The-Unix-Shell-Teaching-Notes

6.5 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

Solutions

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

Episode 6 Exercise 6