11.4 The for-do-done Loop

The for-do-done loop is executed on a list of elements. The list of elements is assigned to a variable one-by-one. The value of this variable is processed inside the loop. The loop continues to execute until all of the list elements are processed and there are no more elements in the list. The general syntax of the for-do-done loop is:

 for var in list do    command block done

The for-do-done loop flow diagram is shown in Figure 11-3.

Figure 11-3. The `for-do-done` loop.

graphics/11fig03.gif

As an example of the use of this loop, if you want to list all executable files in your home directory, you can use the following program ( script-23 ) for this purpose.

 #!/usr/bin/sh echo "List of all executable files in home directory" cd $HOME for F in * do    if [ -x $F ]    then       ll $F    fi done

The asterisk character represents all files in this directory. When you run this program, the result is shown as follows . You may have a different result on your system. There may be other uses of this program. You can utilize this script to find all files that have the SUID bit set or some other type of file with slight modifications.

 $  ./script-23  List of all executable files in home directory -rwxr-xr-x 1 boota   users  267 Oct 18 19:23 script-00 -rwxr-xr-x 1 boota   users  131 Oct 18 19:53 script-01 -rwxr-xr-x 1 boota   users  198 Oct 18 20:01 script-02 -rwxr-xr-x 1 boota   users  100 Oct 18 20:07 script-03 -rwxr-xr-x 1 boota   users  121 Oct 18 20:16 script-04 -rwxr-xr-x 1 boota   users  132 Oct 18 21:25 script-05 -rwxr-xr-x 1 boota   users  232 Oct 18 23:11 script-06 -rwxr-xr-x 1 boota   users  177 Oct 18 22:04 script-07 -rwxr-xr-x 1 boota   users  142 Oct 19 17:43 script-08 -rwxr-xr-x 1 boota   users  170 Oct 19 18:04 script-09 -rwxr-xr-x 1 boota   users  638 Oct 19 18:30 script-10 -rwxr-xr-x 1 boota   users  313 Oct 19 19:31 script-11 -rwxr-xr-x 1 boota   users  195 Oct 20 23:16 script-20 -rwxr-xr-x 1 boota   users  193 Oct 20 23:00 script-21 -rwxr-xr-x 1 boota   users  195 Oct 21 17:04 script-22 -rwxr-xr-x 1 boota   users  140 Oct 21 17:07 script-23 $

The script-24 is another example of the for-do-done loop, where a list is provided to the for command. This list contains the names of weekdays, which the program reads one-by-one and prints them on your terminal screen.

 #!/usr/bin/sh for DAY in Sunday Monday Tuesday Wednesday Thursday Friday Saturday do    echo "The day is : $DAY" done

The result of this program is:

 $  ./script-24  The day is : Sunday The day is : Monday The day is : Tuesday The day is : Wednesday The day is : Thursday The day is : Friday The day is : Saturday $

Changing File Access Date and Time

Let's suppose you want to change the access time of all files in your current directory to the current time. You can use the touch command with a small shell script as shown here.

 for FILE in * do    touch $FILE done

Accessing Command Line Parameters

To process all command line parameters one-by-one using a for-do-done loop, the following code segment may be used.

 for ARG in $* do    echo $ARG done

You can replace the echo command with any command or a block of commands to get a desired result.

Study Break

Use of Shell Loops

All of the three shell loops have their own applications. However, the while-do-done and until-do-done loops can be used interchangeably in many cases. Let's have some practice with these loops. Using a while-do-done loop, write a shell program that takes a number as input and then prints its table from 1 to 10. Now change the while-do-done loop to an until-do-done loop to get the same functionality. Use the for-do-done loop and pass a list of numbers from 1 to 10 to the for statement. Again print the table with this arrangement.

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11.4 The for-do-done Loop