temperature, the bigger the fluctuations. The temperature gradually gets decreased and the fluctuations get smaller and smaller. Calculating the energy of the system and requiring it to be minimal at each step the temperature gets decreased eventually leads to a solution.

Here is how the solutions for 10, 11, 12, 13, 14 and 15 particles looks like, found using simulated annealing algorithms:

dipole system

Notice how interestingly and non-intuitively the particles position themselves in a case when another particle gets added to a system of 12 particles. Instead of positioning the new particle somewhere in the middle as in transition from 11 particles to 12, the system decides to put it on the border.

In my thesis I will be using genetic algorithms to arrive at the solution for this problem.

Here is a general (not related to my topic of thesis) description of what genetic algorithms are.

Genetic Algorithms 101

Genetic algorithms mimic the evolution by natural selection. The basic idea of genetic algorithms is very simple. Genetic algorithms feature populations of individuals which evolve with the use of the principles of selection, variation and inheritance.

One of the ways to implement this idea in computer programs is to represent individuals as strings of binary digits. Each bit in the string represents one gene. Each individual is assigned a numerical evaluation of its merit by a fitness function. The fitness function determines how each gene of an individual will be interpreted and, thus, what specific problem the population will evolve to solve.

Once all individuals in the population have been evaluated, their fitness values are used for selection. Individuals with low fitness get eliminated and the strongest get selected. Inheritance is implemented by making multiple copies of high-fitness individuals. The high-fitness individuals get mutated and they crossover to produce a new population of individuals. Mutation is implemented as flipping individual bits in the binary string representation of an individual and crossover happens as an exchange of binary substrings of two individuals to obtain a new offspring.

By transforming the previous set of individuals to a new one, the algorithm generates a new set of individuals that have better fitness than the previous set of individuals. When the transformations are applied over and over again, the individuals in the population tend to represent improved solutions to whatever problem was posed in the fitness function.

Here is an illustration how a genetic algorithm might work:

operation of the genetic algorithm programming

I look forward to finding what other interesting projects that I can make using the very useful information I have learnt.

PS. I am writing blog posts less often now because I am really, really busy with studies. Sorry about that.

I was doing a WordPress installation the other day when I noticed how insecure the default generated password was.

On line 38 in wp-admin/includes/upgrade.php (wordpress version 2.3.1) I found that a 6 character password is generated this way:

$random_password = substr(md5(uniqid(microtime())), 0, 6);

The md5 function returns a 32 character hexadecimal number and substr chops off first six characters. Doing elementary combinatorics we can find that the number of possible passwords is 166 (16 to the power 6) or 16,777,216, or roughly just 16.7 million passwords!

I am more than sure that most people doing WP installations never change the default password. If you're on a good connection and can do just 100 password checks per second, then you can crack a WordPress installation in worst case time of 16,777,216/100 seconds, which is 46.6 hours! Most likely you'd crack the password in half of that time, so you can crack any WordPress installation that has a default password in about 24 hours!

bash readline emacs editing mode default keyboard shortcut cheat sheetWhen you are working in a shell you certainly don't want to waste your time using arrow keys or home/end keys to navigate around the command line. One of the most popular shells, bash - Bourne Again SHell, uses GNU's Readline library for reading the command line.

The GNU Readline library provides a set of functions for use by applications that allow users to edit command lines as they are typed in. The readline library also includes functions to maintain a list of previously-entered command lines, to recall and perhaps reedit those lines, and perform csh-like history expansion on previous commands. Both emacs and vi editing modes are available.

I have mastered both of the editing modes and have created cheat sheets for both of them (and a tiny separate one for readline's history expansion).

This is a cheat sheet for the default, emacs, editing mode.

Here are a few examples with screenshots on how to use this editing mode.

Let '[]' be the position of cursor in all the examples.

Example 1: movement basics

Suppose you are at the end of the line and want to move 3 words backwards.

$ echo word1 word2 word3 word4 word5 word6[]

If you hit M-3 followed by M-b, you would end up exactly where you wanted:

$ echo word1 word2 word3 []word4 word5 word6

An alternative is to hit M-b three times in a row: M-b M-b M-b

If you look up on the cheat sheet what M-3 does, it sets the numeric-argument to 3 which in this case acts as a counter how many times should M-b command be repeated. The M-b command calls backward-word function which does the obvious.

The numeric-argument can also be negative, which makes the argument to be applied in the opposite direction.

Other shortcuts of interest are M-f to move forward and C-a, and C-e to move to the beginning and end of line.

Example 2: command history

Suppose you used a pretty complex command a while ago and now you remember just a few arguments of this command. You want to find this command and call it with a few arguments modified.

If you hit C-r readline will put you in an incremental reverse history search mode. Typing a part of the arguments you remember, will locate the previously executed command matching the typed text. Hitting C-r again will locate any other command which matches your typed text.

To put the found command on command line for editing hit C-j.

Example 3: completing

Suppose you want to quickly list all the users on the system.

Hit C-x ~ and read-line will attempt username completion and output all the usernames to the terminal.

$ []
adm        catonmat   ftp        halt       mailnull   nobody     root       smmsp      vcsa
apache     cpanel     games      lp         mysql      nscd       rpc        sshd
bin        daemon     gopher     mail       named      operator   rpm        sync
cat        dbus       haldaemon  mailman    news       picurls    shutdown   uucp
$ []

Suppose you now want to quickly list all the users on the system starting with 'm'. You can type 'm' followed by the same C-x ~ to do that.

$ m[]
mail      mailman   mailnull  mysql
$ m[]

The other interesting completions are:

  • C-x / which lists possible filename completion,
  • C-x $ which lists possible bash variable completion,
  • C-x @ which lists possible hostname completion and,
  • C-x ! which lists possible command completion.


  • Meta-/ which does filename completion,
  • Meta-$ which does bash variable completion,
  • Meta-@ which does hostname completion and,
  • Meta-! which does command completion.

Example 3: killing and yanking

Suppose you have to type a-long-word-like-this a couple of times.

The easiest way to do this is to kill the word, which puts it into the kill ring. Contents of the kill ring can be accessed by yanking.

For example, type 'a-long-word-like-this' in the shell:

$ command a-long-word-like-this []

Now press C-w to kill one word backward:

$ command []

Press C-y to yank (paste) the word as many times as you wish (I pressed it 3 times here:)

$ command a-long-word-like-this a-long-word-like-this a-long-word-like-this []

The kill ring does not contain just the one latest killing. It can be filled with a number of kills and rotated with M-y shortcut.

Another example:

Suppose you typed a longer command and you noticed that part of the THE TEXT GOT TYPED IN CAPITAL LETTERS. Without knowing the readline shortcuts you would erase the text and probably type it again. Now you can use the readline keyboard shortcuts and change the case very, very quickly.

You can use the following shortcuts to accomplish this:

1) M-l (Meta-l (on your computer, probably ESC-l)) shortcut is bound to readline's downcase-word function which lowercases the current word.
2) M-b shortcut is bound to readline's backward-word function which moves the cursor one word backwards.
3) M-<number> shortcut is bound to readline's numeric-argument function which in some cases acts as how many times should the following command be repeated.

Here is a real word example, suppose we have typed the following ([] is the cursor):


To get to the beginning of 'THE' we might repetitively hit M-b seven times or we could set the numeric argument to seven by typing M-7 and then hit M-b once.

After doing this the cursor would have moved before the word 'THE':


Now, by setting the numerical argument to 7 again and by pressing M-l or by pressing M-l seven times, we turn the text all in lower case.

$ echo the text. the text got typed in capital letters[]

Actually what we did in this example was not as efficient as it could have been. The numeric-argument shortcut accepts negative arguments which turn the direction of the following command in other direction. We could have turned the text in lower case by hitting M--7 and M-l

If you really want to be more productive, I suggest you play around with the commands in the cheat sheet for a while.
My previous article on being more productive on the command line was screen's cheat sheet which allows to emulate multiple terminals in a single window. You can take a look at it as well!

Download Emacs Editing Mode Cheat Sheet

PDF format (.pdf):
Download link: readline emacs cheat sheet (.pdf)
Downloaded: 49424 times

ASCII .txt format:
Download link: readline emacs cheat sheet (.txt)
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LaTeX format (.tex):
Download link: readline emacs cheat sheet (.tex)
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This cheat sheet is released under GNU Free Document License.

The next cheat sheet will be readline's vi editing mode's default keyboard shortcut cheat sheet! :)

extract mp3 audio track from youtube videoA few days ago my blog reader, Ankush Agarwal, on the comments of downloading youtube videos with gawk article asked:

I've seen tools available to download just the audio from a youtube video, in various formats; but as per your explanation it seems, that the audio is integrated with the video in the .swf file. How can we extract only the audio part and have it converted to a format like mp3?

As I have written a few articles before on how to download YouTube videos with Perl, gawk and VBScript, and how to convert the downloaded flash video files (flv) to divx or xvid, or any other format with ffmpeg, it was very easy to help this guy.

This is a guide that explains how to extract audio tracks from any videos, not just YouTube.

First, lets download the ffmpeg tool (that's for Windows Operating System. If you are using linux operating system, you can get the ffmpeg tool as a package distribution) and open the ffmpeg documentation in another window.

Lets choose a sample video which we will extract the audio track from. I found some music video clip "My Chemical Romance - Famous Last Words" (

Now, lets download the music video. If you are on a windows machine, you may use my VBScript program to download the video (download vbscript youtube video downloader, read how to use it here), or if you are on linux, you may use gawk program to download the video (download gawk youtube video downloader, read how to use it here).

After downloading the video, I ended up with a file named My_Chemical_Romance_-_Famous_Last_Words.flv.

Once you have downloaded the video, just for the sake of interest, lets find out the audio quality of this You Tube audio video.
The ffmpeg documentation does not tell us about a switch which would just output the audio parameters of the input file. After experimenting a little with the ffmpeg tool, it can be found that by just specifying '-i' switch and the input video file, the ffmpeg will output input streams information and quit.

Here is an example of how it looks:

c:\> ffmpeg.exe -i My_Chemical_Romance_-_Famous_Last_Words.flv

Seems that stream 1 comes from film source: 1000.00 (1000/1) -> 24.00 (24/1)
Input #0, flv, from 'My_Chemical_Romance_-_Famous_Last_Words.flv':
  Duration: 00:04:27.4, start: 0.000000, bitrate: 64 kb/s
  Stream #0.0: Audio: mp3, 22050 Hz, mono, 64 kb/s
  Stream #0.1: Video: flv, yuv420p, 320x240, 24.00 fps(r)
Must supply at least one output file

From this information (2nd line in bold) we can read that the audio bitrate of a YouTube video is 64kbit/s, sampling rate is 22050Hz, the encoding is mp3, and it's a mono audio.

You will be surprised how easy it is to extract the audio part as it is in the video. By just typing:

c:\> ffmpeg.exe -i My_Chemical_Romance_-_Famous_Last_Words.flv famous_last_word.mp3

the ffmpeg tool will extract it to an mp3 audio file!

That's it! After running this command you should have 'famous_last_words.mp3' file in the same folder/directory where the downloaded video file was!

We can go a little further and look up various audio switches on the documentation of ffmpeg. For example, if we had some fancy alarm clock which can be stuffed an mp3, you might not need the whole 64kbit/s of bitrate. You might want to convert the audio to a lower bitrate, say 32kbit/s.

The Section 3.5 - Audio Options of the ffmpeg documentation says:

`-ab bitrate' - Set the audio bitrate in bit/s (default = 64k).

So, by specifying a command line switch '-ab 32k' the audio will be converted to a lower bitrate of 32kbit/s.

Here is the example of running this command:

c:\> ffmpeg.exe -i My_Chemical_Romance_-_Famous_Last_Words.flv -ab 32k famous_last_word.32kbit.mp3
Seems that stream 1 comes from film source: 1000.00 (1000/1) -> 24.00 (24/1)
Input #0, flv, from 'My_Chemical_Romance_-_Famous_Last_Words.flv':
  Duration: 00:04:27.4, start: 0.000000, bitrate: 64 kb/s
  Stream #0.0: Audio: mp3, 22050 Hz, mono, 64 kb/s
  Stream #0.1: Video: flv, yuv420p, 320x240, 24.00 fps(r)
Output #0, mp3, to 'famous_last_word.32kbit.mp3':
  Stream #0.0: Audio: mp3, 22050 Hz, mono, 32 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
size=    1045kB time=267.6 bitrate=  32.0kbits/s
video:0kB audio:1045kB global headers:0kB muxing overhead 0.000000%

The line in bold indicates that the output audio indeed was at a bitrate of 32kbit/s.

Some other things you can do are - changing the codec of the audio (-acodec option (find all codecs with -formats option)) or cut out a part of the audio (-t and -ss options) you are interested in.

This technique actually involved re-encoding the audio which was already in the movie file. If you read closely the audio option documentation, you will find that the -acodec option says:

`-acodec codec' - Force audio codec to codec. Use the copy special value to specify that the raw codec data must be copied as is.

If the input video file was from YouTube or it already had mp3 audio stream, then using the following command line, the audio will be extracted much, much faster:

c:\> ffmpeg.exe -i My_Chemical_Romance_-_Famous_Last_Words.flv -acodec copy famous_last_words.mp3

Have fun ripping your favorite music off YouTube! :)

ps. Do you have something cool and useful you would like to accompish but do not have the necessary computer skills? Let me know in the comments and I will see if I can write an article about it!