Linux shell 条件表达式

Conditional judge

the result of conditional judge is  true  or false.

0 --- true  and  Non-0  --- false

test  cond_expr or [ cond_expr ]

cond_expr:

file type and the permission of the file

string content and the length

compare values

Expression

expression If expression is true, !expression is false

expression1 –a expression2 Both expression1 and expression2 are true

Expression1 –o expression2 Either expression1 or expression2 are true

String

-n string The length of string is nonzero

-z string The length of string is zero

string1 = string2 The strings are equal

string1 != string2 The strings are not  equal

string1 The string1 is not null

Integer

interger1 –eq interger2 Interger1 is equal to interger2

interger1 –ge interger2 Interger1 is greater than or equal to interger2

interger1 –gt interger2 Interger1 is greater than interger2

interger1 –le interger2 Interger1 is less than or equal to interger2

interger1 –lt interger2 Interger1 is less than interger2

interger1 –ne interger2 Interger1 is not equal to interger2

File

file1 –ef file2 file1 and file2 have the same device and inode number

file1 –nt file2 file1 is newer(modification date) than file2

file1 –ot file2 file1 is older than file2

-b –c –d file file exits and is block/character/directory type

-e file exits

-s file exists and the length bigger than 0

-f file is a normal file

-r –w -x file exits and is readable/writable/executable

-L  -h file exists and is a symbolic link