Intro to the Unix Shell > 7. Shell Scripts > g. Exercises II

g. Exercises II

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?

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

Solution

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:

$ bash longest.sh shell-lesson-data/exercise-data/proteins pdb

would print the name of the .pdb file in shell-lesson-data/exercise-data/proteins that has the most lines.

Feel free to test your script on another directory e.g.

$ bash longest.sh shell-lesson-data/exercise-data/writing txt

Solution

Script Reading Comprehension

For this question, consider the shell-lesson-data/molecules 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.

# Script 1
echo *.*

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

# Script 3
echo $@.pdb

Solution

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.

cubane.pdb ethane.pdb methane.pdb octane.pdb pentane.pdb propane.pdb.pdb

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 stats for data files.
for datafile in "$@"
do
	echo $datfile
	bash goostats.sh $datafile stats-$datafile
done

When you run it:

$ bash do-errors.sh NENE*A.txt NENE*B.txt

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

bash -x do-errors.sh NENE*A.txt NENE*B.txt

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

Solution