4 Working with text

Learning Objectives

Inspect the content of text files (head, tail, cat, zcat, less).
Use the * wildcard to work with multiple files at once.
Redirect the output of a command to a file (>, >>).
Find a pattern in a text file (grep).

4.1 Looking Inside Files

Often we want to investigate the content of a file, without having to open it in a text editor. This is especially useful if the file is very large (as is often the case in bioinformatic applications).

For example, let’s take a look at the cubane.pdb file in the molecules directory. We will start by printing the whole content of the file with the cat command, which stands for “concatenate” (we will see why it’s called this way in a little while):

cd molecules
cat cubane.pdb

COMPND      CUBANE
AUTHOR      DAVE WOODCOCK  95 12 06
ATOM      1  C           1       0.789  -0.852   0.504  1.00  0.00
ATOM      2  C           1      -0.161  -1.104  -0.624  1.00  0.00
ATOM      3  C           1      -1.262  -0.440   0.160  1.00  0.00
ATOM      4  C           1      -0.289  -0.202   1.284  1.00  0.00
ATOM      5  C           1       1.203   0.513  -0.094  1.00  0.00
ATOM      6  C           1       0.099   1.184   0.694  1.00  0.00
ATOM      7  C           1      -0.885   0.959  -0.460  1.00  0.00
ATOM      8  C           1       0.236   0.283  -1.269  1.00  0.00
ATOM      9  H           1       1.410  -1.631   0.942  1.00  0.00
ATOM     10  H           1      -0.262  -2.112  -1.024  1.00  0.00
ATOM     11  H           1      -2.224  -0.925   0.328  1.00  0.00
ATOM     12  H           1      -0.468  -0.501   2.315  1.00  0.00
ATOM     13  H           1       2.224   0.892  -0.134  1.00  0.00
ATOM     14  H           1       0.240   2.112   1.251  1.00  0.00
ATOM     15  H           1      -1.565   1.730  -0.831  1.00  0.00
ATOM     16  H           1       0.472   0.494  -2.315  1.00  0.00
TER      17              1
END

Sometimes it is useful to look only at only the top few lines of a file (especially for very large files). We can do this with the head command:

head cubane.pdb

COMPND      CUBANE
AUTHOR      DAVE WOODCOCK  95 12 06
ATOM      1  C           1       0.789  -0.852   0.504  1.00  0.00
ATOM      2  C           1      -0.161  -1.104  -0.624  1.00  0.00
ATOM      3  C           1      -1.262  -0.440   0.160  1.00  0.00
ATOM      4  C           1      -0.289  -0.202   1.284  1.00  0.00
ATOM      5  C           1       1.203   0.513  -0.094  1.00  0.00
ATOM      6  C           1       0.099   1.184   0.694  1.00  0.00
ATOM      7  C           1      -0.885   0.959  -0.460  1.00  0.00
ATOM      8  C           1       0.236   0.283  -1.269  1.00  0.00

By default, head prints the first 10 lines of the file. We can change this using the -n option, followed by a number, for example:

head -n 2 cubane.pdb

COMPND      CUBANE
AUTHOR      DAVE WOODCOCK  95 12 06

Similarly, we can look at the bottom few lines of a file with the tail command:

tail -n 2 cubane.pdb

TER      17              1
END

Finally, if we want to open the file and browse through it, we can use the less command:

less cubane.pdb

less will open the file and you can use ↑ and ↓ to move line-by-line or the Page Up and Page Down keys to move page-by-page. You can exit less by pressing Q (for “quit”). This will bring you back to the console.

Searching Text with less

When you open a file with the less program, you can also search for text within the file. To do this, press / and you will notice the bottom of the terminal changes to /. Now, type the word (or part of a word) that you want to search for and press Enter ↵.
Less will search of the word in the file and highlight it for you. If you want to move to the next match press n and to move to the previous match press Shift + n.

4.2 Count Words/Lines/Characters

Often it can be useful to count how many lines, words and characters a file has. We can use the wc command for this:

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

In this case, we used the * wildcard to count lines, words and characters (in that order, left-to-right) of all our PDB files. Often, we only want to count one of these things, and wc has options for all of them:

-l counts lines only.
-w counts words only.
-c counts characters only.

For example, the following counts only the number of lines in each file:

wc -l *.pdb

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

4.3 Combining several files

Earlier, we said that the cat command stands for “concatenate”. This is because this command can be used to concatenate (combine) several files together. For example, if we wanted to combine all PDB files into one:

cat *.pdb

4.4 Redirecting Output

The commands we’ve been using so far, print their output to the terminal. But what if we wanted to save it into a file? We can achieve this by redirecting the output of the command to a file using the > operator.

wc -l *.pdb > number_lines.txt

Now, the output is not printed to the console, but instead sent to a new file. We can check that the file was created with ls.

If we use > and the output file already exists, its content will be replaced. If what we want to do is append the result of the command to the existing file, we can use >> instead. Let’s see this in practice in the next exercise.

4.5 Finding Patterns

Something it can be very useful to find lines of a file that match a particular text pattern. We can use the tool grep (“global regular expression print”) to achieve this.
Going back to our molecules directory (cd ../molecules), let’s find the word “ATOM” in our cubane.pdb molecule file:

grep "ATOM" cubane.pdb

ATOM      1  C           1       0.789  -0.852   0.504  1.00  0.00
ATOM      2  C           1      -0.161  -1.104  -0.624  1.00  0.00
ATOM      3  C           1      -1.262  -0.440   0.160  1.00  0.00
ATOM      4  C           1      -0.289  -0.202   1.284  1.00  0.00
ATOM      5  C           1       1.203   0.513  -0.094  1.00  0.00
ATOM      6  C           1       0.099   1.184   0.694  1.00  0.00
ATOM      7  C           1      -0.885   0.959  -0.460  1.00  0.00
ATOM      8  C           1       0.236   0.283  -1.269  1.00  0.00
ATOM      9  H           1       1.410  -1.631   0.942  1.00  0.00
ATOM     10  H           1      -0.262  -2.112  -1.024  1.00  0.00
ATOM     11  H           1      -2.224  -0.925   0.328  1.00  0.00
ATOM     12  H           1      -0.468  -0.501   2.315  1.00  0.00
ATOM     13  H           1       2.224   0.892  -0.134  1.00  0.00
ATOM     14  H           1       0.240   2.112   1.251  1.00  0.00
ATOM     15  H           1      -1.565   1.730  -0.831  1.00  0.00
ATOM     16  H           1       0.472   0.494  -2.315  1.00  0.00

We can see the result is all the lines that matched this word pattern.

grep has many other options available, which can be useful depending on the result you want to get. Some of the more useful ones are illustrated below.

Illustration of the `grep` command by Julia Evans

4.6 Exercises

Exercise 1 - Redirection

Level:

Move to the directory sequencing and do the following:

List the files in the run1/ directory. Save the output in a file called sequencing_files.txt.
Inspect the content of the file, e.g. using cat.
What happens to the content of that file after you run the command ls run2 > sequencing_files.txt?
The operator >> can be used to append the output of a command to an existing file. Run the following code and inspect the file content again. What happens now?
```
ls run1/ > sequencing_files.txt
ls run2/ >> sequencing_files.txt
```

Answer

Task 1

To list the files in the directory we use ls, followed by > to save the output in a file:

ls run1 > sequencing_files.txt

Task 2

We can check the content of the file:

cat sequencing_files.txt

sampleA_1.fq.gz
sampleA_2.fq.gz
sampleB_1.fq.gz
sampleB_2.fq.gz
sampleC_1.fq.gz
sampleC_2.fq.gz
sampleD_1.fq.gz
sampleD_2.fq.gz

Task 3

If we run ls run2/ > sequencing_files.txt, we will replace the content of the file:

cat sequencing_files.txt

sampleE_1.fq.gz
sampleE_2.fq.gz
sampleF_1.fq.gz
sampleF_2.fq.gz

Task 4

If we start again from the beginning, but instead use the >> operator the second time we run the command, we will append the output to the file instead of replacing it:

ls run1/ > sequencing_files.txt
ls run2/ >> sequencing_files.txt
cat sequencing_files.txt

sampleA_1.fq.gz
sampleA_2.fq.gz
sampleB_1.fq.gz
sampleB_2.fq.gz
sampleC_1.fq.gz
sampleC_2.fq.gz
sampleD_1.fq.gz
sampleD_2.fq.gz
sampleE_1.fq.gz
sampleE_2.fq.gz
sampleF_1.fq.gz
sampleF_2.fq.gz

Exercise 2 - Counting lines

Level:

In the directory coronavirus/variants/, there are several CSV files with information about Coronavirus samples that were classified according to variants (e.g. “Alpha”, “Delta”, “Omicron”).

Inspect the top and bottom few lines of one of the files.
Each sample is represented by one line in the file. How many samples are there in each file?

Answer

Task 1

We can use head and tail to inspect the top and bottom few lines of one of the files, e.g.:

head -n 3 uk_variants.csv

Sample,clade
GB01,20I (Alpha; V1)
GB02,20I (Alpha; V1)

tail -n 3 uk_variants.csv

GB46,21J (Delta)
GB47,21J (Delta)
GB48,21J (Delta)

Task 2

We can use wc -l to count the number of lines in a single file:

wc -l uk_variants.csv

49 uk_variants.csv

To count the number of lines in all files at once, we can use the * wildcard:

wc -l *_variants.csv

  49 india_variants.csv
  49 ireland_variants.csv
  49 southafrica_variants.csv
  60 switzerland_variants.csv
  49 uk_variants.csv
 256 total

Exercise 3 - Combining files and pattern matching

Level:

In the directory coronavirus/variants/, there are several CSV files with information about SARS-CoV-2 virus samples that were classified according to clades (these are also commonly known as coronavirus variants).

Combine all files into a new file called all_countries.csv.

Hint
You can use cat to combine multiple text files. You can use > to redirect the output of a command to a new file.
Create another file called alpha.csv that contains only the Alpha variant samples.

Hint
You can use grep to find a pattern in a file. You can use > to redirect the output of a command to a new file.
How many Alpha samples are there in total?

Answer

Task 1

We can use cat to combine all the files into a single file:

cat *_variants.csv > all_countries.csv

Task 2

We can use grep to find a pattern in our text file and use > to save the output in a new file:

grep "Alpha" all_countries.csv > alpha.csv

We could investigate the output of our command using less alpha.csv.

Task 3

We can use wc to count the lines of the newly created file:

wc -l alpha.csv

Giving us 38 as the result.

4.7 Summary

Key points

The head and tail commands can be used to look at the top or bottom of a file, respectively.
The less command can be used to interactively investigate the content of a file. Use ↑ and ↓ to browse the file and Q to quit and return to the console.
The cat command can be used to combine multiple files together. The zcat command can be used instead if the files are compressed.
The > operator redirects the output of a command into a file. If the file already exists, it’s content will be overwritten.
The >> operator also redictects the output of a command into a file, but appends it to any content that already exists.
The grep command can be used to find the lines in a text file that match a text pattern.