Shell Scripts
Last updated on 2023-06-06 | Edit this page
Estimated time: 45 minutes
Overview
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.
Not only will writing shell scripts make your work faster, but also you won’t have to retype the same commands over and over again. It will also make it more accurate (fewer chances for typos) and more reproducible. If you come back to your work later (or if someone else finds your work and wants to build on it), you will be able to reproduce the same results simply by running your script, rather than having to remember or retype a long list of commands.
Let’s start by going back to alkanes/ and creating a new
file, middle.sh which will become our shell script:
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 5This is a variation on the pipe we constructed earlier, which selects
lines 11-15 of the file octane.pdb. Remember, we are
not running it as a command just yet; we are only incorporating
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
alkanes 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:
OUTPUT
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 0.00Sure 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, which expects
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 typing the command out again in the shell and
executing it with a new file name. Instead, let’s edit
middle.sh and make it more versatile:
Now, within “nano”, replace the text octane.pdb with the
special variable called $1:
head -n 15 "$1" | tail -n 5Inside a shell script, $1 means ‘the first filename (or
other argument) on the command line’. We can now run our script like
this:
OUTPUT
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 0.00or on a different file like this:
OUTPUT
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 0.00Double-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.
Currently, we need to edit middle.sh each time we want
to adjust the range of lines that is returned. Let’s fix that by
configuring our script to instead use three command-line arguments.
After the first command-line argument ($1), each additional
argument that we provide will be accessible via the special variables
$1, $2, $3, which refer to the
first, second, third command-line arguments, respectively.
Knowing this, we can use additional arguments to define the range of
lines to be passed to head and tail
respectively:
head -n "$2" "$1" | tail -n "$3"We can now run:
OUTPUT
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 0.00By changing the arguments to our command, we can change our script’s behaviour:
OUTPUT
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 1This 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:
# 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:
because wc -l lists the number of lines in the files
(recall that wc stands for ‘word count’, adding the
-l option 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
arguments to the shell script’. We also should put $@
inside double-quotes to handle the case of arguments containing spaces
("$@" is special syntax and is equivalent to
"$1" "$2" …).
Here’s an example:
# Sort files by their length.
# Usage: bash sorted.sh one_or_more_filenames
wc -l "$@" | sort -nOUTPUT
9 methane.pdb
12 ethane.pdb
15 propane.pdb
20 cubane.pdb
21 pentane.pdb
30 octane.pdb
163 ../creatures/basilisk.dat
163 ../creatures/minotaur.dat
163 ../creatures/unicorn.dat
596 totalList 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,1An example of this type of file is given in
shell-lesson-data/exercise-data/animal-counts/animals.csv.
We can use the command
cut -d , -f 2 animals.csv | sort | uniq to produce the
unique species in animals.csv. In order to avoid having to
type out this series of commands every time, a scientist may choose to
write a shell script instead.
Write a shell script called species.sh that takes any
number of filenames as command-line arguments and uses a variation of
the above command to print a list of the unique species appearing in
each of those files separately.
Suppose we have just run a series of commands that did something useful — for example, creating 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:
The file redo-figure-3.sh now contains:
297 bash goostats.sh NENE01729B.txt stats-NENE01729B.txt
298 bash goodiff.sh 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.shAfter 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.
Why Record Commands in the History Before Running Them?
If you run the command:
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?
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.
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
Nelle’s supervisor insisted that all her analytics must be reproducible. The easiest way to capture all the steps is in a script.
First we return to Nelle’s project directory:
She creates a file using nano …
…which contains the following:
BASH
# Calculate stats for data files.
for datafile in "$@"
do
echo $datafile
bash goostats.sh $datafile stats-$datafile
doneShe saves this in a file called do-stats.sh so that she
can now re-do the first stage of her analysis by typing:
She can also do this:
so that the output is just the number of files processed rather than the names of the files that were processed.
One thing to note about Nelle’s script is that it lets the person running it decide what files to process. She could have written it as:
BASH
# Calculate stats for Site A and Site B data files.
for datafile in NENE*A.txt NENE*B.txt
do
echo $datafile
bash goostats.sh $datafile stats-$datafile
doneThe advantage is that this always selects the right files: she
doesn’t have to remember to exclude the ‘Z’ files. The disadvantage is
that it always selects just those files — she can’t run it on
all files (including the ‘Z’ files), or on the ‘G’ or ‘H’ files her
colleagues in Antarctica are producing, without editing the script. If
she wanted to be more adventurous, she could modify her script to check
for command-line arguments, and use NENE*A.txt NENE*B.txt
if none were provided. Of course, this introduces another tradeoff
between flexibility and complexity.
Variables in Shell Scripts
In the alkanes directory, imagine you have a shell
script called script.sh containing the following
commands:
While you are in the alkanes directory, you type the
following command:
Which of the following outputs would you expect to see?
- All of the lines between the first and the last lines of each file
ending in
.pdbin thealkanesdirectory - The first and the last line of each file ending in
.pdbin thealkanesdirectory - The first and the last line of each file in the
alkanesdirectory - An error because of the quotes around
*.pdb
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:
BASH
$ 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.pdbThe 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.
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 arguments, and
prints out the name of the file with the most lines in that directory
with that extension. For example:
would print the name of the .pdb file in
shell-lesson-data/exercise-data/alkanes that has the most
lines.
Feel free to test your script on another directory e.g.
BASH
# 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 1The first part of the pipeline, wc -l $1/*.$2 | sort -n,
counts the lines in each file and sorts them numerically (largest last).
When there’s more than one file, wc also outputs a final
summary line, giving the total number of lines across all
files. We use tail -n 2 | head -n 1 to throw away this last
line.
With wc -l $1/*.$2 | sort -n | tail -n 1 we’ll see the
final summary line: we can build our pipeline up in pieces to be sure we
understand the output.
Script Reading Comprehension
For this question, consider the
shell-lesson-data/exercise-data/alkanes directory once
again. This contains a number of .pdb files in addition to
any other files you may have created. Explain what each of the following
three scripts would do when run as bash script1.sh *.pdb,
bash script2.sh *.pdb, and
bash script3.sh *.pdb respectively.
In each case, the shell expands the wildcard in *.pdb
before passing the resulting list of file names as arguments to the
script.
Script 1 would print out a list of all files containing a dot in their name. The arguments passed to the script are not actually used anywhere in the script.
Script 2 would print the contents of the first 3 files with a
.pdb file extension. $1, $2, and
$3 refer to the first, second, and third argument
respectively.
Script 3 would print all the arguments to the script (i.e. all the
.pdb files), followed by .pdb. $@
refers to all the arguments given to a shell script.
OUTPUT
cubane.pdb ethane.pdb methane.pdb octane.pdb pentane.pdb propane.pdb.pdbDebugging Scripts
Suppose you have saved the following script in a file called
do-errors.sh in Nelle’s north-pacific-gyre
directory:
BASH
# Calculate stats for data files.
for datafile in "$@"
do
echo $datfile
bash goostats.sh $datafile stats-$datafile
doneWhen you run it from the north-pacific-gyre
directory:
the output is blank. To figure out why, re-run the script using the
-x option:
What is the output showing you? Which line is responsible for the error?
The -x option 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
- Save commands in files (usually called shell scripts) for re-use.
-
bash [filename]runs the commands saved in a file. -
$@refers to all of a shell script’s command-line arguments. -
$1,$2, etc., refer to the first command-line argument, the second command-line argument, etc. - Place variables in quotes if the values might have spaces in them.
- Letting users decide what files to process is more flexible and more consistent with built-in Unix commands.