Chapter 6 variables and references of shell Learning

Variable

Local variable: similar to local variables, only valid for the current Shell Process

Environment variable: applicable to all sub-processes generated by login Processes

Location Parameter: used to pass parameters to shell scripts, read-only

Variable replacement and assignment

The referenced variable value is called variable replacement, and $ is the variable replacement symbol. For example, if a is the variable name, $ A or $ {A} is the variable value.

Assigning a value to a variable name is called a value assignment. The format is variable = value or $ {Variable = value}. If the value contains spaces, you must use ""

Clear the variable value:

Unset variable name

Set Read-Only variables:

Variable = Value

Readonly variable

You can also use declare and typeset to implement

View all read-only variables in the system:

[[Email protected] TMP] # readonly

Declare-ar bash_versinfo = '([0] = "3" [1] = "2" [2] = "25" [3] = "1" [4] =" release "[5] =" x86_64-redhat-linux-gnu ")'

Declare-ir euid = "0"

...

Variable assignment mode

1. Variable :? Value or variable? Value

Displays System Errors for unassigned variable

[[Email protected] TMP] # echo $ {:? Blue}

Bash: A: Blue

[[Email protected] TMP] # A = black

[[Email protected] TMP] # echo $ {:? Blue}

Black

2. Variable: = Value

Assign the value to the variable that is not assigned a value.

[[Email protected] TMP] # A = black

[[Email protected] TMP] # echo $ {A: = blue}

Black

[[Email protected] TMP] # unset

[[Email protected] TMP] # echo $ {A: = blue}

Blue

[[Email protected] TMP] # echo $

Blue

3. Variable:-Value

Returns the value for the variable that is not assigned a value, but the variable value is not assigned.

[[Email protected] TMP] # unset

[[Email protected] TMP] # echo $ {A:-blue}

Blue

[[Email protected] TMP] # echo $ {:? Blue}

Bash: A: Blue

Shell script variables with no type

Shell script variables are non-typed and have both numeric and numeric assignment values. They can be directly used without pre-defined variables. The initial value of numeric type is 0, and the initial value of numeric type is null.

[[Email protected] TMP] # Cat A. Sh

#! /Bin/sh

C = ""

Echo "c = $ C"

Let "C + = 1" # Let command is used to execute arithmetic operations. Let "C + = 1" is equivalent to C + = 1

Echo "c = $ C"

Echo "E = $ e"

Let "e + = 1"

Echo "E = $ e"

E = "hello" # the numeric value of this variable changes to 0 again.

Echo "E = $ e"

Let "e + = 2"

Echo "E = $ e"

[[Email protected] TMP] #./A. Sh

C =

C = 1

E =

E = 1

E = Hello

E = 2

Environment Variable

The environment variables of the parent process can be passed to the child process, but the environment variables of the child process (or the environment variables defined by the parent process are modified) cannot be passed or affect the parent process.

Define environment variables:

Environ-variable = Value

Export environ-Variable

Common environment variables are capitalized.

Export indicates that this variable is an environment variable.

Clear environment variables:

Unset environ-Variable

Important environment variables:

1. pwd and oldpwd

PWD records the current directory, and oldpwd records the old working directory

[[Email protected] TMP] # echo $ pwd

/Tmp

[[Email protected] TMP] # cd

[[Email protected] ~] # Echo $ pwd

/Root

[[Email protected] ~] # Echo $ oldpwd

/Tmp

2. Path

Shell searches for the directory list in path for each input command.

[[Email protected] ~] # Echo $ path

/Usr/Kerberos/sbin:/usr/Kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin: /usr/bin:/usr/x11r6/bin:/root/bin

[[Email protected] ~] # Export Path = $ path:/tmp/bin # Add a new path

[[Email protected] ~] # Echo $ path

/Usr/Kerberos/sbin:/usr/Kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin: /usr/bin:/usr/x11r6/bin:/root/bin:/tmp/bin

3. Home

Path of the user's home directory

[[Email protected] ~] # Echo $ home

/Root

4. Shell

Save the default shell value (/bin/bash)

5. User and uid

The name and ID of the logged-in user

[[Email protected] ~] # Echo $ user $ uid

Root 0

6. ppid and $

$ Ppid: the parent process ID of the current process. $ it is the process ID of the current process.

7. PS1 and PS2

The START prompt and branch prompt respectively (that is, the prompt when the command is input in multiple lines)

[[Email protected] ~] # Echo $ PS1

[\ [Email protected] \ H \ W] \ $

[[Email protected] ~] # Echo $ PS2

>

\ U represents the current user name

\ H indicates the Host Name

\ H Represents the Host Name and domain name

\ W indicates the name of the current working directory

\ W indicates the complete path of the current working directory

\ $ If the UID is 0, # is displayed; otherwise, $ is printed.

8. Ifs

Specifies the input domain Separator of shell. The default Delimiter is space.

[[Email protected] ~] # Export ifs = ""

[[Email protected] ~] # Echo $ path

/Usr/Kerberos/sbin:/usr/Kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin: /usr/bin:/usr/x11r6/bin:/root/bin:/tmp/bin

[[Email protected] ~] # Export ifs =:

[[Email protected] ~] # Echo $ path

/Usr/Kerberos/sbin/usr/Kerberos/bin/usr/local/sbin/usr/local/bin/sbin/bin/usr/sbin/usr/bin/usr/x11r6 /bin/root/bin/tmp/bin

Environment variable configuration file

Hide a file in the root directory of the user

1. bash_profile

The user logs on automatically and initializes the environment variable. If the file does not exist, the user executes the/etc/profile command.

You can add new environment variables and execute. bash_profile or source. bash_profile to take effect immediately (you can also log out)

The source command is equivalent to the. Command

Directly execute the file to start a sub-shell, and the environment variables set by the sub-shell script cannot affect the current shell script. the soure command (or. Command) allows the short script to be executed in the current shell.

[[Email protected] ~] # Cat. bash_profile

#. Bash_profile

# Get the aliases and functions

If [-f ~ /. Bashrc]; then

.~ /. Bashrc

Fi

# User specific environment and startup programs

Path = $ path: $ home/bin

2. bashrc

Sets the environment variables of the sub-shell to separate the environment variable settings during user login from the environment variable settings of the sub-shell, improving flexibility.

3. bash_logout

When you log out, you can write commands that identify certain environment variables or do not exist.

Export path

Unset Username

Parent process, child process environment variable relationship

The child process can inherit the environment variables of the parent process, but the environment variables modified by the child process are only valid for the child process and do not affect the environment variables of the parent process, the parent process of the Environment Variable created by the child process cannot receive it, while the local variable does not affect each other.

Example:

[[Email protected] TMP] # Cat father. Sh

#! /Bin/bash

Fatherlocal = "fatherlocal"

Fatherenviron = "fatherenviron"

Export fatherenviron

Echo "in fahter. Sh"

Echo "Father process ID is $"

Echo "fatherlocal is $ fatherlocal"

Echo "fatherenviron is $ fatherenviron"

$ PWD/child. Sh

Echo "return to Father. Sh"

Echo "fatherlocal is $ fatherlocal"

Echo "fatherenviron is $ fatherenviron"

[[Email protected] TMP] # Cat child. Sh

#! /Bin/bash

Echo "in child. Sh"

Echo "child process ID is $ and my father is $ ppid"

Echo "fatherlocal is $ fatherlocal"

Echo "fatherenviron is $ fatherenviron"

Echo "Change faterenviron"

Export "fatherenviron = redefinfatherenviron"

Echo "New fatherenviron is $ fatherenviron"

Result:

[[Email protected] TMP] #./father. Sh

In fahter. Sh

Father process ID is 29171

Fatherlocal is fatherlocal

Fatherenviron is fatherenviron

In child. Sh

Child process ID is 29172 and my father is 29171

Fatherlocal is

Fatherenviron is fatherenviron

Change faterenviron

New fatherenviron is redefinfatherenviron

Return to Father. Sh

Fatherlocal is fatherlocal

Fatherenviron is fatherenviron

Location parameters

$0 Script Name

$1 first Parameter

$ {10} the tenth parameter, starting from 10 to add {}

$ # Number of parameters

$ * Or [email protected] All parameters, excluding $0

$ Process ID

$ Ppid parent process ID

$? Exit status. "0" indicates no error, and "non-0" indicates an error.

Reference

Reference is to block the special meaning of a special character and interpret it as literal meaning.

"" References all characters except the dollar sign ($), backquotes (''), and backslash (\)

''References all characters''

''. Shell interprets the content in the quotation marks as a system command.

\ Backslash to block the special meaning of the next character

You can retain multiple spaces (with line breaks) for variables with double quotation marks instead of the field separator (IFS is empty by default). Double quotation marks prevent variables from changing separators.

Example:

[[Email protected] TMP] # A = "a B C"

[[Email protected] TMP] # echo $

A B C

[[Email protected] TMP] # echo "$"

A B C

[[Email protected] TMP] # echo '$'

$

[[Email protected] TMP] # echo 'Why can't I'm you're'

Why cant IM you are

[[Email protected] TMP] # echo "Why can't I'm you are"

Why can't I'm you are

Command replacement

Replace the standard output of a command with the location of the command.

Back quotes ('') and $ ()

$ () Commands can be nested

$ () Is \, and '(') is interpreted as a space.

The result of execution in other languages can be assigned to shell variables using reverse quotation marks, so as to be processed by shell.

Example:

[[Email protected] TMP] # echo \\

\

[[Email protected] TMP] # echo $ (echo \\)

\

[[Email protected] TMP] # echo 'echo \\'

# Blank lines

Command 'echo '# command represents any command, which is equivalent to no Parameter

Command "'echo '" # null string parameter

Command 'echo x y' # contains two parameters: X and Y.

Command "'echo x y'" # With a parameter, X Y

Escape

Purpose:

1. Remove the meaning of some special characters

2. A second-level prompt appears for a single \ in the command line, which is also applicable in shell scripts.

3. Use echo, sed, awk, and other commands to make some letters express special meanings.

1.

Variable = "() \\{}\$ \""

Echo $ variable

Echo "$ variable"

Ifs = '\' # change the separator of input variables

Echo $ variable

Echo "$ variable" # Double quotation marks prevent variables from changing the separator and retain the variable separator.

[[Email protected] TMP] #./a3.sh

() \ {}$"

() \ {}$"

() {}$"

() \ {}$"

2.

2.1

[[Email protected] TMP] # echo \

> Hello World

Hello World

2.2

[[Email protected] TMP] # echo \ # You can use "or" Echo ".

> "Hello World"

Hello World

2.3

[[Email protected] TMP] # echo "\" # note that there is an essential difference between this and the previous two. Here, the \ escape "causes missing" command to end after completion, same as 2.4

> Hello World"

"

Hello World

2.4

[[Email protected] TMP] # echo"

> Hello

> World"

Hello

World

2.5

[[Email protected] TMP] # variable = \

> Hello World

Bash: World: Command not found # assign values to variables using quotation marks if spaces exist.

2.6

[[Email protected] TMP] # variable = \

> "Hello World"

[[Email protected] TMP] # echo $ variable

Hello World

2.7

[[Email protected] TMP] # Cat a5.sh

#! /Bin/bash

Echo \ # single/also applies in shell scripts

Hello World

[[Email protected] TMP] #./a5.sh

Hello World

3.

Special meanings of letters followed:

\ N new row

\ R return

\ T Tab key

\ V or \ f line feed, but the cursor stays at the original position (with the same horizontal coordinates)

\ B Return key

\ A sends an alarm

Characters corresponding to \ 0xx ASCII code 0xx

Example:

[[Email protected] TMP] # echo Hello \ B \ 101

Hellob101

[[Email protected] TMP] # echo $ 'Hello \ B \ 000000' # is basically equivalent to-e"

Hella

[[Email protected] TMP] # echo-e "Hello \ B \ 0101" # note that 0 of \ 0101 cannot be parsed as ASCII code without 0

Hella

[[Email protected] TMP] # echo-e "Hello \ B \ 101"

Hell/101

[[Email protected] TMP] # echo-e Hello \ B \ 101 # Note-E is not "" and cannot be parsed

Hellob101

Note:

Echo [Option] [String]

Option-e indicates that special characters formed after the escape character (\) are interpreted as special characters.

Option-N indicates no line feed after the output text:

[[Email protected] TMP] # echo-n "hello"

Hello [[email protected] TMP] #

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