Linux [Electronic resources]

For Loops

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
command s
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/*