Intro to the Unix Shell > 5. Loops > c. Exercises I

c. Exercises I

Variables in Loops

This exercise refers to the shell-lesson-data/molecules directory. ls gives the following output:

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

What is the output of the following code?

$ for datafile in *.pdb
do
	ls *.pdb
done

Now, what is the output of the following code?

$ for datafile in *.pdb
do
ls $datafile
done

Why do these two loops give different outputs?

Solution

The first code block gives the same output on each iteration through the loop. Bash expands the wildcard *.pdb within the loop body (as well as before the loop starts) to match all files ending in .pdb and then lists them using ls. The expanded loop would look like this:

$ for datafile in cubane.pdb  ethane.pdb  methane.pdb  octane.pdb  pentane.pdb  propane.pdb
do
	ls cubane.pdb  ethane.pdb  methane.pdb  octane.pdb  pentane.pdb  propane.pdb
done

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

The second code block lists a different file on each loop iteration. The value of the datafile variable is evaluated using $datafile, and then listed using ls.

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

Limiting Sets of Files

What would be the output of running the following loop in the shell-lesson-data/molecules directory?

$ for filename in c*
do
	ls $filename
done

No files are listed.
All files are listed.
Only cubane.pdb, octane.pdb and pentane.pdb are listed.
Only cubane.pdb is listed.

Solution

How would the output differ from using this command instead?

$ for filename in *c*
do
	ls $filename
done

The same files would be listed.
All the files are listed this time.
No files are listed this time.
The files cubane.pdb and octane.pdb will be listed.
Only the file octane.pdb will be listed.

Solution