Shell_ 03_ environment variable

environment variable

environment variable ： Replace settings or retain data with a set of strings （ Stored in memory ）

1） Environment variables are divided into ： Global variables 、 local variable

Global and local variables are for parent / Son Shell The difference between ：

1） Global variables for all Shell And his son Shell Can be called ;

2） A local variable can only be created by Shell call ;

Variable call form ：${ Variable name }

1） Can also pass “$ Variable name ” Form call of

2） If you call the form of the specified value in the array variable ：${ Variable name [ Index value ]}

When calling variables , Usable symbols ：

Symbol	explain
“ ” （ Double quotes ）	Enable the system to recognize variables Realize escape （ Such as ：“/*”）
‘ ’ （ Single quotation marks ）	Make the system unable to recognize variables Can't escape （ Only the contents of single quotation marks will be output as is ）
` ` （ The quotation marks ）	Assign the data obtained after executing the command in the script to the variable Form the enclosed content into a complete expression
\ （ The backslash ）	Convert special symbols to general characters Put it in front of a special character to indicate escape

set command ： List the current shell All environment variables in the environment

Command format ：set

1） Also list global variables 、 Local variables and user-defined variables ;

2） The order listed is alphabetical ;

Such as ： Inquire about set Variables of the command prompt in the file

Symbol	meaning
\d	Show “ week month Japan ” Information
\H	Displays the full host name
\h	Display only the name before the first decimal point of the host name
\t	Display time 24 Hours “HH：MM：SS” The format of
\u	Displays the name of the current user
\v	Show BASH Version information
\w	Displays the complete current working directory name
\W	Only the last directory name of the current working directory is displayed
\#	Displays the number of commands executed by the current terminal
\$	Display user identifier

System variables

Bash Shell At system startup , Special environment variables will be created by default as system variables

1） The name of the system environment variable is all uppercase by default （ Distinguish user variables ）

env command （environment）： List the current system global variables

Command format ：env

Such as ： List the current system global variables

1） And export Command similar , but export Than env More content

common Bash Shell The environment variables are as follows ：

Variable name	explain
*	All command line arguments （ Single valued form ）
@	All command line arguments （ Multivalued form ）
#	Number of command line arguments
?	The last exit status code of the foreground process
-	The current command line option tag
$	At present Shell The process of PID
!	Of the recently executed background process PID
0	The command name used in the command line
_	Shell The absolute pathname of
BASH	call Shell The absolute path of
BASHOPTS	Allow colon separated list form Shell Options
BASHPID	At present Bash Shell The process of ID
BASH_ALIASED	Array of currently used aliases
BASH_ARGC	The number of parameters in the subfunction
BASH_ARGV	Specify an array of command line parameters
BASH_CMDS	An array of internal hash tables of the command
BASH_COMMAND	The name of the command currently being executed
BASH_ENV	Shell Before the script runs , File executed by default
BASH_LINENO	Shell An array of line numbers for each command in the script
BASH_REMATCH	An array of text elements that match the specified regular expression
BASH_SOURCE	Shell Array of source file names where functions are declared in
BASH_SUBSHELL	At present Shell Generated sub Shell number
BASH_VERSINFO	At present Shell An array of major version numbers and minor version numbers of
BASH_VERSION	At present Shell Version number of
COLUMNS	At present Shell The width of the terminal
COMP_CWORD	Current cursor position
COMP_KEY	Key to call the completion function
COMP_LINE	The current command line
COMP_POINT	The index of the current cursor position relative to the starting position of the current command
COMP_TYPE	Complete the integer value corresponding to the intensity
COMP_WORDBREAKS	When completing words , Characters used as word separators
COMP_WORDS	An array of all words on the current command line
COMPREPLY	from Shell An array of possible completion codes generated by the function
COPROC	For anonymous Ctrip I/O Array of file descriptors
DIRSTACK	An array of the current contents of the directory stack
ENV	Shell With POSIX When the mode is called , File executed by default
EUID	The current user's valid UID
FCEDIT	fc The default editor used by the command
FIGNORE	Colon separated list of suffixes , Ignored in completion
FUNCNAME	Currently executed Shell The name of the function
FUNCNEST	The highest level of nested functions
GLOBIGNORE	File names in the colon delimited list of patterns are ignored when expanded
GROUPS	An array of groups to which the current user belongs
histchars	Control the characters of history expansion
HISTCMD	The number of the current command in history
HISTCONTROL	Control which commands remain in the history list
HISTFILE	preservation Shell The file name of the history list （ The default is ~/.bash_history）
HISTFILESIZE	The maximum number of lines saved in the history file
HISTIGNORE	Commands in the colon delimited pattern list are not saved in history
HISTSIZE	The maximum number of commands that can be recorded
HISTIMEFORMAT	The format string that determines the timestamp of the history file entry
HOSTFILE	Shell When completing the hostname , Read file name
HOSTNAME	Host name
HOSTTYPE	function Shell Terminal
IGNOREEOF	Shell On exit , Must receive continuous EOF The number of characters The default value is 1
INPUTRC	readline initialization
LANG	Shell The language family of
LC_ALL	Shell Overall language environment （ Cover LANG）
LC_COLLATE	When sorting string values , The language family used
LC_CTYPE	When the file name extension matches the pattern , The language family used
LC_MESSAGES	Parsing prefix $ Double quoted string , The language family used
LC_NUMERIC	When formatting numbers , The language family used
LINENO	Shell The line number of the currently executing code in the script
LINES	Defines the number of rows visible on the terminal
MACHTYPE	“CPU- company - System ” Format definition system type
MAILCHECK	Shell Check the cycle of new messages （ The default is 60s）
OLDPWD	Shell Previous working directory
OSTYPE	function Shell Operating system of
PIPESTATUS	An array of foreground process exit status codes
POSIXLY_CORRECT	Bash Whether or not to POSIX mode
PPID	Bash Shell Parent process PID
PROMPT_COMMAND	Commands executed before the command line main prompt
PS1	The string of the main prompt
PS2	String of secondary prompt
PS3	select Command prompt
PWD	working directory
RANDOM	return 0~32767 The random number
READLINE_LINE	preservation readline Contents of the line buffer
READLINE_POINT	At present readline The insertion point position of the row buffer
REPLY	read The default variable for the command
SECONDS	Shell Elapsed time （ second ）
SHELL	Shell The absolute path of
SHELLOPTS	Together with Bash Shell A list of options （ Colon separated ）
SHLVL	At present Shell Hierarchy
TIMEFORMAT	Shell Display the format of the time value
TMOUT	select and read Command without input , Waiting time
TMPDIR	Shell Directory of temporary files
UID	function Shell Users of UID

Such as ： Use $? Variables determine the execution of commands

Shell Script default variables ： The implementation is similar to Linux Options in built-in commands

The essence ： Let the system pass options to commands through variables

Accept variables ：

${0} // Order itself 

${1} // First option 

${2} // The second option 

${N} // The first N An option 

（1） Also available “$N” The form

1） But when N Greater than 9 when , You have to use “${N}” form

（2） among 3 Special system variables

1）$#： Number of parameters （ There are several options , be equal to N）

2）[email protected]： representative “$1” “$2” “$3” “$N”

3）$*： representative “$1 Separator $2 Separator $3 Separator $N”

Such as ：test13.sh The script can accept three external options and output

1） To write test13.sh Script files

2） call test13.sh Script files

// If the passed in parameter contains spaces , Use double quotation marks

shift command ： Parameter variable offset

Command format ：shift N

1）N Is the number , Specify the number of offset digits （ If not specified N, The default offset is one digit ）

2） The essence ：$1 And the movement of subsequent variables ,$0 It's fixed

Such as ： Use shift_paras.sh Script file printing 1、2、3、4、5、6

1） To write shift_paras.sh Script files

2） call shift_paras.sh Script files

Such as ： The implementation script file can accept both options and parameters

1） To write test17.sh Script files


//Linux By default, the double slope polyline is used as the separation of options and parameters
2） call test17.sh Script files

Custom variable

Variable name rules ：

1） Only English letters 、 Numbers and underscores （ The first character cannot be a number ）;

2） Cannot contain spaces and punctuation , Special keywords cannot be used ;

3） Variable names are case sensitive , And the length should not exceed 20 Characters ;

// It is recommended that the custom variable name be lowercase （ Distinguish system environment variables ）

The assignment form of variables ： Variable name = A variable's value

1） There should be no spaces or other symbols on the left and right sides of the equal sign ;

2） When an undefined variable is called , By default, its content is empty ;

3） If the variable value contains special contents, you can use single / Double quotation marks are reserved or converted ;

4） If you define an array variable , You can enclose the array values in parentheses , And use spaces to separate

Assign the command to the variable form ： Variable name =` command `

1） Also available “ Variable name =$（ command ）” form ;

Variable value append form ： Variable name =${${ Variable name } The new content }

1） Two variable names can also be different , You can assign a value to a specified variable after adding content to it ;

Such as ： Set up test2.sh The script can print the current time

1） Father Shell The custom variable of cannot be in sub Shell Used internally

// Father Shell The global variable of is a child Shell serviceable

2） Son Shell after , Son Shell All variables of will be unregistered （ Will not be passed back to the father Shell）

export command ： Make a local variable a global variable

Command format ：export Variable name

1） If there is no variable name , All environment variables are displayed by default

2） Custom variables are local variables by default

Such as ： Set up name Variables and assign values in subroutine calls

Special custom variables

read command ： Define input variables

Command format ：read Options Variable name

Options	meaning
-p	Set input prompt Before entering variables , Display prompt message
-t	Set the waiting time If the time ends , End input directly
-n	Set the number of characters accepted If the number of accepted characters reaches the specified value , The default is to start execution
-s	Hide input

1） If there is a space in the prompt message , It needs to be enclosed in single quotes ;

2） The input variable ： Wait for user input , Continue with the script ;

3） if read The command does not specify variables , By default, the input data is stored in REPLY variable ;

Such as ：test4.sh The script can be based on the path entered by the user , Create a file under this path （ And list ）

readonly command ： Define read-only variables or functions

Command format ：readonly Options Variable name

Options	meaning
-f	Define read-only functions
-a	Define system array variables
-p	Display all read-only variables in the system

1） Don't follow the options , Read only variables are defined by default

2） A read-only variable （readonly）： Variables can only be read , Can't assign a value

Such as ：test3.sh In the script a The variable is assigned to 1, Set as a read-only variable and assign a value of 2

unset command ： Delete the specified variable

Command format ：unset Variable name

1） When deleting a value in an array variable , The index bit of this value in the array will be set to null （ Still occupied ）;

// When indexing variables , The index bit still needs to be calculated

declare command ： Declare variable type

Command format ：declare Options Variable name

Options	meaning
-a （array）	Defined as an array variable
-i （integer）	Defined as an integer variable
-x	Defined as an environment variable
-r	Defined as readonly type And can't be unset
-p	List the types of variables

1）declare and typeset Is exactly the same use and function ;

2） The variable type defaults to string ;

3）“-” become “+”, Is to cancel the corresponding variable type （-p With the exception of ）;

4） The variable is set to readonly when , The type of variable can be restored by logging out and then logging in