Linux shell Bash built-in variable reference

• • Special Shell variables
  • Test operation: Two Yuan comparison
    • Arithmetic comparison
    • string comparison
  • File type operations
  • Parameter substitution and extension
  • String manipulation
  • Summary of some structures

Special Shell variables

variables	meaning
$	Foot Name
$-$9	Position parameter 1-9
${10}	Position parameter 10
$#	Number of positional parameters
"$*"	All positional parameters (as a single string)
"[Email protected]"	All positional parameters (each as a separate string)
${#*}	The number of command-line arguments passed to the script
${#*}	The number of command-line arguments passed to the script
$?	return value
$$	PID of the script process
$-	The identity passed to the script
$_	Last parameter of the previous command
$!	Process ID (PID) of the last job running in the background

Test operation: Two USD compare arithmetic comparison

operator	meaning
-eq	Equals
-ne	Not equal to
-lt or ((<))	Less than
-le or ((<=))	Less than or equal
-gt or ((>))	Greater than
-ge	Greater than or equal

string comparison

operator	meaning
=
==
!=
<	Less than (ASCII)
>	Greater than (ASCII)
-Z	String is empty
-N	string is not empty

If you use it in a double-bracket [[...]] test structure, you do not need to use the escape character \.

File type operations

operator	meaning
-E	File is
-F	Standard file
-D	Directory
-H	Hard Link Link
-L	Link
-B	Block devices
-C	Character device
-P	Pipeline
-S	Socket sockets
-T	File is associated with a terminal
-N	The file has been modified since the last time it was read
-O	The document belongs to the Lord himself.
-G	Group ID is consistent with performer
-S	The file size is not 0
-R	File has Read permission
-W	File has Write permissions
-X	File has Execute permissions
-G	Set the Sgid tag
-U	Set the suid tag
-K	Set the paste position
File1-nt File2	File1 than file File2 new
File1-ot File2	File1 than file2 old
File1-ef File2	Fiel1 and Fiel2 are hard links to the same file

Parameter substitution and extension /tr>

expression	meaning
${va R_default}	if Var is not declared, then use $default as its value *
${var=default}	if Var is not declared, Then use $default as its value *
${var:-default}	if Var is not declared, or its value is empty, then use $default as its value *
${var:=default}	if Var is not declared, or its value is empty, use $default as its value *
${var+ot Her}	if Var is declared, then its value is $other, otherwise it will be a null string
${var:+other}	if Var is set, then its The value is $other, otherwise the null string
${var? ERR_MSG}	if Var is not declared, print $err_msg*
${var:? ERR_MSG}	if Var is not set, print $err_msg*
${!varprefix*}	match before all start with Varprefix Declared variable
${[email protected]}	matches all variables declared at the beginning of Varprefix

String manipulation

An expression	meaning
${#string}	Length of $string
${string:position}	In $string, start extracting substrings from position $position
${string:position:length}	In $string, a substring of length $length is extracted starting from position $position
${string#substring}	Delete the substring of the shortest match $substring from the beginning of the variable $string
${string##substring}	Delete the substring of the longest matching $substring from the beginning of the variable $string
${string%substring}	Delete the substring of the shortest match $substring from the end of the variable $string
${string%%substring}	Delete the substring of the longest matching $substring from the end of the variable $string
${string/substring/replacement}	Use $replacement to replace the first matching $substring
${string//substring/replacement}	Use $replacement instead of all matching $substring
${string/#substring/replacement}	If the $string prefix matches the $substring, then the $replacement is used instead of the matching $substring
${string/%substring/replacement}	If the $string suffix matches the $substring, then the $replacement is used instead of the matching $substring
expr match "$string" ‘$substring‘	Matches the length of the $substring* at the beginning of the $string
expr "$string" : ‘$substring‘	Matches the length of the $substring* at the beginning of the $string
expr index "$string" $substring	The position in $string where the first character of the $substring is matched to
expr substr $string $position $length	Extracting substrings of length $length from position $position in $string
expr match "$string" ‘\($substring\)‘	Extract the $substring* from the $string's opening position
expr "$string" : ‘\($substring\)‘	Extract the $substring* from the $string's opening position
expr match "$string" ‘.*\($substring\)‘	Extract $substring* from the end of $string
expr "$string" : ‘.*\($substring\)‘	Extract $substring* from the end of $string

$substring is a regular expression.

Summary of some structures

An expression	meaning
if [CONDITION]	Test structure
if [[CONDITION]]	Extended Test structure
Array[1]=element1	Array initialization
[A-z]	The character range of a regular expression
${!variable}	Indirect variable Reference
{Command1; command2;.. commandn;}	code block
{String1,string2,string3,...}	Curly brace Extension
(Command1; command2)	Command groups executed in a child shell
array= (element1 element2 element3)	Array initialization
result=$ (COMMAND)	Executes the command in the child shell and assigns the result to the variable
> (COMMAND)	Process substitution
< (COMMAND)	Process substitution
(var = 78)	Integral type operation
var=$ ((20 + 5))	Integer operation and assigns the result to the variable
"$variable"
result=COMMAND	Run a command in a child shell and assign the result to a variable

Reference http://blog.csdn.net/u012749933/article/details/48030503

Linux shell Bash built-in variable reference