Loops

Lesson

Wildcards and tab completion are two ways to reduce typing (and typing mistakes). Another is to tell the shell to do something over and over again. Suppose we have several hundred genome data files named basilisk.dat, unicorn.dat, and so on. When new files arrive, we'd like to rename the existing ones to original-basilisk.dat and original-unicorn.dat. We can't use:

mv *.dat original-*.dat

because that would expand (in the two-file case) to:

mv basilisk.dat unicorn.dat

This wouldn't back up our files: it would replace the content of unicorn.dat with whatever's in basilisk.dat.

Instead, we can use a loop to do some operation once for each thing in a list. Here's a simple example that displays the first three lines of each file in turn:

$ for filename in basilisk.dat unicorn.dat
> do
>    head -3 $filename
> done
COMMON NAME: basilisk
CLASSIFICATION: basiliscus vulgaris
UPDATED: 1745-05-02
COMMON NAME: unicorn
CLASSIFICATION: equus monoceros
UPDATED: 1738-11-24

When the shell sees the keyword for, it knows it is supposed to repeat a command (or group of commands) once for each thing in a list. In this case, the list is the two filenames. Each time through the loop, the name of the thing currently being operated on is assigned to the variable called filename. Inside the loop, we get the variable's value by putting $ in front of it: $filename is basilisk.dat the first time through the loop, unicorn.dat the second, and so on. Finally, the command that's actually being run is our old friend head, so this loop prints out the first three lines of each data file in turn.

Follow the Prompt

The shell prompt changes from $ to > and back again as we were typing in our loop. The second prompt, >, is different to remind us that we haven't finished typing a complete command yet.

We have called the variable in this loop filename in order to make its purpose clearer to human readers. The shell itself doesn't care what the variable is called; if we wrote this loop as:

for x in basilisk.dat unicorn.dat
do
    head -3 $x
done

or:

for temperature in basilisk.dat unicorn.dat
do
    head -3 $temperature
done

it would work exactly the same way. Don't do this. Programs are only useful if people can understand them, so using meaningless names (like x) or misleading names (like temperature) increases the likelihood of the program being wrong.

Here's a slightly more complicated loop:

for filename in *.dat
do
    echo $filename
    head -100 $filename | tail -20
done

The shell starts by expanding *.dat to create the list of files it will process. The loop body then executes two commands for each of those files. The first, echo, just prints its command-line parameters to standard output. For example:

echo hello there

prints:

hello there

In this case, since the shell expands $filename to be the name of a file, echo $filename just prints the name of the file. Note that we can't write this as:

for filename in *.dat
do
    $filename
    head -100 $filename | tail -20
done

because then the first time through the loop, when $filename expanded to basilisk.dat, the shell would try to run basilisk.dat as a program. Finally, the head and tail combination selects lines 81-100 from whatever file is being processed.

Spaces in Names

Filename expansion in loops is another reason you should not use spaces in filenames. Suppose our data files are named:

basilisk.dat
red dragon.dat
unicorn.dat

If we try to process them using:

for filename in *.dat
do
    head -100 $filename | tail -20
done

then the shell will expand *.dat to create:

basilisk.dat red dragon.dat unicorn.dat

With older versions of Bash, or most other shells, filename will then be assigned the following values in turn:

basilisk.dat
red
dragon.dat
unicorn.dat

That's a problem: head can't read files called red and dragon.dat because they don't exist, and won't be asked to read the file red dragon.dat.

We can make our script a little bit more robust by quoting our use of the variable:

for filename in *.dat
do
    head -100 "$filename" | tail -20
done

but it's simpler just to avoid using spaces (or other special characters) in filenames.

Going back to our original file renaming problem, we can solve it using this loop:

for filename in *.dat
do
    mv $filename original-$filename
done

This loop runs the mv command once for each filename. The first time, when $filename expands to basilisk.dat, the shell executes:

mv basilisk.dat original-basilisk.dat

The second time, the command is:

mv unicorn.dat original-unicorn.dat

Measure Twice, Run Once

A loop is a way to do many things at once—or to make many mistakes at once if it does the wrong thing. One way to check what a loop would do is to echo the commands it would run instead of actually running them. For example, we could write our file renaming loop like this:

for filename in *.dat
do
    echo mv $filename original-$filename
done

Instead of running mv, this loop runs echo, which prints out:

mv basilisk.dat original-basilisk.dat
mv unicorn.dat original-unicorn.dat

without actually running those commands. We can then use up-arrow to redisplay the loop, back-arrow to get to the word echo, delete it, and then press "enter" to run the loop with the actual mv commands. This isn't foolproof, but it's a handy way to see what's going to happen when you're still learning how loops work.

Nelle's Pipeline: Processing Files

Nelle is now ready to process her data files. Since she's still learning how to use the shell, she decides to build up the required commands in stages. Her first step is to make sure that she can select the right files—remember, these are ones whose names end in 'A' or 'B', rather than 'Z':

$ cd north-pacific-gyre/2012-07-03

$ for datafile in *[AB].txt
do
    echo $datafile
done
NENE01729A.txt
NENE01729B.txt
NENE01736A.txt
...
NENE02043A.txt
NENE02043B.txt

Her next step is to decide what to call the files that the goostat analysis program will create. Prefixing each input file's name with "stats" seems simple, so she modifies her loop to do that:

$ for datafile in *[AB].txt
do
    echo $datafile stats-$datafile
done
NENE01729A.txt stats-NENE01729A.txt
NENE01729B.txt stats-NENE01729B.txt
NENE01736A.txt stats-NENE01736A.txt
...
NENE02043A.txt stats-NENE02043A.txt
NENE02043B.txt stats-NENE02043B.txt

She hasn't actually run goostats yet, but now she's sure she can select the right files and generate the right output filenames.

Typing in commands over and over again is becoming tedious, though, and Nelle is worried about making mistakes, so instead of re-entering her loop, she presses the up arrow. In response, the shell redisplays the whole loop on one line (using semi-colons to separate the pieces):

$ for datafile in *[AB].txt; do echo $datafile stats-$datafile; done

Using the left arrow key, Nelle backs up and changes the command echo to goostats:

$ for datafile in *[AB].txt; do goostats $datafile stats-$datafile; done

When she presses enter, the shell runs the modified command. However, nothing appears to happen—there is no output. After a moment, Nelle realizes that since her script doesn't print anything to the screen any longer, she has no idea whether it is running, much less how quickly. She kills the job by typing Control-C, uses up-arrow to repeat the command, and edits it to read:

$ for datafile in *[AB].txt; do echo $datafile; goostats $datafile stats-$datafile; done

When she runs her program now, it produces one line of output every five seconds or so:

NENE01729A.txt
NENE01729B.txt
NENE01736A.txt
...

1518 times 5 seconds, divided by 60, tells her that her script will take about two hours to run. As a final check, she opens another terminal window, goes into north-pacific-gyre/2012-07-03, and uses cat stats-NENE01729B.txt to examine one of the output files. It looks good, so she decides to get some coffee and catch up on her reading.

Those Who Know History Can Choose to Repeat It

Another way to repeat previous work is to use the history command to get a list of the last few hundred commands that have been executed, and then to use !123 (where "123" is replaced by the command number) to repeat one of those commands. For example, if Nelle types this:

$ history | tail -5
  456  ls -l NENE0*.txt
  457  rm stats-NENE01729B.txt.txt
  458  goostats NENE01729B.txt stats-NENE01729B.txt
  459  ls -l NENE0*.txt
  460  history

then she can re-run goostats on NENE01729B.txt simply by typing !458.