Pipes and FIFOs

Overview

The basic design of the Unix command line world is the same one which makes it so powerful and which many people swear by, namely the pipes and FIFOs design. The basic idea is that you have many simple commands which take an input and produce an output, and then to string them together into something that will give the desired effect.

Standard IO Streams or Pipes

To start out, let me give a rough explanation of the IO pipes. These are Standard Input, Standard Output and Standard Error. Standard Error and Standard Output are both output streams writing out whatever the application puts into to them. Standard Input is an input stream giving to the application whatever is written into it. Each program when run has each of these 3 streams available to it by default.

From this point forward I’ll refer to the 3 streams as STDOUT for Standard Output, STDERR for Standard Error and STDIN for Standard Input. These are the general short form or constant names for these streams.

Take for example the echo and cat commands. The echo command takes all text supplied as arguments on it’s command line and writes it out STDOUT. For example, the following command will print the text “Hi There” to the STDOUT stream, which by default is linked to the terminal’s output.

echo Hi There

Then, in it’s simplest form the cat command takes all data it reads from it’s STDIN stream and writes it back out to STDOUT exactly as it was received. You can also instruct cat to read in the contents of one or more files and write it back out to STDOUT. For example, to read in the contents of a file name namelist, and write it to STDOUT (the terminal) you can do:

cat namelist

To see cat in it’s purest form, simply run it without arguments, as:

cat

Each line of input typed in will be duplicated. This is because the input you type is sent to STDIN. This input is then received by cat which will write it back to STDOUT. The end of your input can be indicated by pressing Ctrl+D, which is the EOF or End of File key. Pressing Ctrl+D will close the STDIN stream, and will be handled by the program the same as if it was reading a file and came to the end of that file.

Pipes and Redirects

Now, all command line terminals allow you to do some powerful things with these IO pipes. Each type of shell has it’s own syntax, so I will be explaining these using the syntax for the Bash shell.

You could for instance redirect the output from a command into a file using the greater than or > operator. For example, to redirect the STDOUT of the echo command into a file called message, you would do:

echo Hi There > message

You could also read this file back into a command using the less than or < operator. This will take the contents of the file and write it to the command’s STDIN stream. For example, reading the above file into the cat program, would have it written back to STDOUT. So this has the same effect as supplying the filename as an argument to cat, but instead uses the IO pipes to supply the data.

cat < message

Where things really get powerful is when you start stringing together commands. You can take the STDOUT of one command and pipe it into the STDIN of another command, with as many commands as you want. For example, the following command pipes the message “Pipes are very useful” into the cut command, instructing it to give us the 4th word of the line. This will result in the text “useful” being printed to the terminal.

echo Pipes are very useful | cut -f 4 -d " "

As you can see, commands are stringed together with the pipe or | operator. The pipe operator by itself makes many powerful things possible.

Using the pipe (|) and redirect (>) operator, let’s give a more complex example. Let’s say we want to get the PID and user name of all running processes, sorted by the PID and separated by a comma. We can do something like this:

ps -ef | tail -n+2 | awk '{print $2 " " $1}' | sort -n | sed "s/ /,/"

To give an idea of what happens here, let me explain the purpose of each of these commands with the output each one produces (which becomes the input of the command that follows it).

Command	Description
ps -ef	Gives us a list of processes with many columns of data, of these the 1st column being the user and the 2nd column being the PID. Output: UID PID PPID C STIME TTY TIME CMD root 4222 443 0 20:14 ? 00:00:00 udevd quintin 3922 2488 0 20:14 pts/2 00:00:00 /bin/bash quintin 4107 2496 0 20:18 pts/0 00:00:00 vi TODO
tail -n+2	Takes the output of ps and gives us all the lines from line 2 onwards, effectively stripping the header. Output: root 4222 443 0 20:14 ? 00:00:00 udevd quintin 3922 2488 0 20:14 pts/2 00:00:00 /bin/bash quintin 4107 2496 0 20:18 pts/0 00:00:00 vi TODO
awk ‘{print $2 ” ” $1}’	Takes the output of tail, and prints the PID first, a space and then the user name. The rest of the data is discarded here. Output: 4222 root 3922 quintin 4107 quintin
sort -n	This sorts the lines received from awk numerically. Output: 3922 quintin 4107 quintin 4222 root
sed “s/ /,/”	Replaces the space separating the PID and user name with a comma. Output: 3922,quintin 4107,quintin 4222,root

Some Example Useful Commands

The above should give you a basic idea of what it’s all about. If you feel like experimenting, here are a bunch of useful commands to mess around with.

I’ll be describing the commands from the perspective of the standard IO streams. So even though I don’t mention it, some of these commands also support reading input from files specified as command line arguments.

To get more details about the usage of these commands, see the manual page for the given command by running:

man [command]

.

Command	Description
echo	Writes to STDOUT the text supplied in command line arguments.
cat	Writes to STDOUT the input from STDIN.
sort	Sorts all lines of input from STDIN.
uniq	Strips duplicate lines. The input needs to be sorted first, thus same basic effect can be achieved with just sort -u
cut	Cuts a string by a specified character and returns requested parts.
grep	Search for a specified pattern or string in the data supplied via STDIN.
gzip	Compresses the input from STDIN and writes the result to STDOUT. Uses gzip compression.
gunzip	Uncompresses the gzip input from STDIN and writes the results to STDOUT. Basically the reverse of gzip.
sed	Stream editor applying basic processing and filtering operations to STDIN and writes to STDOUT.
awk	Pattern scanning and processing langauge. Powerful script-like processing of lines/words from input.
column	Takes the input from STDIN and formats it into columns, writing the result to STDOUT. Useful for displaying data.
md5sum	Takes the input from STDIN and produces a md5sum of the data
sha1sum	Takes the input from STDIN and produces a sha1sum of the data
base64	Takes the input from STDIN and base64 encodes or decodes it
xargs	Takes input from STDIN and a uses it as arguments to a specified command
wc	Count the number of lines, words or characters read from input.
tee	Read input and write it to both STDOUT as well as a specified file.
tr	Translate or delete characters read from input

Conclusion

I would recommend anyone to get comfortable with these aspects of the Linux terminal as well as Bash scripting. Not knowing this, you might not even realize how many of your common tasks could be automated/simplified by it. Also remember that automation not only makes your tasks be completed quicker, but also reduces the chances for errors/mistakes that come from doing repetitive tasks by hand.

So Why Love Linux? Because the pipes and FIFOs pattern gives you a lot of power for building complex instructions.