The logical operators of the shell involve the following types, so as long as the appropriate choice, can solve our many complex judgments, to achieve a multiplier effect.
first, the logical operator
Logical Volume Label |
Express meaning |
1. |
about the file and directory of the detection of Logical volume label! |
-F |
Common! Detect if "Archive" exists eg:if [-f filename] |
-D |
Common! Detects if the directory exists |
-B |
Detects if it is a "block file" |
-C |
Detects if it is a "character file" |
-S |
Detects if it is a "socket tag file" |
-L |
Detects if it is a "symbolic Link's profile" |
-E |
Detect if "something" exists! |
2. |
about the logical volume label of the program! |
-G |
Detect whether the program owned by the GID |
-O |
Detection is owned by the program executed by the UID |
-P |
Detects if a name pipe or FIFO is being transmitted between programs (to be honest, this is not a good idea!). ) |
3. |
about the property detection of the file! |
-R |
Detect if a property is readable |
-W |
Detects if a property can be written |
-X |
Detect whether the property is executable |
-S |
Detects if it is a "non-blank file" |
-U |
Detect if a property with "SUID" is |
-G |
Detect if a property with "SGID" is |
-K |
Detect if a property with "sticky bit" is |
4. |
judgments and comparisons between two files ; for example [Test file1-nt file2] |
-nt |
The first file is newer than the second one. |
-ot |
The first file is older than the second one. |
-ef |
The first file is the same file as the second file (link and other files) |
5. |
Logical AND (and) "or (OR)" |
&& |
The meaning of the logical and |
|| |
The meaning of a logical OR |
Operation symbols |
Representative meaning |
= |
Equal to: Integer or string comparison if in [], only the string |
!= |
Not equal to: integer or string comparison if in [], only the string |
< |
Less than applied: integer comparison in [], cannot use the representation string |
> |
Greater than applied: integer comparison in [], cannot use the representation string |
-eq |
equals applies To: integer comparison |
-ne |
Not equal to: integer comparison |
-lt |
Less than applied to: integer comparison |
-gt |
Greater than applied to: integer comparison |
-le |
Less than or equal to: integer comparison |
-ge |
Greater than or equal to: integer comparison |
-A |
Both sides established (and) logical Expressions –a logical expressions |
-O |
Single-sided (or) logical expression –o logical expression |
-Z |
Empty string |
-N |
Non-empty string |
Ii. Logical Expressions
How to use:test EXPRESSION
Such as:
[root@localhost ~]# Test 1 = 1 && echo ' OK '
Ok
[Root@localhost ~]# test-d/etc/&& echo ' OK '
Ok
[Root@localhost ~]# Test 1-eq 1 && echo ' OK '
Ok
[Root@localhost ~]# if test 1 = 1; Then echo ' OK '; Fi
Ok
Note: All characters are separated from the logical operators directly with "spaces" and cannot be joined together.
[Root@localhost ~]# [1-eq 1] && echo ' OK '
Ok
[ [email protected] ~]# [2 < 1] && echo ' OK '
-bash:2: No such file or directory
[ [email protected] ~]# [2 \< 1] && echo ' OK '
[ [email protected] ~]# [2-GT 1-a 3-lt 4] && echo ' OK '
OK
[[Email protected] ~]# [2-GT 1 && 3-lt 4] && echo ' OK '
-bash: [: Missing '] '
Note: In the [] expression, the common >,< need to add an escape character, which represents the string size comparison, as compared to the Acill code position. Do not directly support the <> operator, there are logical operators | | && it needs to be represented by-a[and]–o[or]
[[Email protected] ~]# [1-eq 1] && echo ' OK '
Ok
[[Email protected] ~]$ [[2 < 3]] && echo ' OK '
Ok
[[Email protected] ~]$ [[2 < 3 && 4 > 5]] && echo ' OK '
OK
Note: The [[]] operator is only an extension of the [] operator. The ability to support the <,> symbol operation does not require an escape character, it is also a string comparison size. Logical operators are supported inside: | | &&
Third, performance comparison
There are three almost equivalent symbols and commands in the conditional expression of bash: test,[] and [[]]. Usually, everyone is accustomed to using if [];then this form. The appearance of [[]], according to ABS, is to be compatible with operators such as ><. The following is a comparison of their performance, found [[]] is the fastest.
$ time (for M in {1..100000}; do test-d.; Done;)
Real 0m0.658s
User 0m0.558s
SYS 0m0.100s
$ time (for M in {1..100000}; do [-D.]; Done;)
Real 0m0.609s
User 0m0.524s
SYS 0m0.085s
$ time (for M in {1..100000}; do [[-D.]]; Done;)
Real 0m0.311s
User 0m0.275s
SYS 0m0.036s
Regardless of the low version of Bash and the compatibility of SH, with [[]] is strong compatibility, and performance is relatively fast, in the case of a conditional operation, you can use the operator.
Linux shell logical operators, logical expressions