Shell common knowledge (1)

1. system environment variables after users log on to the system

$ HOME user's own directory
$ PATH: the directory to be searched during Command Execution
$ TZ Time Zone
$ MAILCHECK the number of seconds to check whether a new letter exists
$ PS1 prompt number in the Command Column
$ PS2 indicates the prompt number when Shell requests to re-input when the command has not been completed
$ MANPATH man command search path
Ii. Special Variables

$0 execution name of the program
$ N the nth parameter value of this program, n = 1 .. 9
$ * All parameters of this program
$ # Number of parameters of this program
$ PID of the program
$! PID used to execute the previous command
$? The Return Value of the previous command.
3. Variable in shell

* Any string
? Any character
[Abc] One of the three: a, B, and c
[A-n] any character from a to n
4. Special characters

\ B Return
\ C is often used to print a row without line breaks.
\ F form feed
\ R press ENTER
\ T tabulation
\ V vertical tabulation
\ Backslash itself
V. Determining file attributes

Format:-operator filename
-If the file-e exists, 1 is returned; otherwise, 0 is returned.
-If the r file is readable, 1 is returned; otherwise, 0 is returned.
-W file write return 1, otherwise return 0
-1 is returned for executable-x Files; otherwise, 0 is returned.
-O: if the file belongs to the user, 1 is returned; otherwise, 0 is returned.
-If the length of the z file is 0, 1 is returned; otherwise, 0 is returned.
-If the f file is a normal file, 1 is returned; otherwise, 0 is returned.
-If the d file is a directory file, 1 is returned; otherwise, 0 is returned.
6. Test the string

String 1 = string 2 true when two strings are equal
String 1! = String 2: true when two strings are not equal
-N string: true when the string length is greater than 0
-Z string: true when the string length is 0
String is true when the string is not empty
7. Test the relationship between two integers

Number 1-eq Number 2 equal to true
The number 1-ne number 2 is not true.
Digit 1-gt digit 2 digit 1 greater than digit 2 true
Number 1-ge number 2 Number 1 greater than or equal to number 2 true
Number 1-lt number 2 Number 1 less than number 2 true
Number 1-le number 2 Number 1 less than or equal to number 2 true
8. Logic Testing

-A and
-O or
! Non