Shell logic judgment, file attribute judgment, if special usage, case judgment

Logical-judgment logical expressions in shell scripts are used in [] (brackets):

• -lt:=little than less than
• -le:=little && wqual less than equals
• -eq:=equal equals
• -ne:=no Equal Not equal to
• -gt:=greater than greater than
• -ge:=greater && equal greater than or equal

Used in (()) parentheses:

• <,<=,==,!=,>,>=

You need to enclose it in double brackets.

Format One

• if condition; then statement; fi

  #!/bin/bash
  A=5
  If [$a-GT 3]
  Then
  echo "OK"
  Fi
  ~
  [] There are spaces between the variable and the parentheses and the logical expression

Format Two

• if condition; Then statement 1;else statement 2;fi

  #!/bin/bash
  A=5
  If [$a-GT 3]
  Then
  echo "OK"
  Else
  echo "Fault"
  Fi

Format three

• If condition, then statement 1;elif condition 2;then Statement 2;else statement 3;fi

  #!/bin/bash
  A=5
  If [$a-lt 3]
  Then
  echo "A<3"
  elif [$a-GT 6]
  Then
  echo "A>6"
  Else
  echo "Nook"
  Fi

Relationship

• The relationships between the various conditions can use logical connectors
  • Conditions a&& conditions b:a and B
  • Condition a| | Condition B:a or B

File Directory property judgment

• The IF in a shell script is often used to determine the properties of a document, such as whether it is a normal file or a directory file, determine if the file has read and write execution permissions, etc.

File Directory Property Determination options

• -E: Determine if a file or directory exists
• -D: Determine if the directory file is not present
• -F: Determine if the file is not normal and whether it exists
• -r: Determine if Read permission is available
• -W: Determine if Write permission is available

• -X: Determine if EXECUTE permission is available

  #!/bin/bash
  f= "/tmp/test/123456"
  If [-e $f]
  Then
  echo "OK"
  Else
  echo "No"
  Fi
  Determine if the file or directory exists, there is a return OK, there is no return no

If special usage

• If [-Z ' $a]: Indicates what happens when the value of variable A is empty

• If [-n ' $a]: Indicates what happens when the value of variable A is not empty

• -N and-Z do not work on the file. Can only be used for variables.

• -Z and-N are the opposite two conditions

  #!/bin/bash
  n=wc -l /tmp/test.txt
  If [$n-gt 20]
  Then
  Echo 1
  Else
  Echo 0
  Fi
  There is no syntax error in the script, but we preset/tmp/test.txt to exist.
  If the file does not exist, the script will error when it executes

  [Email protected] shell]# sh file.sh
  WC:/tmp/test.txt: No file or directory
  if.sh: Line 3rd: [:-gt: Expecting unary expression

  So, in order to avoid this kind of error, we need to write the script more rigorous
  You need to confirm the existence of the file "/tmp/test.txt" before executing "if [$n-gt 20]"

  #!/bin/bash
  n=wc -l /tmp/test.txt
  If [-Z $n]
  Then
  echo "Error"
  Exit
  If the file does not exist, the script will be rolled out here.
  The following will not be executed
  elif [$n-lt 20]
  Then
  Echo 1
  Else
  Echo 0
  Fi

  Execution Result:
  [Email protected] shell]# sh if.sh
  WC:/tmp/test.txt: No file or directory
  Error

• If grep ' 123 ' test.txt;then indicates what happens when the test.txt contains 123

  To determine that a parameter does not exist
  #!/bin/bash
  If
  Grep-wq ' user1 '/etc/passwd
  Then
  echo "User1 exist."
  Fi
  [Email protected] sbin]# sh if1.sh

  To determine that a parameter does not exist:
  #!/bin/bash
  If
  ! Grep-wq ' user1 '/etc/passwd
  Then
  echo "No User1"
  Fi

  -W: Exact match, filter a word
  -Q: Quiet mode, do not print filter results
  ！ : Take counter

  grep non-quiet mode:
  [Email protected] shell]#./file.sh
  User1:x:1005:1006::/home/user1:/bin/bash
  User1 exist.

  grep Quiet mode:
  [Email protected] shell]#./file.sh
  User1 exist.

Case Judgment

• The case statement is a multi-select statement, which can match a value with a pattern, and execute a matching command if the match succeeds.

  Case value ($ variable name) in
  value1)
  Command
  ;;
  value2)
  Command
  ;;
  VALUE3)
  Command
  ;;
  *)
  Command
  ;;
  Esac

• The case works as shown above, the value must be followed by an in, and each pattern must end with parentheses. The value will detect each pattern that matches. Once the pattern is matched, other modes are no longer resumed after the corresponding command is executed.

• If there is no match to any one of the modes, the command after the * mode is executed.

• In a case, you can use |, the meaning of the expression or in the condition

  2|3)
  Command
  ;;

• Instance

  #!/bin/bash
  Read-p "Please input a number:" N
  If [-Z "$n"]
  Then
  echo "Null,please input a number."
  Exit 1
  Exit 1 indicates the return value after executing the partial command
  That is, when the command finishes executing, use echo $? The value
  Fi

  n1=echo $n | sed ‘s/[0-9]//g‘
  Change the number in variable N to null and assign to N1
  If [-N "$n 1"]
  Determines if the N1 is not empty, then outputs the following result, and is empty to continue execution
  Then
  echo "Please input a number."
  Exit 1
  Fi

  If [$n-lt] && [$n-ge 0]
  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-lt 100]
  Then
  Tag=4
  Else
  Tag=0
  Fi
  tag is the variable name, as a label for easy reference.
  Case $tag in
  1)
  echo "D"
  ;;
  2)
  echo "C"
  ;;
  3)
  echo "B"
  ;;
  4)
  echo "A"
  ;;
  *)
  echo "The number is range 0-100"
  ;;
  Esac

  Case $tag in
  1)
  echo "D"
  ;;
  2)
  echo "C"
  ;;
  3)
  echo "B"
  ;;
  4)
  echo "A"
  ;;
  *)
  echo "The number is range 0-100"
  ;;
  Esac

  The script is to enter the test scores and determine the grade of the test scores.

• Test

  [Email protected] shell]#./case.sh
  Please input a number:95
  A
  [Email protected] shell]#./case.sh
  Please input a number:85
  B
  [Email protected] shell]#./case.sh
  Please input a number:75
  C
  [Email protected] shell]#./case.sh
  Please input a number:55
  D
  [Email protected] shell]#./case.sh
  Please input a number:101
  The number is range 0-100
  [Email protected] shell]#./case.sh
  Please input a number:7y
  Zm,please input a number.
  [Email protected] shell]#./case.sh
  Please input a number:
  Null,please input a number.

Shell logic judgment, file attribute judgment, if special usage, case judgment