First, conditional judgment statement
Meet the criteria to execute the corresponding command
[Condition]: more commonly used in scripts
Cases:
[-f/etc/hosts]
Test condition: More commonly used for Linux character terminals
1. File or directory judgment
-F: Determines whether the file is
-D: Determine if the directory
-E: Determine if there is
-R: Determine if the file/directory has Read permissions
-W: Determines whether the file/directory has write permissions
-X: Determines whether the file/directory has execute permissions
Cases:
[-f/etc/hosts]//Determine if/etc/hosts is a file, as a file, then echo $? must be 0.
[!-f/etc/hosts]; Touch/etc/hosts//Determine if/etc/hosts is a file, such as not a file, echo $? must be 0.
[-D/ETC]//To determine if/etc is a directory, as a directory, then echo $? must be 0.
[-w/etc/hosts]//To determine if/etc/hosts has write permission, such as Write permission, then echo $? must be 0
[-e/boot]//Determine if/boot exists, if present, then echo $? must be 0
2. Numerical comparison
-EQ: Determine whether the values on both sides are equal
-ne: Judging whether the values on both sides are unequal
-GT: Determine if the left value is greater than the right value
-ge: Determine if the left value is greater than or equal to the right value
-LT: Determine if the left value is less than the right value
-le: Determine if the left value is less than or equal to the right value
Cases:
[1-eq 1]//judgment 1 is equal to 1, if Equal then echo $? must be 0
[3-le 5]//judgment 3 is less than or equal to 5, such as less than equals the echo $? Yes 0
Note: Numeric comparisons can only compare integers
3. String comparisons
= =: Determine whether the strings are exactly equal on both sides
-Z: Determine if the value of the required variable is not empty
Cases:
[$PWD = =/root]//Determine if the PWD variable value equals/root, if equal to echo $? must be 0.
[! $PWD = =/root]//Determine if the PWD variable value equals/root, if not equal to echo $? definitely 0.
[-Z $PWD]//Determine if the PWD variable value is empty, or if it is empty, echo $? must be 0.
Note: string refers to letters, numbers, Chinese
4. Logic Test
&& or; Or-A//and relationship, the condition is set up to continue execution; make && make install (continuation of subsequent commands after successful execution); Find/-name A *-a-type f
|| OR-o//or relationship, which satisfies any condition to continue execution; find/-name A *-o-type f
! Conditional inversion
Second, if statement
Single BRANCH statement: satisfies the condition, executes the following command; command effect with [-f/etc/a] && chmod +x/etc/a
If [conditional judgment statement]
Then
Command
Fi
Cases:
if [-f/etc/a]
Then
chmod +x/etc/a
Fi
Two-branch statement: satisfies the condition, then executes the command one, if does not satisfy the condition, executes the command two
If [conditional judgment statement]
Then
Command One
Else
Command two
Fi
Multi-branch statement: satisfies the judgment one executes the command, if does not satisfy then continues to judge two, satisfies then executes the command two, XXXX, if does not satisfy then executes the command x
If [conditional judgment statement I]
Then
Command One
elif [Conditional Judgment Statement II]
Then
Command two
elif [Conditional Judgment Statement III]
Then
Command three
Else
Command x
Fi
#!/bin/bash
Read-p "Please enter your score: [1-100]" A
If [$A-le 100]
Then
If [$A-eq 100]
Then
echo "优"
elif [$A-le] && [$A-ge 70]
Then
echo "良"
elif [$A-le] && [$A-ge 60]
Then
echo "及格"
Else
echo "你退群吧!!!"
Fi
Else
Echo, "You can not do it."
Fi
Note: If first determine whether the user input value within 100, such as within 100 to continue to judge, if not the return "You can not do"
Shell condition judgment statement and if