Shell basic Syntax (ii)

If statementBasic format

Format 1:if condition; Then statement; Fi

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

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

can use && | | Combine multiple conditions

If [$a-gt 5] && [$a-lt 10]; Then

If [$b-gt 5] | | [$b-lt 3]; Then

Arithmetic comparison operators

Num1-eq num2 equals [3-eq $mynum]

Num1-ne num2 Not equal to [3-ne $mynum]

Num1-lt num2 less than [3-lt $mynum]

Num1-le num2 less than or equal to [3-le $mynum]

NUM1-GT num2 greater than [3-GT $mynum]

Num1-ge num2 greater than or equal to [3-ge $mynum]

The judgment of the document

-e filename true if filename exists [-e/var/log/syslog]

-D filename True if filename is a directory [-d/tmp/mydir]

-F filename True if filename is a regular file [-f/usr/bin/grep]

-L filename True if filename is a symbolic link [-l/usr/bin/grep]

-R filename True if filename is readable [-r/var/log/syslog]

-W filename if filename is writable, true [-w/var/mytmp.txt]

-X filename is true if filename is executable [-l/usr/bin/grep]

Special usage

If [-Z ' $a '] This indicates what happens when the value of variable A is empty (develop a good habit, be sure to add "double quotation marks" to the value of the judgment; if "file" can be omitted)

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 []

# if Grep-wq ' Juispan '/etc/passwd; Then echo "Juispan already exists"; Fi

Juispan already exists

Case StatementBasic format

Case variable name in

value1)

Command

;;

value2)

Command

;;

*)

Commond

;;

Esac

If one of the value in the case is the same, you can also write this:

2|3) # # "|" The meaning of an expression or

Command

;;

Shell basic Syntax (ii)