Condition testing and judgment in the shell

Condition test:

To determine whether a demand is satisfied, it needs to be realized by testing mechanism;

How to write a test expression to implement the required tests:

(1) Execute the command, and use the command status return value to judge;

$?: Command Status return value

0: Success

1-255: Failure

(2) test expression

Test EXPRESSION

[EXPRESSION]

[[EXPRESSION]]

Note: You must have a white-space character at both ends of expression, or a syntax error;

Example:

[[email protected] tmp]# Touch A[[email protected] tmp]# [-f a][[email protected] tmp]# echo $?0[[email protected] tmp]# [-D A] [[email protected] tmp]# echo $?1[[email protected] tmp]# test-f a[[email protected] tmp]# echo $?0[[email protected] tmp ]# [[-D $?]] [Email protected] tmp]# echo $?1

Test type of bash:

Numerical test

String test

File test

Numerical test: Numerical comparison

-eq: is equal to; [$num 1-eq $num 2]

-ne: is not equal to;

-GT: Is it greater than;

-ge: is greater than or equal to;

-LT: Is it less than;

-le: is less than or equal to;

String test:

= =: whether equal;

: Is it greater than;

<: Is it less than;

! =: Not equal;

=~: Whether the left string can be matched by the pattern on the right;

-Z "STRING": Determine whether the specified string is empty, empty is true, and not empty is false;

-N "string": Determines whether the specified string is not empty, is true, empty is false;

Attention:

(1) string to be quoted;

(2) to use [[]];

file Test :

Presence Testing

-A FILE

-E FILE

The existence of the file test, the existence is true, otherwise it is false;

Existence and type testing

-B File: Whether it exists and is a block device file;

-C File: Whether it exists and is a character device file;

-D file: Exists and is a directory file;

-F file: exists and is a normal file;

-H file or-L file: exists and is a symbolic link file;

-P file: exists and is a named pipe file;

-S file: exists and is a socket file;

File permission test:

-R FILE: exists and is readable to the current user;

-W FILE: exists and is writable to the current user;

-X FILE: Exists and is executable to the current user;

Special permission tests:

-U FILE: Exists and has suid permissions;

-G FILE: Exists and has sgid permissions;

-K FILE: Exists and has sticky permissions;

Whether the file has content:

-S FILE: whether there is content;

Time stamp:

-N File: Whether the file has been modified since the last read operation;

Dependency testing:

-O File: Whether the current user is the owner of the file;

-G file: Whether the current user belongs to the genus Group of the file;

Binocular test:

File1-ef File2:file1 and FILE2 are hard links to the same inode as the same file system;

Whether File1-nt File2:file1 is new to FILE2;

File1-ot File2:file1 is older than FILE2;

Combination Test conditions:

Logical operation:

The first way:

COMMAND1 && COMMAND2

COMMAND1 | | COMMAND2

! COMMAND

[-o file] && [-R File]

The second way:

Expression1-a EXPRESSION2

Expression1-o EXPRESSION2

! EXPRESSION

[-O file-a-x FILE]

Exercise: Save the current host name to the hostname variable; If the hostname is empty or localhost.localdomain, set it to WLM;

[Email protected] tmp]# hostname=$ (hostName) [[email protected] tmp]# Hostnamelocalhost[[email protected] tmp]# echo $ HostName Localhost[[email protected] tmp]# [-Z "$hostName"-o "$hostName" = = "localhost"-o "$hostName" = = "Localhost.loc Aldomain "] && hostname wlm[[email protected] tmp]# echo $?0[[email protected] tmp]# HOSTNAMEWLM

This article is from the "Wang Liming" blog, make sure to keep this source http://afterdawn.blog.51cto.com/7503144/1863926

Condition testing and judgment in the shell