Use of bash if conditional judgment

In bash, if can choose and execute statements, and if condition tests include three forms: integer test, character test, and file test.

if format

Single branch syntax

if condition; then
    Statement1
    Statement2
    ... 
fi

Double branch syntax

if condition;then
    Statement1
    Statement2
else
    Statement2
fi

Multi-branch syntax

if condition1; then
    Statement1
    Statement2
elif condition2; then
    Statement1
    Statement2
else
    Statement1
    Statement2
fi

Conditional testing

If the judgment conditions can be found in the following forms

• [ express ]
• [[ express ]]
• test express
• bash command (execute the statement if the command is executed successfully)

[],[[]], test functions similarly, basically it can be interchanged when writing bash scripts
Note: [],[[]] space position;
[1 space + expression + 1 space]
[[1 space + expression + 1 space]]

Example: bash command to do if condition

Example 1: Check whether there is a hadoop user in the system. If there is a uid and shell, no return

[root@node1 bash_test]# cat  
#!/bin/bash

if id -u hadoop &> /dev/null;then
    grep "^hadoop" /etc/passwd | cut -d: -f1,7
else
    echo no
fi
[root@node1 bash_test]# ./ 
hadoop:/bin/bash

Notice:

• id -u hadoop &> The function of the /dev/null directive is to execute id -u hadoop to throw away its receipt (not displayed in the terminal)
• When the bash command performs if condition, when the command status return value is true, that is, the value is 0, the condition is satisfied: the command status return value 1-255 means that the condition is not satisfied

Command status return value

We can use $? to view the command status return value of the previous command. The command will return 0 after successful execution, and the command will return 1 after failure execution.

[root@node1 bash_test]# id hadoop
uid=4024(hadoop) gid=4024(hadoop) Group=4024(hadoop),4026(mygrp)
[root@node1 bash_test]# echo $?
0
[root@node1 bash_test]# id hadoop111
id: hadoop111: no such user
[root@node1 bash_test]# echo $?

Of course, in bash scripts, we can use exit to exit the program, and the number followed by represents the command execution status. For example: exit 0 means execution success, exit 1 means execution failure

Integer test

If conditional test is to compare integers, the specific rules are as follows
expression: value 1 comparison symbol value 2
Comparison symbols:
Greater than -gt
greater than or equal to -ge
equal to -eq
Not equal to -ne
Less than -gt
Less than or equal to -ge

Example 1: Read a parameter, if it is less than 10, returns Yes, and returns No if it is greater than 10.

[root@node1 bash_test]# cat  
#!/bin/bash

if [ $1 -lt 10 ];then
  echo Yes
else
  echo No
fi
[root@node1 bash_test]# ./ 5
Yes
[root@node1 bash_test]# ./ 11
No

Note: $1 represents the first parameter passed in by the terminal
The position variables in bash are:
$1,$2,$3…
$@ $* Show all position variables
$@ Five strings are displayed as one string
$* Five strings are displayed separately
$# Displays the number of position variables
$? Save the status return value of the command just executed
shift implements positional parameter rotation
Combining shfit can use a $1 to get all variables
shift 2 Specify rotation 2

For example: execute command./ 5
$0 is ./
$1 is 5
@ and @ and @ and * are both "./ 5"
$# is 1

Example 2: Write a script to generate two numbers randomly and compare their sizes

[root@node1 bash_test]# cat  
#!/bin/bash

A=$RANDOM
B=$RANDOM
result=""
if [ $A -lt $B ];then
  result="A is greater than B"
elif [ $A -gt $B ];then
  result="A is less then B"
else
  result="A equals B"
fi
echo "A = $A, B = $B, $result"

[root@node1 bash_test]# ./ 
A = 6646, B = 3889, A is less then B

$RANDOM generates a random number between 0-32767

Character Test

If the condition is the size comparison of characters, the size comparison is done by default with the ASCII code table.
In addition to using the symbols -lt, -gt, -le, -ge, -eq, -ne for judgment, we can also use the following symbols for judgment.

> : Greater than
< : Small
== : equal to
!= : Not equal to
Monogram test
-z $STRING The length of the string, if it is empty, it is true, if it is not empty, it is false
-n $STRING is empty, it is false, and if it is not empty, it is true
=~ Determine whether the string on the left can be matched by the pattern on the right, usually used in double[[  ]]
Generally, the beginning of the line is used, and the end of the line is anchored $, and the beginning of the word cannot be anchored\< \>
Cannot use quotes

Generally, we use == more often to determine whether two strings are equal.

Example 1: When writing a script, you can accept a parameter, and its usage form is as follows:
{start|stop|restart|status}
in:
If the parameter is start, create an empty file /var/lock/subsys/script and display "starting script successfully."
If the parameter is stop, delete the file /var/lock/subsys/script and display "Stop script successfully."
If the parameter is restart, delete the file /var/locksubsys/script and recreate it, and then "Restarting script successfully."
If the parameter is status, then: if the file /var/lock/subsys/script exists, "Script is running..." is displayed, otherwise, "Script is stopped."
Any other parameters; display "{start|stop|restart|status}"

#!/bin/bash
# echo $1
if [ $1 == "start" ];then
    if [ -e /var/lock/subsys/script ];then
        echo  "script is already running."
    else
        mkdir /var/lock/subsys/script
        echo  "Starting script successfully."
    fi
elif [ $1 == "stop" ];then
    if [ -e /var/lock/subsys/script ];then
        rm -rf /var/lock/subsys/script
        echo "Stop script finished."
    else
        echo "script is stopped yet."
    fi
elif [ $1 == "restart" ];then
    rm -rf /var/lock/subsys/script
    mkdir /var/lock/subsys/script
    echo  "Restarting script successfully."
elif [ $1 == "status" ];then
    if [ -e /var/lock/subsys/script ];then
        echo "script is running."
    else
        echo "script is stopped."
    fi
else
    echo " {start|stop|restart|status}"
fi

File Testing

There are mainly the following types of operations on files

Monogram test
-f Test whether it is a normal file, that is, a file whose file type is - when ls -l
-d Test whether it is a directory file, that is, the directory with the file type of - when ls -l
-e Test whether the file exists, and existence is true
-r,-w,-x are all the three judging file permissions and whether they have read, write, and execute permissions
-s Test whether the file size is not empty. If it is not empty, it is true. If it is empty, it is false.
-l -b -c determines whether the file is of the corresponding type

Example: If /tmp/test10 does not exist, create it

[root@node1 bash_test]# cat  
#!/bin/bash

if [ ! -e /tmp/test10 ];then
    mkdir /tmp/test10
fi
[root@node1 bash_test]# ls -ld /tmp/test10
ls: Unable to access/tmp/test10: There is no file or directory
[root@node1 bash_test]# ./ 
[root@node1 bash_test]# ls -ld /tmp/test10
drwxr-xr-x 2 root root 6 8moon   1 21:21 /tmp/test10

Combination condition test

When there are multiple judgments to be made, you can use the following to connect

If using independent commands to combine conditions, use &&,|| or,!

Example 1: Write a script, given the user, if there is no exit script

[root@node1 bash_test]# cat  
#!/bin/bash

if ! id $1 > /dev/null;then
    echo "no find this user"
    exit 6
else
    id $1
fi
[root@node1 bash_test]# ./ hadoop
uid=4024(hadoop) gid=4024(hadoop) Group=4024(hadoop),4026(mygrp)
[root@node1 bash_test]# ./ hadoop1
id: hadoop1: no such user
no find this user

Example 2: Pass a string to the script through parameters. If the passed string is "memory" or "Memory", the memory information of the current host is displayed in MB: Otherwise, the contents of the /proc/uptime file are displayed.

#!/bin/bash

if [[ $1 == "memory" || $1 == "Memory" ]]; then
   free -tm
else
   cat /proc/uptime
fi

Notice:
|| Short circuit operation, the front is true, and the back will not be run
&& The front is false, no calculation is required

If we need to have the previous one as false, run the subsequent program, use ||
If we need the front to be true, run the later program, use &&

Example 1: Create /tmp/test10 if it does not exist

[ -e /tmp/test10 ] || mkdir /tmp/test10

Example 2: If the /tmp/test10 folder exists, create a hello file under that folder

[ -d /tmp/test10 ] && touch /tmp/test10/hello

Example 3: If the /tmp/test10/hello file exists, it will show 'exist'; if it does not exist, it will show 'not exist'

[ -f /tmp/test10/hello ] && echo "exist" || echo "not exist"

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