Shell script programming for statement, if statement use introduction _linux shell

Upper:

Process oriented:
Sequential execution
Select execution: If, case
Loop execution: For, while, until

One, for statement

Format:
for variable in list;
Statement 1;
Statement 2;
...
Done
Example 1, write a script, add 10 users, and let each user's password with the user name "

Copy Code code as follows:

#!/bin/bash
For I in {1..10}; Todo
Useradd user$i;
echo user$i | passwd--stdin user$i
Done

Example 2, write a script that shows the timestamp of the/tmp/1.dir/tmp/2.dir/tmp/3.dir three files, and change the time of the three files to 201003030303.03

Copy Code code as follows:

#!/bin/bash
For Dir In/tmp/1.dir/tmp/2.dir/tmp/3.dir; Todo

Stat $Dir
Touch-m-T 201003030303.03 $Dir
Stat $Dir
Done

Types of arguments for bash:
Local variables
Local variables
Environment variables
Position variables: $, $, $, $ ....
Special variables:
$?: The state return value used to save the command just executed;
0: Successful execution; 1-255: Failure, 1,2,127
You can use the Exit command to customize the script execution state return value in a script, or, if not defined, the script execution status returned
The return value depends on the state of the statement that was last executed before the end of the script execution;

$@, $*: all positional parameters;
$#: Number of positional parameters;
$: script Name

Two, if statement

1. Single Branch if statement:
Format:
if condition; Then
Statement 1
Statement 2
...
Fi
Example 3, write a script to achieve the following functions: If the user exists, it means that the existence;

Copy Code code as follows:

#!/bin/bash
Username=user1
If grep "^ $UserName \>"/etc/passwd &>/dev/null; Then
echo "$UserName exists."
Fi

Example 4, write a script to achieve the following functions: If the device/dev/sda3 has been mounted, display its mount point;

Copy Code code as follows:

#!/bin/bash
Device= '/dev/sda3 '
If Mount | grep "^ $Device" &>/dev/null; Then
Mount | grep "/dev/sda3" | Cut-d '-f3
Fi

Example 5, write a script to achieve the following functions: If there is a blank line in the/etc/rc.d/rc.sysinit, it shows the number of blank lines;

Copy Code code as follows:

#!/bin/bash
File= '/etc/rc.d/rc.sysinit '
If grep "^$" $File &>/dev/null; Then
grep "^$" $File | Wc-l
Fi

2, Double branch if statement:

Format:
if condition; Then
Statement 1
Statement 2
...
Else
Statement 1
Statement 2
...
Fi

Example 6, write a script to achieve the following functions:

If the device/dev/sda3 is already mounted, the mount point is displayed, otherwise, it is not mounted or the device does not exist;

Copy Code code as follows:

#!/bin/bash
Device= '/dev/sda3 '
If Mount | grep "^ $Device" &>/dev/null; Then

Mount | grep "/dev/sda3" | Cut-d '-f3
Else
echo "$Device not mounted or not exist."
Fi

3, multiple branch if statement:
Format:
If condition 1; Then
Statement 1
Statement 2
...
Elif Condition 2; Then
Statement 1
Statement 2
...
elif condition 3; Then
Statement 1
Statement 2
...
Else
Statement 1
Statement 2
...
Fi
Example 7, write a script:
Determine the CPU manufacturer for the current host, with information in the Vendor ID row in the/proc/cpuinfo file.
If its manufacturer is Genuineintel, it is Intel Corporation;
If its manufacturer is AUTHENTICAMD, it will show AMD company;
Otherwise, the display is not recognizable;

Copy Code code as follows:

#!/bin/bash
Vendor= ' grep ' vendor_id '/proc/cpuinfo | Uniq | cut-d:-f2 '
if [[$Vendor =~ [[: space:]]*genuineintel$]]; Then

echo "Intel"
elif [$Vendor =~ [[: space:]]*authenticamd$]]; Then
echo "AMD"
Else
echo "Unknown"
Fi

Lower:

One, bash condition test:
Integer test [expression]
character test [[expression]]
Conditional testing test expression Test returns 0 (true) or 1 (false) as a result of an expression evaluation

1. Integer test: Numerical comparison
-GT Greater than
-ge is greater than or equal to
-eq equals
-lt less than
-le less than or equal to
-ne is not equal to

Example 1: Write a script that generates two random numbers, compares their size, and displays large numbers;

Bash has a built-in variable: $RANDOM

Copy Code code as follows:

#!/bin/bash
A= $RANDOM
b= $RANDOM
If [$A-ge $B]; Then
echo "Max number is $A."
Else
echo "Max number is $B."
Fi

Example 2: Write a script, randomly generate an integer, determine, show its parity;

Copy Code code as follows:

#!/bin/bash
#
A= $RANDOM
If [$[$A%2]-eq 0]; Then
echo "$A: Even"
Else
echo "$A: Odd"
Fi

Example 3, write a script:

Calculate the sum of all odd numbers and all even number within 100, and show them separately;

Copy Code code as follows:

#!/bin/bash
Evensum=0
Oddsum=0
For I in {1..100}; Todo
If [$[$I%2]-eq 0]; Then
evensum=$[$EvenSum + $I]
Else
oddsum=$[$OddSum + $I]
Fi
Done
echo "Evensum is: $EvenSum. Oddsum is: $OddSum. "

Example 4, to compute the sum of all odd numbers and all even in N, and to show that n is a positive integer passed through the parameter;

Copy Code code as follows:

#!/bin/bash
Evensum=0
Oddsum=0
For I in ' seq 1 $ ';d o
If [$[$I%2]-eq 1]; Then
oddsum=$[$OddSum + $I]
Else
evensum=$[$EvenSum + $I]
Fi
Done
echo "Evensum: $EvenSum."
echo "Oddsum: $OddSum."
echo "Sum: $[$EvenSum + $OddSum]"

Example 5, write a script to complete the following requirements:

1, add 10 users user1, User2, ..., User10, but first to determine whether the user exists, does not exist and then add;
2, after the completion of the addition, the display has added a total of several users, of course, can not be included because of prior existence without adding;
3, the final display of the current system on a total number of users;

Copy Code code as follows:

#!/bin/bash
Count=0
For I in {1..10}; Todo
If ID user$i &>/dev/null; Then
echo "user$i exists."
Else
Useradd user$i
echo "Add user$i successfully."
count=$[$Count +1]
Fi
Done
echo "Add $Count new users."
echo "Total users: ' Wc-l/etc/passwd | Cut-d '-f1 '.

2. Bash character test:
": Greater Than
: Less than
= =: equal To
=~: Determines whether the string on the left can be matched by the pattern on the right, usually used [[]];
[[$opt 1 =~ $opt 2]]] generally do the beginning of the line, the end of the anchor; do not quote
Monocular:
-Z $STRING: null is true, not empty is false;
-N $STRING: null is false, not empty is true;

Example 6, write a script to determine whether the user's shell is bash;

Copy Code code as follows:

#!/bin/bash
Shell= ' grep ' ^$1: "/etc/passwd | cut-d:-f7 '
If ["$Shell" = = "/bin/bash"]; Then
echo "Bash User."
Ret=0
Else
echo "Not Bash User."
Ret=9
Fi
Exit $Ret

Example 7, according to the user shell terminator is sh to determine whether it is a logged-on user:

Copy Code code as follows:

#!/bin/bash
Shell= ' grep ' ^$1: "/etc/passwd | cut-d:-f7 '
If [-Z $Shell]; Then
echo "No shell."
Exit 3
Fi
if [["$Shell" =~ sh$]]; Then
echo "Login User."
Ret=0
Else
echo "None Login User."
Ret=4
Fi
Exit $Ret

Second, bash test file test:

Operator file path
-F: Test whether it is a normal file, that is, ls-l file type-file;
-D: Test whether it is a directory file, that is, ls-l file type D file;
-e: Test file exists, exists as true, otherwise false;
-R: Tests whether the file is readable to the current user;
-W: Test file is writable for the current user;
-X: Test whether the file is executable for the current user;
-S: Test file size is not empty, is true, empty is false;
Short-circuit operation: As long as the first half of the final paragraph can be determined, the second half of the operation is no longer;
and operations:
True && true = True
True && false = False
False && {true | false} = FALSE

OR operation:
Fake | | 0 = 0
Fake | | 1 = 1
true | | =1

Example 8, given a path, judge if it is a normal file, show it;
Otherwise, say cannot recognize;

Copy Code code as follows:

#!/bin/bash
if [!-e $]; Then
echo "No such file."
Exit 7
Fi
If [F $]; Then
echo "Common file."
elif [D $]; Then
echo "Directory."
Else
echo "Unknown file."
Fi

Comprehensive Example:
Write a script: You can accept a parameter with the following form:
script.sh {Start|stop|restart|status}
If the argument is start, create an empty file/var/lock/subsys/script and display "starting script successfully." ；
If the argument is stop, the file/var/lock/subsys/script is deleted and the Stop script finished is displayed. ；
If the parameter is restart, the file/var/lock/subsys/script is deleted and the restarting script successfully is displayed. ；
If the parameter is status, then:
If the/var/lock/subsys/script file exists, it is displayed as "the script is running."
Otherwise, it is displayed as "The script is stopped."
Any other parameter: "script.sh {start|stop|restart|status}" is displayed

Copy Code code as follows:

#!/bin/bash
If [$ = start]; then
Touch/var/lock/subsys/script
echo "Starting script successfully."
elif [$ = stop]; Then
Rm-f/var/louk/subsys/script
echo "Stop script finished."
elif [$ = = Restart];then
Rm-f/var/louk/subsys/script
Touch/var/lock/subsys/script
echo "Restarting script successfully."
elif [$ = = Status];then
[-e/var/lock/subsys/script]&& echo "script is running." | | echo "Script is stopped."
Else
echo "script.sh {start|stop|restart|status}"
Fi