For Loops | Spring Into Linux

A for loop repeats commands once for each item in a list of arguments. The general format of a for loop is:

 for index in list do     commands done

The index is a variable name. It takes on a value from the list with each iteration of the loop. The list is a list of arguments through which for loops. A simple for loop is:

 for name in John Sam Paul do     echo $name done

Items in the list can be separated by a space or by the end of a line.

You can provide the for loop with a list of arguments by executing a command. For instance, the following script processes the output of the date command.

 #!/bin/bash # Script name: splitdate - outputs parts of the date in a column for part in `date` do     echo $part done

When you run the script, you see:

 ./splitdate Mon Jan 10 15:02:34 PST 2005

Another way to create a list of arguments is to store them in a file and read the file, as follows:

 for item in `cat file1` do     echo $item done

If you leave out the in parameter, the script looks for the list in the numbered variables, assuming you entered a list on the command line. Thus, you can use the following syntax to enter your list at the command line:

 for name  do     echo $name done

Then run the script with:

 ./listnames John Sam Paul John Sam Paul

The for loop is useful for processing each file in a directory. The following for loop echoes each file in the current directory:

 for file in * do     if [ ! -s $file ]       then          echo Removing $file          rm $file     fi done

The code tests whether the file is empty. If it's empty, the filename displays and the file is deleted.

The * represents all files in the current directory. You can specify a different directory with the line below:

 for file in /home/janet/*