Conditional expressions in the shell

by Francis_hao Oct 1,2017

This article extracts the Man Handbook from bash

Conditional expressions

The conditional expression is composed of a composite command [[or] built-in command test and [used for testing file properties and string or numeric comparisons. The expression form is shown in the following table, and if the file form is/dev/fd/n, then the document descriptor Nwill be tested, if it is/dev/stdin,/dev/stdout, or/dev/stderr. Then the file descriptor 0,1 and 2 will be tested. Unless otherwise specified, testing a connection is a test of the file it points to, not the connection itself.

An expression	Description
File Properties Test
-A file	True if file exists
-B File	True if file exists and is block
-C file	True if file exists and is character files
-D File	True if file exists and is a directory
-E File	True if file exists
-F File	True if file exists and is normal
-G file	If file exists and the set setting-group ID is true, see Section 4.4, 12.3 of the reference "1"
-H file	True if file exists and is a symbolic connection
-K File	If file exists and its sticky position is true, refer to man chmod
-P File	True if file exists and is a command pipeline (FIFO)
-R File	True if file exists and is readable
-S file	True if file exists and its length is greater than 0
-T FD	True if the file descriptor fd is open and points to a terminal
-U file	If file exists and is set-user-id is true, see Section 4.4 of Reference "1"
-W File	True if file exists and is writable
-X File	If file exists and is executable
-G file	True if file exists and is owned by a valid group ID, see Reference "1" section 4.4
-L File	True if file exists and is a symbolic connection
-N File	True if file exists and has been modified since the last read (modified)
-O File	True if file exists and is owned by a valid user ID, see section 4.4, "1"
-S file	True if file exists and is a socket
File1-ef File2	True if File1 and file2 are pointing to the same device's inode
File1-nt File2	If file1 is newer than file (modified), or file1 exists file2 does not exist in the true
File1-ot File2	If File1 is older than file (modified), or file1 exists file2 does not exist in the true
Shell-related
-O optname	If the shell option Optname is true, refer to the-O option for the built-in instruction set
-V varname	True if the shell variable varname is set
String test
-Z String	True if string length is 0
String -N String	True if string length is not 0
string1 = = string2 string1 = string2	True if string1 and string2 are equal, = should only be used by test
String1! = string2	True if the strings are not equal
String1 < string2	True if the dictionary order string1 before string2
String1 > string2	True if the dictionary order string1 after string2
Numerical test
Arg1 OP arg2	OP is one of-eq,-ne,-lt,-le,-GT, or-ge, and the value can be negative.

Compound command [[

[[ expression ]]

returns 0 or 1 based on the result of the conditional expression expressions. in [[and]], Word segmentation and path expansion are not performed, other extensions are executed, and conditional operators, like-F, must be non-referenced to be recognized as operators. The < and > operators use the current locale for dictionary sorting.

When using the = = and! = operators, the operands on the right can be a pattern (pattern Matching), and if you want to match the pattern itself, use quotation marks.

Another available two-dollar operand, =~, has the same precedence as = = and! =. When it is used, the operand on the right is considered an extended regular expression (see reference "2"), and if the option to enable Shell is nocasematch, then the expression is not case sensitive.

Expression can be combined using the following operators, with precedence ordered from high to low

Operator	Description
(expression)	Used to change the priority level
! Expression	The result of expression is reversed
Expression1 && expression2	Logic and, if expression1 enough to show the true and false of the whole expression, then expression2 will not be evaluated
expression1 | | Expression2	Logical OR, if expression1 is sufficient to indicate the true and false of the entire expression, then expression2 will not be evaluated

Built-in command test and [

Test Expr
[ expr ]

Depending on the result of the conditional expression expr returns 0 or 1, each operator and operand must be a separate parameter, and test does not accept any options, even the end of the argument--or not.

The expression can be combined by the following operators, with precedence ordered from high to low

Operator	Description
! Expr	The result of expr is reversed
(expr)	Used to change the priority level
Expr1-a EXPR2	Logic and
Expr1-o EXPR2	Logical OR

The < and > operators use ASCII for dictionary sorting.

This article was authored by Liu Yinghao and licensed under the Creative Commons Attribution-NonCommercial use-Share 3.0 Chinese mainland license agreement in the same way. Welcome reprint, please specify the source:
Reprinted from: http://www.cnblogs.com/yinghao1991/p/7617965.html

Reference

"1" W.richard Stevens, Stephen a.rago Hu Zhengwei Zhangya Yingyu translation "Advanced Programming of UNIX Environment (3rd edition)" Beijing: People's post and post publishing house 2014.6

"2" blog Park http://www.cnblogs.com/yinghao1991/p/7617004.html posix.2 Regular expression

"3" Man sh

Conditional expressions in the shell