linux[basic]16-shell Judging user parameters [01]

Shell to determine user parameters

Conditional test statements are able to test the characteristics of an expression that is true

The return value is 0 when the condition is set

Otherwise, return other values

To test the statement format:

[ conditional expression ] There should be a space on both sides

The test statements are:

File test

Logic test

Integer value Comparison

string comparison

File test:

[ operator file or directory name ]

Parameters	Role
-D	Whether the test is a directory
-E	Test whether a file or directory exists
-F	Determine if the file is
-R	Tests whether the current user is authorized to read
-W	Test whether the current user can write
-X	Test whether the current user is executable

Instance:

[[Email protected] ~]# [-D/ETC] # to determine if it is a directory

[[email protected] ~]# echo $?

0

[[Email protected] ~]# [-f/etc/inittab] # to determine if it is a file

[[email protected] ~]# echo $?

0

[[Email protected] ~]# [-r/etc/inittab] # to determine if the current user is readable

[[email protected] ~]# echo $?

0

[[Email protected] ~]# [-w/etc/inittab] # Determine if the current user can write

[[email protected] ~]# echo $?

0

[[Email protected] ~]# [-x/etc/inittab] # Determine if the current user can execute

[[email protected] ~]# echo $?

1

Example 2:

[Email protected] ~]# [-e/dev/cdrom] && echo "Exist"

Exist

Logic test:

[ expression 1] operator [ expression 2]

Operator	Role
&&	Logical AND, "and" meaning
||	Logical OR, "or" of the above
!	Logic's No

Example 1:

Root User under:

[[Email protected] ~]# [$USER! = root] && echo "USER"

[Email protected] ~]#

Under normal User:

[[Email protected] ~]$ [$USER! = root] && echo "USER"

User

[Email protected] ~]$

Example 2:

Root User under:

[[Email protected] ~]# [$USER! = root] && echo "USER" | | echo "Root"

　　　　　　Root

[Email protected] ~]#

Integer value comparison:

[ integer 1 operator integer 2]

Operator	Role
-eq	Determine if it equals
-gt	Judging if it is greater than
-lt	Judging if it is less than
-le	Determine if it equals less than
-ge	Determine if it equals greater than

Example 1:

[Email protected] ~]# [10-GT] # to determine if it is more than ten

[[email protected] ~]# echo $?

1 no

Example 2:

[Email protected] ~]# [10-eq] # to determine if it equals ten

[[email protected] ~]# echo $?

0 is

Example 3:

[Email protected] ~]# freemem= ' free-m | grep cache: | awk ' {print $} '

[Email protected] ~]# echo $FreeMem

205

[[Email protected] ~]# [$FreeMem-lt 1024x768] && echo "Out of memory "

Not enough memory

[Email protected] ~]#

string comparison:

[ string 1 operator string 2]

Operator	Role
=	Compare strings for the same content
!=	Compare strings for different content
-Z	Determines whether the string contents are empty

Example 1:

[[Email protected] ~]# [-Z $String] # Determine if the variable is empty

[[email protected] ~]# echo $?

0

Example 2:

[Email protected] ~]# [$LANG! = "en. US "] && echo" not en. US "

# determine if the system language is en.us

Not en. US

linux[basic]16-shell Judging user parameters [01]