Shell Note 2

1.Linux default shell is bash,unix default shell is sh

Bash is fully compatible with SH

2. Define variables:

When defining a variable, the variable name does not have a dollar sign ($), and a variable is used before the variable name plus $

There can be no spaces between the variable name and the equals sign

Variable names can use characters that are numbers, letters, and underscores only

The first character must be a letter

The curly braces outside the variable name are optional, plus the curly braces are used to help the interpreter identify the bounds of the variable, for example: ${skill}script

3. Thereadonly command can define a variable as a read-only variable

unset command to delete a variable

4.

List of special variables

variables	meaning
$	File name of the current script
$n	Arguments passed to the script or function. N is a number that represents the first few parameters. For example, the first parameter is $ $, and the second argument is $ A.
$#	The number of arguments passed to the script or function.
$*	All parameters passed to the script or function.
[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.
$?	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.

5.shell variable Substitution

ECHO-E represents the substitution of escape characters:

1. A=ten
2. echo- e "Value of A is $a \ n"

Output:

   Value of A is 10

Command substitution:

' Command ' (note is the anti-quote, not single quote, this key is located below the ESC key.) )

6. Arithmetic operations using expr

Expr 3 + 2 (spaces between expressions and operators)

Expr 3-2

Expr 3 \* 2 (multiplication sign front plus backslash \)

Expr 3/2

Output is: 4

7. Conditional expressions

List of 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.

Conditional-expression Syntax:

 1  if  [$a-lt 100 -a $b-gt 15  ]  2  then  3  echo   "  $a-lt 100-a $b-gt 15:returns true   " 4  else  5  echo   "  $a-lt 100-a $b-gt 15:returns false   6  fi  

Conditional expressions are placed between square brackets and have spaces, such as [$a = = $b] is wrong and must be written as [$a = = $b]

List of Boolean operators

operator	Description	Example
!	Non-operation, the expression is true returns False, otherwise true.	[! false] returns TRUE.
-O	Or operation, there is an expression of true to return true.	[$a-lt 20-o $b-GT 100] returns TRUE.
-A	With an operation, two expressions are true to return true.	[$a-lt 20-a $b-GT 100] returns FALSE.

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.

Shell Note 2