c. 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:

Exercise: 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

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.

note

Redirecting to the same file = Bad

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.

Exercise: What Does >> Mean? (Hint: It means “append”)

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

Exercise: 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