2018-4-18 Linux Learning Notes

20.5 logical judgments in shell scripts

Format 1:if condition; Then statement; Fi

Format 2:if condition; Then statement; else statement; Fi

Format 3:if ...; Then ...; Elif ...; Then ...; else ...; Fi

Logical judgment Expression:
If [$a-gt $b];
If [$a-lt 5];
If [$b-eq 10]
-GT (>); -lt (<); -ge (>=); -le (<=);-eq (= =); -ne (! =) 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

20.6 File Directory property 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

20.7 If special usage

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 []

20.8/20.9 Case Judgment

Format case variable name in
Value1
Command
;;
value2)
Command
;;
*)
Commond
;;
Esac
In a case program, you can use a |, meaning, or means in a condition, such as
2|3)
Command
;;

2018-4-18 Linux Learning Notes