Logical judgment File directory attribute in shell script determine if special usage case judgment

I. Logical judgments in shell scripts

In the shell script, many will logically judge, judge a certain value, judge a file, or a directory, we do some action against the result of the judgment, if there is no judgment, there is no way to do some operations
Format 1:if condition; then statement; fi
Example:
[[Email protected] ~]# if [$a-ge 3]//branch Write it like this

Then
Echo OK
Fi
Ok
[Email protected] ~]# if [$a-ge 3]; then echo OK; FI//This is a one-line write format
Explanation:-gt means greater than the meaning, the format is special: square brackets must have a space on both sides of the-GT must have a space on both sides, conditional statements are everywhere space
We can write the above command as a script:
[Email protected] ~]# CD shell/
[Email protected] shell]# VI if1.sh
#!/bin/bash
A=5
If [$a-GT 3]
Then
Echo OK
Fi
[[Email protected] shell]# sh if1.sh//Execute script to view script execution results
Ok
Most scripts use this format, using logic to determine if, then

Format 2:if condition; then statement; Else statement; fi
Example:
[email protected] shell]# CP if1.sh if2.sh
[Email protected] shell]# VI if2.sh
#!/bin/bash
A=1
If [$a-GT 3]
Then
Echo OK
Else
Echo Nook
Fi
[[Email protected] shell]# sh if2.sh//execution added the judgment of the Else statement, output Nook
Nook
[[email protected] shell]# sh-x if2.sh//-x can see the detailed execution of the script

• A=1
• ' [' 1-gt 3 '] '//1 and 3 comparison is not greater than 3, the judgment is else
• Echo Nook
  Nook

Format 3:if ... ; ; elif. ; ; else ... ; Fi
Example:
[[[email protected] shell]# cp if2.sh if3.sh
[[email protected] shell]# vi if3.sh
#!/bin /bash
a=5
If [$a-gt 1]//If A is greater than 1
then
echo ">1"//Output >1
Elif [$a-lt 6]//Less than 6,elif on the basis of greater than 1 this condition can be judged Write multiple
Then
echo "<6 && >1"//Output <6 && >1
Else//else to indicate conditions other than the above two conditions
Echo Nook
fi
[[email protected] shell]# sh-x if3.sh//Use the-X option to view the script verbose execution process

• A=5
• ' [' 5-GT 1 '] '
• Echo ' >1 '

  1
  Above this if3.sh the logic of this script may be problematic and needs to be improved
  Logical judgment Expression:
  If [$a-gt $b] indicates if a>b;
  If [$a-lt 5] indicates if a<5;
  If [$b-eq 10] indicates if b=10;
  If [$b-ne 10] Indicates if B is not = 10;
  -GT (>)
  -lt (<)
  -ge (>=)
  -le (<=)
  -eq (= =)
  -ne (! =)
  If you want to use the greater than sign directly, you can write in a different format
  [Email protected] shell]# a=3
  [[Email protected] shell]# if (($a >1)); then echo OK; FI//using two brackets, but cumbersome to use
  Ok
  can use && | | Combine multiple conditions,&& representation and, | | Indicate or
  If [$a-gt 5] && [$a-lt 10]; then//If A is greater than 5 and less than 10,then
  If [$b-gt 5] | | [$b-lt 3]; then//if B is greater than 5 or less than 3,then

Second, the file directory attribute judgment

1, [-F file] to determine whether the files are ordinary files, and there is
Example: Creating a file1.sh Script
[Email protected] shell]# VI file1.sh
#!/bin/bash
f= "/tmp/aminglinux"
If [-F $f]
Then
echo $f exist
Else
Touch $f
Fi
[[email protected] shell]# sh-x file1.sh//view script execution process

• F=/tmp/aminglinux//View file path
• ' ['-f/tmp/aminglinux '] '//To compare files that do not exist
• Touch/tmp/aminglinux//Does not exist to create
  [[email protected] shell]# sh-x file1.sh//re-execute script hint file already exists
• F=/tmp/aminglinux
• ' ['-f/tmp/aminglinux '] '
• Echo/tmp/aminglinux exist
  /tmp/aminglinux exist

2, [-D file] to determine whether the document is a directory, and there is
Instance:
[[email protected] shell]# CP file1.sh file2.sh//Copy under File1,sh script to modify
[Email protected] shell]# VI file2.sh
#!/bin/bash
f= "/tmp/aminglinux"
If [-D $f]//change here to-D, judging is not
Then
echo $f exist
Else
Touch $f
Fi
[Email protected] shell]# sh-x file2.sh

• F=/tmp/aminglinux
• ' ['-d/tmp/aminglinux '] '//comparison is not a directory
• Touch/tmp/aminglinux//Create a directory

3. [-e file] Determines whether a file or directory exists
Instance
[Email protected] shell]# VI file2.sh
#!/bin/bash
f= "/tmp/aminglinux"
If [-e $f]//-E determine whether it is a file or directory
Then
echo $f exist
Else
Touch $f
Fi
[Email protected] shell]# sh-x file2.sh

• F=/tmp/aminglinux
• ' ['-e/tmp/aminglinux '] '
• Echo/tmp/aminglinux exist
  /tmp/aminglinux exist
  directories and files can be touch, if the touch when the file or directory is present, then you can change the file of three time, respectively, Atime,ctime,mtime

4, [-R File] to determine whether the document is readable
Instance:
[Email protected] shell]# VI file2.sh
#!/bin/bash
f= "/tmp/aminglinux"
If [-R $f]
Then
echo $f Readable
Fi
[[Email protected] shell]# sh file2.sh//execute script, prompt readable
/tmp/aminglinux readable

5, [-W file] to determine whether the file is writable
Instance:
[Email protected] shell]# VI file2.sh
#!/bin/bash
f= "/tmp/aminglinux"
If [w $f]
Then
Echo $f writeable
Fi
[[Email protected] shell]# sh file2.sh////Execute script, prompt to write
/tmp/aminglinux writeable
That it determines that readable writable is actually the current user who is judging the execution of this shell script, we are executing the script as root

6, [-X file] to determine whether the file is executable
Instance:
[Email protected] shell]# VI file2.sh
#!/bin/bash
f= "/tmp/aminglinux"
If [-X $f]
Then
Echo $f exeable
Fi
[[Email protected] shell]# sh file2.sh//Because there is no execute permission, our script does not define else, so there is no output information

This script is modified below
The most used in the work is: usually first determine that the file exists, if it exists, it will delete the file,&& means that the previous command after the successful execution of the subsequent command
So [-F $f] && rm-f $f This line of command is equivalent to the following four lines of command
If [-F $f]
Then
Rm-f $f
Fi
[-F $f] | | Touch $f indicates that the following touch $f command is performed if the $f file is not executed successfully
[-F $f] | | Touch $f is equivalent to the following four lines
if [!-F $f]
Then
Touch $f
Fi

Third, if special usage

1. If [-Z "$a"] This indicates what happens when the value of variable A is empty
Example:
[Email protected] shell]# VI if4.sh
#!/bin/bash
n=wc -l /tmp/lalal
If [-Z "$n"]
Then
echo Error
Exit
elif [$n-GT 100]
Then
Echo abcdef
Fi
If [-Z ' $n '] means when the value of the variable n is empty
[[email protected] shell]# sh-x if4.sh//Execute script to view the execution process
+ + wc-l/tmp/lalal
WC:/tmp/lalal:no such file or directory

• n=
• ' ['-Z '] '
• echo Error
  Error
• Exit

Complete the if4.sh script again:
[Email protected] shell]# VI if4.sh
#!/bin/bash
if [!-f/tmp/lalal]
Then
echo "/tmp/lalal not exist."
Exit
Fi
n=wc -l /tmp/lalal
If [-Z "$n"]
Then
echo Error
Exit
elif [$n-GT 100]
Then
Echo abcdef
Fi
[[Email protected] shell]# sh if4.sh//Execute script, if the/tmp/lalal file does not exist, exit the script directly
/tmp/lalal not exist.

2, if [-N "$a"] means that the value of variable A is not empty
Instance:
[[Email protected] shell]# if [-N 01.sh]; then echo OK; FI//If 01.sh is not empty, output OK
Ok
[Email protected] shell]# if [-N "$b"]; then echo $b; else echo "B is null"; Fi
B is null
If the variable b value is not empty, the output is $b, otherwise the output "B is null"
Note that if [-N 01.sh] is a file, you do not need to use quotation marks, if it is a variable in [-N "$b"], you need to use quotation marks

3, if Grep-q ' 123 ' 1.txt; Then what if the 1.txt contains a ' 123 ' row?
Example:
Requirements: In the script, you can use the results of a command as a condition to determine whether a file contains a certain string, such as the user in the system to determine whether there is user1 this user, we can use the following command to determine
[[email protected] shell]# grep-w ' user1 '/etc/passwd//-w option to match more precisely
User1:x:1000:1000::/home/user1:/bin/bash
With such a way of thinking, you can judge
[Email protected] shell]# if Grep-wq ' user1 '/etc/passwd; Then echo "User1 exist"; Fi//-q option to not display filtered content
User1 exist

4, if [!-e file]; Then what happens when the file doesn't exist?
5, if (($a <1)); Then ... Equivalent to if [$a-lt 1]; Then ...
6, [] can not use <,>,==,!=,>=,<= such a symbol

Iv. Case Judgment

A double semicolon indicates the end of the first judgment and enters the next judgment
Shell Script instance:
[Email protected] shell]# VI case.sh
#!/bin/bash
Read-p "Please input a number:" N//Ask the user to enter a digit, n as a captured variable what output value does the user enter?
If [-Z ' $n]//If the value of n is null, enter a number
Then
echo "Please input a number."
Exit 1//Exit to capture the value of variable 1
Fi

n1=echo $n|sed ‘s/[0-9]//g‘//Determine if the number entered contains letters or all letters, use sed to empty all numbers
if [!-Z $n 1]//This line or use if [-N ' $n 1 "] instead, if the judgment is not empty, exit to prompt the user to enter a number
Then
echo "Please input a number."
Exit 1
FI//If empty, the input is valid, continue to execute the following script content

If [$n-lt] && [$n-ge 0]//If it is a number, you need to determine the range of the numbers
Then
Tag=1
elif [$n-ge] && [$n-LT 80]
Then
tag=2
elif [$n-ge] && [$n-LT 90]
Then
Tag=3
elif [$n-ge] && [$n-le 100]
Then
Tag=4
Else
Tag=0
Fi
Case $tag in
1)
echo "Not OK"
;;
2)
echo "OK"
;;
3)
echo "Ook"
;;
4)
echo "Oook"
;;
*)
echo "The number range is 0-100."
;;
Esac

Logical judgment File directory attribute in shell script determine if special usage case judgment