Solutions

Write your own loop

jupyter-user$ for loop_variable in 0 1 2 3 4 5 6 7 8 9
> do
>     echo $loop_variable
> done
0
1
2
3
4
5
6
7
8
9

Variables in loops

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

For the first loop 4 is the correct answer. * matches zero or more characters, so any file name starting with the letter c, followed by zero or more other characters will be matched.

For the second loop 4 is also the correct answer. * matches zero or more characters, so a file name with zero or more characters before a letter c and zero or more characters after the letter c will be matched.

Saving to a file in a loop - Part one

1. The text from each file in turn gets written to the alkanes.pdb file. However, the file gets overwritten on each loop iteration, so the final content of alkanes.pdb is the text from the propane.pdb file.

Saving to a file in a loop - Part two

1. is the correct answer.>> appends to a file, rather than overwriting it with the redirected output from a command. Given the output from the cat command has been redirected, nothing is printed to the screen.

Creating backups

jupyter-user:$ for filename in *.dat
> do
>     cp $filename original-$filename
> done