The-Unix-Shell-Teaching-Notes

5.4 Saving to a File in a Loop - Part One

In the shell-lesson-data/molecules directory, what is the effect of this loop?

> for alkanes in *.pdb
> do
>    echo $alkanes
>    cat $alkanes > alkanes.pdb
> done
  1. Prints cubane.pdb, ethane.pdb, methane.pdb, octane.pdb, pentane.pdb and propane.pdb, and the text from propane.pdb will be saved to a file called alkanes.pdb.
  2. Prints cubane.pdb, ethane.pdb, and methane.pdb, and the text from all three files would be concatenated and saved to a file called alkanes.pdb.
  3. Prints cubane.pdb, ethane.pdb, methane.pdb, octane.pdb, and pentane.pdb, and the text from propane.pdb will be saved to a file called alkanes.pdb.
  4. None of the above.
Solution 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.

Episode 5 Shell Script For Loop