Pipes and Filters

Overview

Teaching: 20 min
Exercises: 10 min

Questions

• How can I combine existing commands to do new things?

Objectives

• Redirect a command’s output to a file.

• Process a file instead of keyboard input using redirection.

• Construct command pipelines with two or more stages.

• Explain what usually happens if a program or pipeline isn’t given any input to process.

• Explain Unix’s ‘small pieces, loosely joined’ philosophy.

Now that we know a few basic commands, we can finally look at the shell’s most powerful feature: the ease with which it lets us combine existing programs in new ways. We’ll start with the directory called shell-lesson-data/molecules that contains six files describing some simple organic molecules. The .pdb extension indicates that these files are in Protein Data Bank format, a simple text format that specifies the type and position of each atom in the molecule.

$ ls molecules

cubane.pdb    ethane.pdb    methane.pdb
octane.pdb    pentane.pdb   propane.pdb

Let’s go into that directory with cd and run an example command wc cubane.pdb:

$ cd molecules
$ wc cubane.pdb

20  156 1158 cubane.pdb

wc is the ‘word count’ command: it counts the number of lines, words, and characters in files (from left to right, in that order).

If we run the command wc *.pdb, the * in *.pdb matches zero or more characters, so the shell turns *.pdb into a list of all .pdb files in the current directory:

$ wc *.pdb

  20  156  1158  cubane.pdb
  12  84   622   ethane.pdb
   9  57   422   methane.pdb
  30  246  1828  octane.pdb
  21  165  1226  pentane.pdb
  15  111  825   propane.pdb
 107  819  6081  total

Note that wc *.pdb also shows the total number of all lines in the last line of the output.

If we run wc -l instead of just wc, the output shows only the number of lines per file:

$ wc -l *.pdb

  20  cubane.pdb
  12  ethane.pdb
   9  methane.pdb
  30  octane.pdb
  21  pentane.pdb
  15  propane.pdb
 107  total

The -m and -w options can also be used with the wc command, to show only the number of characters or the number of words in the files.

Why Isn’t It Doing Anything?

What happens if a command is supposed to process a file, but we don’t give it a filename? For example, what if we type:

$ wc -l

but don’t type *.pdb (or anything else) after the command? Since it doesn’t have any filenames, wc assumes it is supposed to process input given at the command prompt, so it just sits there and waits for us to give it some data interactively. From the outside, though, all we see is it sitting there: the command doesn’t appear to do anything.

If you make this kind of mistake, you can escape out of this state by holding down the control key (Ctrl) and typing the letter C once and letting go of the Ctrl key. Ctrl+C

Capturing output from commands

Which of these files contains the fewest lines? It’s an easy question to answer when there are only six files, but what if there were 6000? Our first step toward a solution is to run the command:

$ wc -l *.pdb > lengths.txt

The greater than symbol, >, tells the shell to redirect the command’s output to a file instead of printing it to the screen. (This is why there is no screen output: everything that wc would have printed has gone into the file lengths.txt instead.) The shell will create the file if it doesn’t exist. If the file exists, it will be silently overwritten, which may lead to data loss and thus requires some caution. ls lengths.txt confirms that the file exists:

$ ls lengths.txt

lengths.txt

We can now send the content of lengths.txt to the screen using cat lengths.txt. The cat command gets its name from ‘concatenate’ i.e. join together, and it prints the contents of files one after another. There’s only one file in this case, so cat just shows us what it contains:

$ cat lengths.txt

  20  cubane.pdb
  12  ethane.pdb
   9  methane.pdb
  30  octane.pdb
  21  pentane.pdb
  15  propane.pdb
 107  total

Output Page by Page

We’ll continue to use cat in this lesson, for convenience and consistency, but it has the disadvantage that it always dumps the whole file onto your screen. More useful in practice is the command less, which you use with less lengths.txt. This displays a screenful of the file, and then stops. You can go forward one screenful by pressing the spacebar, or back one by pressing b. Press q to quit.

Filtering output

Next we’ll use the sort command to sort the contents of the lengths.txt file. But first we’ll use an exercise to learn a little about the sort command:

What Does sort -n Do?

The file shell-lesson-data/numbers.txt contains the following lines:

10
2
19
22
6

If we run sort on this file, the output is:

10
19
2
22
6

If we run sort -n on the same file, we get this instead:

2
6
10
19
22

Explain why -n has this effect.

Solution

The -n option specifies a numerical rather than an alphanumerical sort.

We will also use the -n option to specify that the sort is numerical instead of alphanumerical. This does not change the file; instead, it sends the sorted result to the screen:

$ sort -n lengths.txt

  9  methane.pdb
 12  ethane.pdb
 15  propane.pdb
 20  cubane.pdb
 21  pentane.pdb
 30  octane.pdb
107  total

We can put the sorted list of lines in another temporary file called sorted-lengths.txt by putting > sorted-lengths.txt after the command, just as we used > lengths.txt to put the output of wc into lengths.txt. Once we’ve done that, we can run another command called head to get the first few lines in sorted-lengths.txt:

$ sort -n lengths.txt > sorted-lengths.txt
$ head -n 1 sorted-lengths.txt

  9  methane.pdb

Using -n 1 with head tells it that we only want the first line of the file; -n 20 would get the first 20, and so on. Since sorted-lengths.txt contains the lengths of our files ordered from least to greatest, the output of head must be the file with the fewest lines.

Redirecting to the same file

It’s a very bad idea to try redirecting the output of a command that operates on a file to the same file. For example:

$ sort -n lengths.txt > lengths.txt

Doing something like this may give you incorrect results and/or delete the contents of lengths.txt.

What Does >> Mean?

We have seen the use of >, but there is a similar operator >> which works slightly differently. We’ll learn about the differences between these two operators by printing some strings. We can use the echo command to print strings e.g.

$ echo The echo command prints text

The echo command prints text

Now test the commands below to reveal the difference between the two operators:

$ echo hello > testfile01.txt

and:

$ echo hello >> testfile02.txt

Hint: Try executing each command twice in a row and then examining the output files.

Solution

In the first example with >, the string ‘hello’ is written to testfile01.txt, but the file gets overwritten each time we run the command.

We see from the second example that the >> operator also writes ‘hello’ to a file (in this casetestfile02.txt), but appends the string to the file if it already exists (i.e. when we run it for the second time).

Appending Data

We have already met the head command, which prints lines from the start of a file. tail is similar, but prints lines from the end of a file instead.

Consider the file shell-lesson-data/data/animals.txt. After these commands, select the answer that corresponds to the file animals-subset.txt:

$ head -n 3 animals.txt > animals-subset.txt
$ tail -n 2 animals.txt >> animals-subset.txt

1. The first three lines of animals.txt
2. The last two lines of animals.txt
3. The first three lines and the last two lines of animals.txt
4. The second and third lines of animals.txt

Solution

Option 3 is correct. For option 1 to be correct we would only run the head command. For option 2 to be correct we would only run the tail command. For option 4 to be correct we would have to pipe the output of head into tail -n 2 by doing head -n 3 animals.txt | tail -n 2 > animals-subset.txt

Passing output to another command

In our example of finding the file with the fewest lines, we are using two intermediate files lengths.txt and sorted-lengths.txt to store output. This is a confusing way to work because even once you understand what wc, sort, and head do, those intermediate files make it hard to follow what’s going on. We can make it easier to understand by running sort and head together:

$ sort -n lengths.txt | head -n 1

  9  methane.pdb

The vertical bar, |, between the two commands is called a pipe. It tells the shell that we want to use the output of the command on the left as the input to the command on the right.

This has removed the need for the sorted-lengths.txt file.

Combining multiple commands

Nothing prevents us from chaining pipes consecutively. We can for example send the output of wc directly to sort, and then the resulting output to head. This removes the need for any intermediate files.

We’ll start by using a pipe to send the output of wc to sort:

$ wc -l *.pdb | sort -n

   9 methane.pdb
  12 ethane.pdb
  15 propane.pdb
  20 cubane.pdb
  21 pentane.pdb
  30 octane.pdb
 107 total

We can then send that output through another pipe, to head, so that the full pipeline becomes:

$ wc -l *.pdb | sort -n | head -n 1

   9  methane.pdb

This is exactly like a mathematician nesting functions like log(3x) and saying ‘the log of three times x’. In our case, the calculation is ‘head of sort of line count of *.pdb’.

The redirection and pipes used in the last few commands are illustrated below:

Piping Commands Together

In our current directory, we want to find the 3 files which have the least number of lines. Which command listed below would work?

1. wc -l * > sort -n > head -n 3
2. wc -l * | sort -n | head -n 1-3
3. wc -l * | head -n 3 | sort -n
4. wc -l * | sort -n | head -n 3

Solution

Option 4 is the solution. The pipe character | is used to connect the output from one command to the input of another. > is used to redirect standard output to a file. Try it in the shell-lesson-data/molecules directory!

Tools designed to work together

This idea of linking programs together is why Unix has been so successful. Instead of creating enormous programs that try to do many different things, Unix programmers focus on creating lots of simple tools that each do one job well, and that work well with each other. This programming model is called ‘pipes and filters’. We’ve already seen pipes; a filter is a program like wc or sort that transforms a stream of input into a stream of output. Almost all of the standard Unix tools can work this way: unless told to do otherwise, they read from standard input, do something with what they’ve read, and write to standard output.

The key is that any program that reads lines of text from standard input and writes lines of text to standard output can be combined with every other program that behaves this way as well. You can and should write your programs this way so that you and other people can put those programs into pipes to multiply their power.

Pipe Reading Comprehension

A file called animals.txt (in the shell-lesson-data/data folder) contains the following data:

2012-11-05,deer
2012-11-05,rabbit
2012-11-05,raccoon
2012-11-06,rabbit
2012-11-06,deer
2012-11-06,fox
2012-11-07,rabbit
2012-11-07,bear

What text passes through each of the pipes and the final redirect in the pipeline below?

$ cat animals.txt | head -n 5 | tail -n 3 | sort -r > final.txt

Hint: build the pipeline up one command at a time to test your understanding

Solution

The head command extracts the first 5 lines from animals.txt. Then, the last 3 lines are extracted from the previous 5 by using the tail command. With the sort -r command those 3 lines are sorted in reverse order and finally, the output is redirected to a file final.txt. The content of this file can be checked by executing cat final.txt. The file should contain the following lines:

2012-11-06,rabbit
2012-11-06,deer
2012-11-05,raccoon

Pipe Construction

For the file animals.txt from the previous exercise, consider the following command:

$ cut -d , -f 2 animals.txt

The cut command is used to remove or ‘cut out’ certain sections of each line in the file, and cut expects the lines to be separated into columns by a Tab character. A character used in this way is a called a delimiter. In the example above we use the -d option to specify the comma as our delimiter character. We have also used the -f option to specify that we want to extract the second field (column). This gives the following output:

deer
rabbit
raccoon
rabbit
deer
fox
rabbit
bear

The uniq command filters out adjacent matching lines in a file. How could you extend this pipeline (using uniq and another command) to find out what animals the file contains (without any duplicates in their names)?

Solution

$ cut -d , -f 2 animals.txt | sort | uniq

Which Pipe?

The file animals.txt contains 8 lines of data formatted as follows:

2012-11-05,deer
2012-11-05,rabbit
2012-11-05,raccoon
2012-11-06,rabbit
...

The uniq command has a -c option which gives a count of the number of times a line occurs in its input. Assuming your current directory is shell-lesson-data/data/, what command would you use to produce a table that shows the total count of each type of animal in the file?

1. sort animals.txt | uniq -c
2. sort -t, -k2,2 animals.txt | uniq -c
3. cut -d, -f 2 animals.txt | uniq -c
4. cut -d, -f 2 animals.txt | sort | uniq -c
5. cut -d, -f 2 animals.txt | sort | uniq -c | wc -l

Solution

Option 4. is the correct answer. If you have difficulty understanding why, try running the commands, or sub-sections of the pipelines (make sure you are in the shell-lesson-data/data directory).

Removing Unneeded Files

Suppose you want to delete your processed data files, and only keep your raw files and processing script to save storage. The raw files end in .dat and the processed files end in .txt. Which of the following would remove all the processed data files, and only the processed data files?

1. rm ?.txt
2. rm *.txt
3. rm * .txt
4. rm *.*

Solution

1. This would remove .txt files with one-character names
2. This is correct answer
3. The shell would expand * to match everything in the current directory, so the command would try to remove all matched files and an additional file called .txt
4. The shell would expand *.* to match all files with any extension, so this command would delete all files

Key Points

• wc counts lines, words, and characters in its inputs.

• cat displays the contents of its inputs.

• sort sorts its inputs.

• head displays the first 10 lines of its input.

• tail displays the last 10 lines of its input.

• command > [file] redirects a command’s output to a file (overwriting any existing content).

• command >> [file] appends a command’s output to a file.

• [first] | [second] is a pipeline: the output of the first command is used as the input to the second.

• The best way to use the shell is to use pipes to combine simple single-purpose programs (filters).