Logical judgments in shell scripts, file directory attribute judgments, special use of if, case judgments

1. Logical judgment in shell scripts

Format 1:if condition; then statement; Fi
Format 2:if condition; then statement; Else statement; Fi
Format 3:if ...; Elif ...; Then ...; else ...; Fi
Logical judgment expression:if [$a-gt $b], if [$a-lt 5], if [$b-eq 10]-gt (>);-lt (<);-ge (>=);-le (<=);-eq (= =); n E (! =) Note that there are spaces everywhere
can use && | | Combine multiple conditions
If [$a-gt 5] && [$a-lt 10]; Then
If [$b-gt 5] | | [$b-lt 3]; Then

Single-Branch judgment:

Single-Branch judgment 2:

Multiple judgments:

#!/bin/bash
A=$1
If [$a-gt 5];then
echo "More than 5"
elif [$a-eq 5];then
echo "number entered is equal to 5"
Else
echo "$ Five Less"
Fi

2. File directory attribute judgment

[-F file] Determines if it is a normal file, and there is
[-D file] Determines if it is a directory and exists
[-E file] to determine whether files or directories exist
[-R File] to determine if the document is readable
[-W file] Determines whether the file is writable
[-X file] Determines whether the file is executable

3. Special use of If

If [-Z ' $a '] This indicates what happens when the value of variable A is empty
If [-n ' $a '] means that the value of variable A is not empty
If Grep-q ' 123 ' 1.txt; Then what happens if the 1.txt contains a ' 123 ' row
if [!-e file]; Then what happens when the file doesn't exist?
if (($a <1)); Then ... Equivalent to if [$a-lt 1]; Then ...
Symbols such as <,>,==,!=,>=,<= cannot be used in []

4. Case Judgment
Usage:

Case $tag in
1)
echo "Not OK"
;;
2)
echo "OK"
;;
3)
echo "Ook"
;;
4)
echo "Oook"
;;
*)
echo "The number range is 0-100."
;;
Esac

Case:

Then use case to judge the script:

Logical judgments in shell scripts, file directory attribute judgments, special use of if, case judgments