Shell Scripting Learning Record

Special variables

Variable	Meaning
$	File name of the current script
$#	The number of arguments passed to the script or function.
$*	All parameters passed to the script or function.
$?	The exit state of the last command, or the return value of the function.
$$	The current shell process ID. For Shell scripts, this is the process ID where the scripts are located.
[Email protected]	All parameters passed to the script or function. When enclosed by double quotation marks (""), it is slightly different from $*, as will be mentioned below.

List of operator relational operators

Operator	Description	Example
-eq	Detects whether two numbers are equal and returns true for equality.	[$a-eq $b] returns TRUE.
-ne	Detects whether two numbers are equal and returns true if they are not equal.	[$a-ne $b] returns TRUE.
-gt	Detects if the number on the left is greater than the right and, if so, returns True.	[$a-gt $b] returns false.
-lt	Detects if the number on the left is less than the right and, if so, returns True.	[$a-lt $b] returns TRUE.
-ge	Detects if the number on the left is large equal to the right, and returns true if it is.	[$a-ge $b] returns false.
-le	Detects if the left number is less than or equal to the right, and returns true if it is.	[$a-le $b] returns TRUE.

List of Boolean operators

List of string operators

Operator	Description	Example
=	Detects whether two strings are equal and returns true for equality.	[$a = $b] returns FALSE.
!=	Detects whether two strings are equal and returns true if they are not equal.	[$a! = $b] Returns TRUE.
-Z	Detects whether the string length is 0 and returns true for 0.	[-Z $a] returns false.
-N	Detects whether the string length is 0 and does not return true for 0.	[-Z $a] returns true.
Str	Detects whether the string is empty and does not return true for null.	[$a] returns TRUE.

List of file test operators

Operator	Description	Example
-B File	Detects if the file is a block device file, and returns True if it is.	[-B $file] returns FALSE.
-C file	Detects if the file is a character device file, and returns True if it is.	[-B $file] returns FALSE.
-D File	Detects if the file is a directory, and returns True if it is.	[-D $file] returns false.
-F File	Detects if the file is a normal file (neither a directory nor a device file), and returns True if it is.	[-F $file] returns TRUE.
-G file	Detects if the file has a SGID bit set, and returns True if it is.	[-G $file] returns false.
-K File	Detects if the file has a sticky bit set (Sticky bit), and returns True if it is.	[-K $file] returns false.
-P File	Detects if the file is a named pipe, and returns True if it is.	[-P $file] returns false.
-U file	Detects if the file has a SUID bit set, and returns True if it is.	[-U $file] returns false.
-R File	Detects if the file is readable and returns true if it is.	[-R $file] returns TRUE.
-W File	Detects if the file is writable and returns true if it is.	[-W $file] returns TRUE.
-X File	Detects if the file can be executed and, if so, returns True.	[-X $file] returns TRUE.
-S file	Detects whether the file is empty (the file size is greater than 0) and does not return true for null.	[-S $file] returns TRUE.
-E File	Detects whether the file (including the directory) exists and, if so, returns True.	[-e $file] returns TRUE.

Local variables

Func5 () {

Locally temp=$[$value +3] Local keyword, defining a locals variable

result=$[$temp * 2]

}

Value=6 by default, any variable defined in the script is a global variable that can be accessed within a function

Exit

Exit is a library of functions that exits the shell and returns the given value

In general, exit 0 means that the program ends normally

If exit is not a value of 0, the program produces an error.

In the case of the shell, after calling your program in the shell, use echo $? command to see the exit value of your program. In a shell script, it is usually based on the last command's $? Value to perform some process control

Cd/home/lftest | | Exit 1 Enter the directory where the script is located or exit

Return

The return value of the function, the statement after return no longer executes

#!/bin/bash

Fun () {

if [$ = "1"];then

echo "Arg=1"

Return 1

Else

Return 2

Fi

echo "345" #不会再执行

}

Fun 1

echo $?

Output: arg=1

1

Shell Scripting Learning Record