Shell Boolean operation

1. The true and false values in bash are not the values 0 and 1 we generally think.

True and false are built-in shell commands that return logical values. For example:

$ True

$ Echo $?

0

$ False

$ Echo $?

1

$? Is a special variable that stores the ending state of the previous program.

In Shell, 0 indicates whether the program is successfully completed.

2. In shell, the logical value indicates the return value of the program. If the program returns successfully,

True. If the return is unsuccessful, the return value is false.

3. The test command can be used for numerical testing, string testing, and file testing.

* Numerical test:

$ If Test 5-EQ 5; then Echo "yes"; else echo "no"; FI

Yes

$ If Test 5-ne 5; then Echo "yes"; else echo "no"; FI

No

* String Testing

$ If test-n "not empty"; then Echo "yes"; else echo "no"; FI

Yes

-N: the length of the test string cannot be zero.

$ If test-z "not empty"; then Echo "yes"; else echo "no"; FI

No

-Z: the length of the test string is zero.

$ If test-z ""; then Echo "yes"; else echo "no"; FI

Yes

* File test

$ If test-F/boot/system. Map; then Echo "yes"; else echo "no"; FI

Yes

-F is used to test whether a file exists and whether it is a common file.

$ If test-D/boot/system. Map; then Echo "yes"; else echo "no"; FI

-D is used to test whether the file exists and whether it is a directory.

4. Test logical combination test

* And, for example:

$ A = 5; B = 4; C = 6;

$ If test $ A-EQ 5-A $ B-EQ 4-A $ C-EQ 6; then Echo "yes"; else echo "no"; FI

Yes

* Or, for example:

$ If test $ A-EQ 5-o $ B-ne 4; then Echo "yes"; else echo "no"; FI

Yes

* Not, for example:

$ If test! -F/etc/profile; then Echo "yes"; else echo "no"; FI

No

5. Test and &, |

$ Str1 = "test"; str2 = "test"

$ If test-z "$ str1"-o "$ str2" = "$ str1"; then Echo "yes"; else echo "no"; FI

Yes

$ If test-z "$ str1" | test "$ str1" = "$ str2"; then Echo "yes"; else echo "no"; FI

Yes

The two test statements are equivalent. But we can see that-O is used inside test, and | used between test

Test can be replaced by []. Note that spaces are required before and after [].

When testing a string, we recommend that all variables be enclosed in double quotation marks to prevent null strings represented by variables.

Note the following when testing with brackets:

$ I = 5

$ If [$ I = 6]; then Echo "yes"; else echo "no"; FI

Yes

$ If [$ I = 6]; then Echo "yes"; else echo "no"; FI

No

6. Command list

* For &, if the preceding command is successfully executed, continue to execute the following command; if it fails, do not execute the following command

* For |, if the preceding command is not successfully executed, continue to execute the following command.

For example:

Ping-C1 www.lzu.edu.cn & Echo "connected"

* The command list is sometimes used to replace branch structures such as if/then, so that some code can be omitted. References ================================ Shell programming example -- by Falcon

Shell Boolean operation