Special characters in the shell

In addition to ordinary characters in the shell, there are many characters with special meanings and functions.

1. Wildcard characters

* : string of any length

? : Matches any single character

[]: Matches any one of these characters, for example:[ABC] matches a or B or C, it can also mean [a- z], but '-' is just a normal character outside [] , * and ? It also becomes a normal character in [].

2. Quotation marks

Single quote ': called a strong reference or full reference, in ' ... ' All characters in are treated as ordinary characters, including ($) (\) ('), but ' cannot appear in ' ... ', or the shell cannot determine the starting position of the single quotation mark.

Double quotation mark ": called a weak reference or partial reference, in the" ... "In addition to ($) (\) (') reserved for its special purpose, the remainder of the word nonalphanumeric as ordinary character processing.

3. Annotation characters

# : Some shell interpreters beginning with # are skipped. except #!.

#!: followed by an executable program (interpreter) that executes this script, and #! must be in the first line of the script.

4. Curly Braces

There are many usages of {} in the shell, most commonly referred to as variable prototypes such as: ${var} (which is the recommended method for referencing variables)

Variable extension of curly braces:

${var}	Take out the variable var value
${var:-default}	If Var is not defined, $default is used as its value
${var:=default}	If Var is not defined, or if the value is NULL, $default is used as its value
${var+value}	If Var is defined, the value is $value, otherwise an empty string
${var:+value}	If Var is defined and is not a null value, the value is $value, otherwise an empty string
${var? MSG}	If Var is not defined, print $msg
${var:? MSG}	If Var is not defined or unassigned, print $msg
${! prefix*} ${[email protected]}	Match all variables that start with prefix
${#STR}	Returns the length of the $STR
${str:position}	Extracting a string from a location $position
${str:position:length}	Extracting a string of length $length from the location $position
${STR#SUBSTR}	Search from the beginning of the variable $str, delete the shortest match $substr string
${str# #SUBSTR}	Search from the beginning of the variable $STR to delete the longest matched $substr string
${STR%SUBSTR}	Search from the end of the variable $STR to delete the shortest matched $substr string
${STR%%SUBSTR}	Search from the end of the variable $STR to delete the longest matched $substr string
${str/substr/replace}	Replace the first matching $substr with $replace
${str//substr/replace}	Replace all matching $substr with $replace
${str/#SUBSTR/replace}	If $str starts with $substr, the $replace is used instead of the matching $SUBSTR
${str/%substr/replace}	If the $str ends with $substr, the $replace is used instead of the matching $SUBSTR

5. Wildcard extension : (possible to match multiple permutation combinations)

such as coordinates: {x1,x2,x3}{y1,y2,y3}

# echo {x1,x2,x3}{y1,y2,y3}x1y1 x1y2 x1y3 x2y1 x2y2 x2y3 x3y1 x3y2 x3y3

such as file name:

# Touch file_{1,2} # ls file_{1,2}file_1  file_2

6. Statement block

Curly braces are also used to construct statement blocks that are separated by carriage returns between statements

7. Control characters :(that is, the ctrl+key combination, used to modify the terminal or text display, "control characters can no longer be used in scripting, can only interact with the shell")

As table:

Ctrl+b	Backspace but do not remove the preceding characters
CTRL + C	End current foreground job
Ctrl+d	Terminator, which can be used to exit the current shell or to end the current input
Ctrl+g	System output a beep
Ctrl+h	Backspace and remove the preceding characters
Ctrl+l	Clear the screen, as with the clear effect
Ctrl+i	Horizontal tab
Ctrl+k	Vertical tab
Ctrl+j	Another line
Ctrl+m	Enter
CTRL + Z	Suspending foreground jobs
CTRL + V	Manipulating Visual Block in vim
Ctrl+u	Remove all characters from the cursor to the beginning of the line

8. Other

1. Anti-quote ': command substitution, which is the same as $ (), that returns the execution result of the current command and assigns a value to the variable
2. Position parameters:
   1. $: script name itself
   2. $, $2...${10}: First argument of script, second parameter .... Tenth parameter
   3. $#: Total number of variables.
   4. $*, [email protected]: Displays all the parameters.
   5. $?: The return value of the previous command's exit.
   6. $!: ID number of the last background process.
3. Exclamation point: Usually a logical inverse, also used to perform a command in the history: for example:!100 "command with line number 100 in Hsitory"

Special characters in the shell