Chapter 6 of linuxshell programming guide ------ control flow structure 1

As mentioned above, expr indicates that this command is used to add a count in a loop. The following uses ls to list the objects and their numbers in the for loop. [Root @ localhosthuangcd] catforcount! Binbashcounter = 0forfilesin * do

In the previous discussion on e x p r, we pointed out that this command is used if the count is to be added during the loop. The following uses l s to list in the f o r loop
File and its number.

[Root @ localhost huangcd] # cat forcount

#! /Bin/bash
Counter = 0
For files in *
Do
Counter = 'expr $ counter + 1'
Done
Echo "there are $ counter files in 'pwd' we need to process"
[Root @ localhost huangcd] # sh forcount
There are 76 files in/home/huangcd we need to process
[Root @ localhost huangcd] # ls | wc-l

The nested loop can embed one f o r loop into another f o r loop:
For variable name 1 in list 1
D o
For variable name 2 in list 2
D o
Command 1
...
D o n e
D o n e

The following script is embedded into the f o r loop. here there are two lists: a p s and s c r I P T S. The first contains the application on the server
The second is the management script running on each application. For each application on the list a p s, the list
The script in s c r I P T S will be run, and the script is actually run in the background. The script uses the t e command to put
So that the output to the screen is also output to a file. View the output result to see how to embed the f o r loop
Use the list s c r I P T S to execute the processing on the list A P S.

[Root @ localhost huangcd] # cat audit_run
#! /Bin/bash
APPS = "/apps/accts/apps/claims/apps/stock/apps/serv"
SCRIPTS = "audit. check report. run cleanup"
LOGFILE = audit. log
MY_DATE = 'date + % H: % M "on" % d/% m % Y'


For loop in $ APPS
Do
For loop2 in $ SCRIPTS
Do
Echo "system $ loop now running $ loop2 at $ MY_DATE" | tee-a $ LOGFILE $ loop $ loop2
Done
Done

U n t I l executes a series of commands cyclically until the condition is true. The processing method of the u n t I l loop and the w h I l e loop is exactly the same
On the contrary. Generally, the w h I l e loop is better than the u n t I l loop, but in some cases, the u n t I l loop is more
Useful.
The format of the u n t I l loop is:
Until condition
Command 1
...
D o n e
The condition can be any test condition, and the test occurs at the end of the loop, so the loop is executed at least once-please note this.

This script constantly searches for the user r o t in the w h o command, and the variable I S-R O T saves the results of the g r e p command.
If the r o t is found, the loop ends, and an email is sent to the user s I o n to notify him that the user r o t has logged on. Note that
Here s l e p command usage, which is often used in the u n t I l loop, because the body must sleep for several seconds before execution,
Otherwise, a large amount of system resources will be consumed.

[Root @ localhost huangcd] # cat until_who
#! /Bin/bash
IS_ROOT = 'Who | grep root'
Until ["$ IS_ROOT"]
Do
Sleep 5
Done
Echo "watch it. roots in" | mail root
[Root @ localhost huangcd] # sh until_who
[Root @ localhost huangcd] # mail
Mail version 8.1 6/6/93. Type? For help.
"/Var/spool/mail/root": 3 messages 1 new 3 unread
U 1 logwatch@localhost.l Sat Oct 26 113/3545 "Logwat"
U 2 logwatch@localhost.l Wed Dec 11 90/3152 "Logwat"
> N 3 root@localhost.local Wed Dec 11 15/646
& 3
Message 3:
From root@localhost.localdomain Wed Dec 11 16:09:29 2013
Date: Wed, 11 Dec 2013 16:09:29 + 0800
From: root
To: root@localhost.localdomain

Watch it. roots in

The w h I l e loop can be used to read keyboard information. In the following example, the input information is set to the variable f I L M, and press <C t r l-
D> end the loop.

[Root @ localhost huangcd] # cat whileread
#! /Bin/bash
Echo "type ctrl + d to terminate"
Echo-n "enter your most like file :"
While read FILE
Do
Echo "yeah, great file the $ FILE"
Done
[Root @ localhost huangcd] # sh whileread
Type ctrl + d to terminate
Enter your most like file: ddfdfdf
Yeah, great file the ddfdfdf
Dfsf
Yeah, great file the dfsf

The w h I l e loop is most commonly used to read data from a file, so writing a script can process such information.
Assume that you want to read information from a file containing the employee name, subordinate department, and I D.

You can use a variable to save each row of data. when no data is read, the condition is true. W h I l e loop using input reset
To ensure that data is read from the file. Note that the entire row of data is set as a single variable $ l I N E ..