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The Unix Shell

Shell Scripts

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Overview

Teaching: 15 min
Exercises: 0 min
Questions

  • How can I save and re-use commands?

Objectives

  • Write a shell script that runs a command or series of commands for a fixed set of files.

  • Run a shell script from the command line.

  • Write a shell script that operates on a set of files defined by the user on the command line.

  • Create pipelines that include shell scripts you, and others, have written.

We are finally ready to see what makes the shell such a powerful programming environment. We are going to take the commands we repeat frequently and save them in files so that we can re-run all those operations again later by typing a single command. For historical reasons, a bunch of commands saved in a file is usually called a shell script, but make no mistake: these are actually small programs.

Let’s start by going back to molecules/ and creating a new file, middle.sh which will become our shell script:

$ cd molecules
$ nano middle.sh

The command nano middle.sh opens the file middle.sh within the text editor “nano” (which runs within the shell). If the file does not exist, it will be created. We can use the text editor to directly edit the file – we’ll simply insert the following line:

head -n 15 octane.pdb | tail -n 5

This is a variation on the pipe we constructed earlier: it selects lines 11-15 of the file octane.pdb. Remember, we are not running it as a command just yet: we are putting the commands in a file.

Then we save the file (Ctrl-O in nano), and exit the text editor (Ctrl-X in nano). Check that the directory molecules now contains a file called middle.sh.

Once we have saved the file, we can ask the shell to execute the commands it contains. Our shell is called bash, so we run the following command:

$ bash middle.sh

ATOM      9  H           1      -4.502   0.681   0.785  1.00  0.00
ATOM     10  H           1      -5.254  -0.243  -0.537  1.00  0.00
ATOM     11  H           1      -4.357   1.252  -0.895  1.00  0.00
ATOM     12  H           1      -3.009  -0.741  -1.467  1.00  0.00
ATOM     13  H           1      -3.172  -1.337   0.206  1.00  0.00

Sure enough, our script’s output is exactly what we would get if we ran that pipeline directly.

Text vs. Whatever

We usually call programs like Microsoft Word or LibreOffice Writer “text editors”, but we need to be a bit more careful when it comes to programming. By default, Microsoft Word uses .docx files to store not only text, but also formatting information about fonts, headings, and so on. This extra information isn’t stored as characters, and doesn’t mean anything to tools like head: they expect input files to contain nothing but the letters, digits, and punctuation on a standard computer keyboard. When editing programs, therefore, you must either use a plain text editor, or be careful to save files as plain text.

What if we want to select lines from an arbitrary file? We could edit middle.sh each time to change the filename, but that would probably take longer than just retyping the command. Instead, let’s edit middle.sh and make it more versatile:

$ nano middle.sh

Now, within “nano”, replace the text octane.pdb with the special variable called $1:

head -n 15 "$1" | tail -n 5

Inside a shell script, $1 means “the first filename (or other parameter) on the command line”. We can now run our script like this:

$ bash middle.sh octane.pdb

ATOM      9  H           1      -4.502   0.681   0.785  1.00  0.00
ATOM     10  H           1      -5.254  -0.243  -0.537  1.00  0.00
ATOM     11  H           1      -4.357   1.252  -0.895  1.00  0.00
ATOM     12  H           1      -3.009  -0.741  -1.467  1.00  0.00
ATOM     13  H           1      -3.172  -1.337   0.206  1.00  0.00

or on a different file like this:

$ bash middle.sh pentane.pdb

ATOM      9  H           1       1.324   0.350  -1.332  1.00  0.00
ATOM     10  H           1       1.271   1.378   0.122  1.00  0.00
ATOM     11  H           1      -0.074  -0.384   1.288  1.00  0.00
ATOM     12  H           1      -0.048  -1.362  -0.205  1.00  0.00
ATOM     13  H           1      -1.183   0.500  -1.412  1.00  0.00

Double-Quotes Around Arguments

For the same reason that we put the loop variable inside double-quotes, in case the filename happens to contain any spaces, we surround $1 with double-quotes.

We still need to edit middle.sh each time we want to adjust the range of lines, though. Let’s fix that by using the special variables $2 and $3 for the number of lines to be passed to head and tail respectively:

$ nano middle.sh

head -n "$2" "$1" | tail -n "$3"

We can now run:

$ bash middle.sh pentane.pdb 15 5

ATOM      9  H           1       1.324   0.350  -1.332  1.00  0.00
ATOM     10  H           1       1.271   1.378   0.122  1.00  0.00
ATOM     11  H           1      -0.074  -0.384   1.288  1.00  0.00
ATOM     12  H           1      -0.048  -1.362  -0.205  1.00  0.00
ATOM     13  H           1      -1.183   0.500  -1.412  1.00  0.00

By changing the arguments to our command we can change our script’s behaviour:

$ bash middle.sh pentane.pdb 20 5

ATOM     14  H           1      -1.259   1.420   0.112  1.00  0.00
ATOM     15  H           1      -2.608  -0.407   1.130  1.00  0.00
ATOM     16  H           1      -2.540  -1.303  -0.404  1.00  0.00
ATOM     17  H           1      -3.393   0.254  -0.321  1.00  0.00
TER      18              1

This works, but it may take the next person who reads middle.sh a moment to figure out what it does. We can improve our script by adding some comments at the top:

$ nano middle.sh

# Select lines from the middle of a file.
# Usage: bash middle.sh filename end_line num_lines
head -n "$2" "$1" | tail -n "$3"

A comment starts with a # character and runs to the end of the line. The computer ignores comments, but they’re invaluable for helping people (including your future self) understand and use scripts. The only caveat is that each time you modify the script, you should check that the comment is still accurate: an explanation that sends the reader in the wrong direction is worse than none at all.

What if we want to process many files in a single pipeline? For example, if we want to sort our .pdb files by length, we would type:

$ wc -l *.pdb | sort -n

because wc -l lists the number of lines in the files (recall that wc stands for ‘word count’, adding the -l flag means ‘count lines’ instead) and sort -n sorts things numerically. We could put this in a file, but then it would only ever sort a list of .pdb files in the current directory. If we want to be able to get a sorted list of other kinds of files, we need a way to get all those names into the script. We can’t use $1, $2, and so on because we don’t know how many files there are. Instead, we use the special variable $@, which means, “All of the command-line parameters to the shell script.” We also should put $@ inside double-quotes to handle the case of parameters containing spaces ("$@" is equivalent to "$1" "$2" …) Here’s an example:

$ nano sorted.sh

# Sort filenames by their length.
# Usage: bash sorted.sh one_or_more_filenames
wc -l "$@" | sort -n

$ bash sorted.sh *.pdb ../creatures/*.dat

9 methane.pdb
12 ethane.pdb
15 propane.pdb
20 cubane.pdb
21 pentane.pdb
30 octane.pdb
163 ../creatures/basilisk.dat
163 ../creatures/unicorn.dat

Why Isn’t It Doing Anything?

What happens if a script is supposed to process a bunch of files, but we don’t give it any filenames? For example, what if we type:

$ bash sorted.sh

but don’t say *.dat (or anything else)? In this case, $@ expands to nothing at all, so the pipeline inside the script is effectively:

$ wc -l | sort -n

Since it doesn’t have any filenames, wc assumes it is supposed to process standard input, 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 script doesn’t appear to do anything.

Suppose we have just run a series of commands that did something useful — for example, that created a graph we’d like to use in a paper. We’d like to be able to re-create the graph later if we need to, so we want to save the commands in a file. Instead of typing them in again (and potentially getting them wrong) we can do this:

$ history | tail -n 5 > redo-figure-3.sh

The file redo-figure-3.sh now contains:

297 bash goostats -r NENE01729B.txt stats-NENE01729B.txt
298 bash goodiff stats-NENE01729B.txt /data/validated/01729.txt > 01729-differences.txt
299 cut -d ',' -f 2-3 01729-differences.txt > 01729-time-series.txt
300 ygraph --format scatter --color bw --borders none 01729-time-series.txt figure-3.png
301 history | tail -n 5 > redo-figure-3.sh

After a moment’s work in an editor to remove the serial numbers on the commands, and to remove the final line where we called the history command, we have a completely accurate record of how we created that figure.

In practice, most people develop shell scripts by running commands at the shell prompt a few times to make sure they’re doing the right thing, then saving them in a file for re-use. This style of work allows people to recycle what they discover about their data and their workflow with one call to history and a bit of editing to clean up the output and save it as a shell script.

Nelle’s Pipeline: Creating a Script

Let’s turn the command we wrote in the last lesson

for datafile in *[GN].txt; do echo $datafile; bash imagestats $datafile stats-$datafile; done

into a script that will take an arbitrary file name as input

Variables in Shell Scripts

In the molecules directory, imagine you have a shell script called script.sh containing the following commands:

head -n $2 $1
tail -n $3 $1

While you are in the molecules directory, you type the following command:

bash script.sh '*.pdb' 1 1

Which of the following outputs would you expect to see?

  1. All of the lines between the first and the last lines of each file ending in .pdb in the molecules directory
  2. The first and the last line of each file ending in .pdb in the molecules directory
  3. The first and the last line of each file in the molecules directory
  4. An error because of the quotes around *.pdb

Solution

The correct answer is 2.

The special variables $1, $2 and $3 represent the command line arguments given to the script, such that the commands run are:

$ head -n 1 cubane.pdb ethane.pdb octane.pdb pentane.pdb propane.pdb
$ tail -n 1 cubane.pdb ethane.pdb octane.pdb pentane.pdb propane.pdb

The shell does not expand '*.pdb' because it is enclosed by quote marks. As such, the first argument to the script is '*.pdb' which gets expanded within the script by head and tail.

List Unique Species

Leah has several hundred data files, each of which is formatted like this:

2013-11-05,deer,5
2013-11-05,rabbit,22
2013-11-05,raccoon,7
2013-11-06,rabbit,19
2013-11-06,deer,2
2013-11-06,fox,1
2013-11-07,rabbit,18
2013-11-07,bear,1

An example of this type of file is given in data-shell/data/animals.txt.

Write a shell script called species.sh that takes any number of filenames as command-line parameters, and uses cut, sort, and uniq to print a list of the unique species appearing in each of those files separately.

Solution

# Script to find unique species in csv files where species is the second data field
# This script accepts any number of file names as command line arguments

# Loop over all files
for file in $@ 
do
	echo "Unique species in $file:"
	# Extract species names
	cut -d , -f 2 $file | sort | uniq
done

Find the Longest File With a Given Extension

Write a shell script called longest.sh that takes the name of a directory and a filename extension as its parameters, and prints out the name of the file with the most lines in that directory with that extension. For example:

$ bash longest.sh /tmp/data pdb

would print the name of the .pdb file in /tmp/data that has the most lines.

Solution

# Shell script which takes two arguments: 
#    1. a directory name
#    2. a file extension
# and prints the name of the file in that directory
# with the most lines which matches the file extension.

wc -l $1/*.$2 | sort -n | tail -n 2 | head -n 1

Why Record Commands in the History Before Running Them?

If you run the command:

$ history | tail -n 5 > recent.sh

the last command in the file is the history command itself, i.e., the shell has added history to the command log before actually running it. In fact, the shell always adds commands to the log before running them. Why do you think it does this?

Solution

If a command causes something to crash or hang, it might be useful to know what that command was, in order to investigate the problem. Were the command only be recorded after running it, we would not have a record of the last command run in the event of a crash.

Script Reading Comprehension

For this question, consider the data-shell/molecules directory once again. This contains a number of .pdb files in addition to any other files you may have created. Explain what a script called example.sh would do when run as bash example.sh *.pdb if it contained the following lines:

# Script 1
echo *.*

# Script 2
for filename in $1 $2 $3
do
    cat $filename
done

# Script 3
echo $@.pdb

Solutions

Script 1 would print out a list of all files containing a dot in their name.

Script 2 would print the contents of the first 3 files matching the file extension. The shell expands the wildcard before passing the arguments to the example.sh script.

Script 3 would print all the arguments to the script (i.e. all the .pdb files), followed by .txt.

Debugging Scripts

Suppose you have saved the following script in a file called do-errors.sh in Nelle’s north-pacific-gyre/2012-07-03 directory:

# Calculate reduced stats for data files at J = 100 c/bp.
for datafile in "$@"
do
    echo $datfile
    bash goostats -J 100 -r $datafile stats-$datafile
done

When you run it:

$ bash do-errors.sh *[AB].txt

the output is blank. To figure out why, re-run the script using the -x option:

bash -x do-errors.sh *[AB].txt

What is the output showing you? Which line is responsible for the error?

Solution

The -x flag causes bash to run in debug mode. This prints out each command as it is run, which will help you to locate errors. In this example, we can see that echo isn’t printing anything. We have made a typo in the loop variable name, and the variable datfile doesn’t exist, hence returning an empty string.

Key Points

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