Shell script syntax-conditional test: test or [command

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Conditional test for shell script Syntax: test or [

Commandtest
Or[
You can test whether a condition is true. If the test result is true, the exit status of the command is 0. If the test result is false, the exit status of the command is 1 (note that the logical representation of the C language is exactly the opposite ). For example, test the relationship between two numbers:

$ VAR=2
$ test $VAR -gt 1
$ echo $?
0
$ test $VAR -gt 3
$ echo $?
1
$ [ $VAR -gt 3 ]
$ echo $?
1

Although it looks strange, the left square brackets[
It is indeed a command name. The parameters passed to the command should be separated by spaces.

, For example,$VAR
,-gt
,3
,]
Yes[
Command, which must be separated by spaces. Commandtest
Or[
The parameter format is the same,test
Command not required]
Parameters. To[
For example, the following table lists common test commands:

Table 31.2. Test commands

[ -d DIR ]	IfDIR If a directory exists and is true
[ -f FILE ]	IfFILE If a common file exists, it is true.
[ -z STRING ]	IfSTRING The length of 0 is true.
[ -n STRING ]	IfSTRING The non-zero length is true.
[ STRING1 = STRING2 ]	True if the two strings are the same
[ STRING1 != STRING2 ]	True if the strings are different
[ ARG1 OP ARG2 ]	ARG1 AndARG2 It should be an integer or a variable with an integer value,OP Yes-eq (Equal)-ne (Not equal)-lt (Less)-le (Less than or equal)-gt (Greater)-ge (Greater than or equal)

Similar to the C language, the test conditions can also be compared with, or, non-logical operations:

Table 31.3. Test commands with and, or, not

[ ! EXPR ]	EXPR It can be any test condition in the preceding table ,! Logical Inversion
[ EXPR1 -a EXPR2 ]	EXPR1 AndEXPR2 It can be any test condition in the preceding table,-a Representation logic and
[ EXPR1 -o EXPR2 ]	EXPR1 AndEXPR2 It can be any test condition in the preceding table,-o Indicates logic or

For example:

$ VAR=abc
$ [ -d Desktop -a $VAR = 'abc' ]
$ echo $?
0

Note: If$VAR
If the variable is not defined in advance, the shell expands to an empty string, causing a syntax error in the test condition (expanded[ -d Desktop -a = 'abc' ]
As a good shell programming habit, the variable value should always be placed in double quotation marks (expanded[ -d Desktop -a "" = 'abc' ]
):

$ unset VAR
$ [ -d Desktop -a $VAR = 'abc' ]
bash: [: too many arguments
$ [ -d Desktop -a "$VAR" = 'abc' ]
$ echo $?
1