UNIX System Management: Shell Programming Loop statements (2)

7.for statement

Cycle through each item in a list

, each time the loop completes, the Var is assigned to the next entry in the list until the end of the last entry loop is completed

Syntax: Example:

For VAR in list $ cat test_for
Do with X in 1 2 3 4 5
List A do
Done Echo ' 2 * $X is \c '
Let X=x*2
Echo $X
Done

$ test_for
2 * 1 is 2
2 * 2 is 4
2 * 3 is 6
2 * 4 is 8
2 * 5 is 10

In the example above, the keyword For,in,do and done,var represent the name of a shell variable whose assignment runs through the execution of the For loop, the list is a string of strings separated by spaces or tabs, and a string is assigned to Var for Each loop execution.

The For loop is executed as follows:

The 1.shell variable var is set equal to the first character in the list.

The commands in 2.list a are executed.

The 3.shell variable var is set equal to the next character in the list.

The commands in 4.list a are executed.

5. The loop continues to execute until the entries in each list have been cycled.

Example of a 8.for loop

Example A:

$ cat Example_a
For NAME in $ (grep home/etc/passwd | cut-f1-d:)
Do
Mail $NAME < Mtg.minutes
echo mailed Mtg.minutes to $NAME
Done

Example B

$ cat Example_b
For FILE in *
Do

If
Test-d $FILE
Then
Ls-f $FILE

Fi

Done

A for structure is a very flexible loop structure that allows loops to run through any list that can be generated. Using command substitution can easily generate a build list, just as the first example uses pipes and filters to produce a list. If you ask to access the same list multiple times, you may want to store it in a file. You can use the Cat command to generate a list for your for loop, as shown in the following example:

$ cat Students
User1
User2
User3
User4

$ cat For_student_file_copy
For NAME in $ (cat students)
Do
CP test.file/home/$NAME
Chown $NAME/home/$NAME/test.file
chmod g-w,o-w/home/$NAME/test.file
Echo Done $NAME
Done

$

accessing command line arguments

You can generate a list from command-line arguments:

For I in $* or for I
Do do
CP $i $HOME/backups CP $i $HOME/backups
Done