Conditional statements of Shell scripts
condition test
1. Document testing
2, integer test
3. String and logic tests
If statement:
if single branch statement
if double branch statement
if multi-branch statement
if nested statement
Test command:
Tests whether a particular expression holds true. When the condition holds, the return value of the test statement is 0, otherwise it is another value.
Format 1: test conditional expression
Format 2:[conditional expression] //note spaces
file test
File testing refers to judging whether a file or directory corresponds to a given path name, or whether a file is readable, writable, executable, etc.
Common test operators:
-d: Test whether it is Directory
-e: Test whether the directory or file exists (Existing)
-f: Test whether it is a file (File)
-r: Test whether the current user has read permission
-w: Tests whether the current user has permission to write
-x: Tests whether the current user has permission to execute (eXcute)
Actual operation:
Format 1:
test -d /etc/sysconfig
echo $? (If the output value is 0, the condition is true)
Format 2:
[ -d /etc/sysconfig ]
echo $? (If the output value is 1, the condition is not true)
Compare integer values:
Format: [integer 1 operator integer 2 ]
Common test operators:
-eq =(Equal)
-ne: not equal
-gt: Greater Than
-lt: Lesser Than
-le: Less or Equal
-ge: Greater or Equal
string comparison
Format 1:[String 1 = String 2 ]
[String 1 ! = string 2 ]
Format 2:[ -z string]
Common test i operators:
=: String content identical
!=：String content is different,! expresses a contrary intention
-z: string content is empty
logic test
Format 1:[Expression 1 ] Operator [Expression 2 ]...
Format 2: Command 1 Operator Command 2...
Common operation options
-a or &&: logical and, meaning "and"
-o or|| logical OR logical OR
!：Logic no means "no."
If statement:
Single branch structure:
if conditional test operation
then
command sequence
fi
Double branch structure:
if conditional test operation
then
Command sequence 1
else
Command Sequence 2
fi
Multi-branch structure:
if conditional test operation
then
Command sequence 1
elif conditional test operation 2
then
Command Sequence 2
else
Command sequence 3
fi
If nested statements:
Objective: Run a race, enter the finals within 10 seconds, enter gender, prompt to enter men's group or women's group respectively, use multiple judgments, nested if to achieve
