Shell programming conditional test statement format plus simple case

Conditional test

• test command
  test whether the expression is established, if it is established, return 0, otherwise return other values
  Format 1: test conditional expression
  Format 2: [conditional expression]
  
  

File test

• File test format
  [operator file or directory]
• Commonly used test operators
  • -d : test is whether the directory (Directory)
  • -e : Test whether the directory or file exists (Exist)
  • -f : test whether it is a file (File)
  • -r : Test whether the current user has permission to read (Read)
  • -w : Test the current user is whether it has permission to write (Write)
  • -x : Test the current user is whether it has permission to perform (eXcute)

Integer test

• Integer value comparison format
  [integer 1 operator integer 2]
• Commonly used test operators
• -eq: equal to (Equal)
• -ne: Not Equal
• -gt: Greater Than
• -lt: Less than (Lesser Than)
• -le: Less than or equal to (Lesser Equal)
• -ge: Greater than or equal to (Greater Equal)

String test

• String comparison
  • Format 1: [String 1 = String 2]
  • Format 2: [-z string]
• Commonly used test operators
  • = : The string content is the same
  • != : The string content is different,! Means the opposite
  • -z : The string content is empty
  • -n : check if there is a string
    

Logic test

• Logic test
  • Format 1: [Expression 1] Operator [Expression 2]…
  • Format 2: Command 1 Operator Command 2…
• Commonly used test operators
• -aOr &&: logical and, meaning "and". The previous execution is successful and the execution continues, and the latter is not executed without success.
• -oOr ||: logical or, meaning "or". Or if the previous execution is successful, the latter will not be executed, and the previous execution will fail to execute the latter
• !: No Logical
  a = 5
  [ $a -ne 1 ] && [ $a !=2 ]equivalent [ $a -ne 1 -a $a !=2 ]
&&and ||the operator can be present in a normal [[ ]]condition judging structure, but if present in [ ]being given structure

if statement

if single branch statement

• Single branch structure
  
  
• For example

if double branch statement

• Double branch structure
  
  
• For example

if multi-branch statement

• Multi-branch structure
  
  
• For example
  

case branch statement

• case multi-branch structure
  
  
• Precautions
  • The case line must start with the word "in", and each mode must end with a single right parenthesis ")"
  • The double semicolon ";;" indicates the end of the command sequence
  • In the pattern string, you can use square brackets to indicate a continuous range, such as "[0-9]"; you can also use vertical bars | to indicate or, such as
    the * at the end of a|b to indicate the default mode, where * is equivalent to Wildcard
• For example:
  
• The second way
  
• Example 2