Common variables, end states, and test expressions under the shell in POSIX

In the open-source Unix system usually for the convenience of management will be a lot of use of shell script, and in peacetime writing script when the fast and effective is a lot of built-in variables can be used, here is a simple summary of the arrangement, so that in peacetime writing script, quickly consult, The built-in variables for the shell in POSIX are as follows:

variables	meaning
#	Number of parameters for the current process
@	Command-line arguments passed to the current process
-	Hyphen, giving shell parameter options when referencing
?	Exit status of the previous command
$	The process number of the shell process, typically using the-bash process
0	Name of the Shel program
！	Process number of the most recent background command
Env	Once referenced, it is used in the interactive shell
HOME	root directory, root user is/root
Ifs	Field divider, equivalent to (tab) key
Lang	Default name of the current locale
Lc_all	The name of the current locale
Lc_collate	The current locale name used to sort the characters
Lc_messages	The name of the current language for the output information
Lineno	The line number of the row that was just executed in the script or function
Nlspath	The directory location for the current language of the output information
PATH	Command Lookup Path
PPID	Process number of the parent process
PS1	The main command prompt string, default is "$", root privilege is "#"
PS2	Line continuation prompt string ">"
PS4	The string of traces executed with Set-x, which is "+" by default, is typically used when testing the trace script with Sh-x *.sh
Pwd	Current working directory

If you don't understand what the above variable is, Echo looks

[[email protected] data]# echo $PWD/data[[email protected] data]# ps-ef|grep ' echo $$ ' root 16174 16172 0 00:37 PTS/1 00:00:00-bashroot 22598 16174 0 13:37 pts/1 00:00:00 ps-efroot 22599 16174 0 13:37 pts/1 00:00:00 gr EP 16174

There are 6 kinds of end states in POSIX:

Value	Significance
0	Command successfully exited
>0	Failure during a redirect or a variable, command, etc unwind
1-125	command does not exit successfully
126	Command found but the file could not be executed
127	Command not Found
>128	The order was killed by the receipt of the signal

Here's a simple example:

[[email protected] test]# lstest1[[email protected] test]# ls-l test1-rw-r--r--1 root root 0 09-16 13:51 test1[[email p    Rotected] test]# echo $?0[[email protected] test]# ls-l test2ls:test2: No file or directory [[email protected] test]# echo $? 2

And the test expression is more important, usually write script when a large number of if judgment will be used to

Operator	If ... is True
String	string is not NULL
-B File	File is a block of device files
-C file	File is a character device files
-D File	File is a directory
-E File	File exists
-F File	File is generic
-G file	File has set Setgid bit
-H file	File is a symbolic connection
-L File	Ditto
-N String	string is non-null
-P File	File is FIFO
-R File	File is readable
-S file	File is the socket
-S file	File is not empty
-T N	File descriptor n points to a terminal
-U file	File has set setuid bit
-W File	File is writable
-X File	File is executable
-Z String	string is null
S1 = s2	String S1 is the same as S2
S1! = S2	String S1 is not the same as S2
N1-eq N2	Integer n1 equals integer n2
N1-ne N2	Integer n1 not equal to integer n2
N1-lt N2	N1 less than N2
N1-GT N2	N1 greater than N2
N1-le N2	N1 less than or equal to N2
N1-ge N2	N1 greater than or equal to N2

This article from "Technical essay" blog, declined reprint!

Common variables, end states, and test expressions under the shell in POSIX