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Author: Thank you! Reading carefully gives you a day to learn about the Shell's basic grammar rules.

1. Create and run shell programs

What is a shell program? The shell program is a simple one that contains several lines

The file for the shell or Linux command.

Like programs that write high-level languages, writing a shell program requires a text editor. such as VI.

In the text editing environment, according to the shell syntax rules, enter some shell/linux command line, form a complete

The entire program file.

There are three ways to execute Shell program files

(1) #chmod +x file

(2) #sh file

(3) #. File

When writing the shell, the first line must indicate that the system needs the shell to interpret your shell program, such as: #!

/bin/bash,

#! /BIN/CSH,/BIN/TCSH, or #!? /bin/pdksh.

Variables in the 2.shell

(1) Common system variables

$ #: Number of Save program command-line arguments

$ ? : Save return code for previous command

$0: Save program Name

$ *: to ("$ $ ...") Saves all input command-line arguments in the form of

$ @: With ("$" "$" ...) Saves all input command-line arguments in the form of

(2) Defining variables

The shell language is a non-type interpreted language, and it is not necessary to declare variables in advance, as in C++/java language programming. Give

A variable assignment is actually defined as a variable.

In all of the shells supported by Linux, you can assign a value to a variable with an assignment symbol (=).

Such as:

Abc=9 (Bash/pdksh cannot leave spaces on both sides of the equals sign)

Set ABC = 9 (tcsh/csh)

Because the shell program's variables are untyped, the user can use the same variable to store characters and sometimes save

Put an integer.

Such as:

NAME=ABC (Bash/pdksh)

Set name = ABC (TCSH)

After assigning a variable, simply precede the variable with a $ reference.

Such as:

Echo $ABC

(3) Position variable

When you run a shell program that supports multiple command-line arguments, the values of these variables are stored separately in the position variable

The first parameter is stored in position variable 1, the second parameter is stored in position variable 2, and so on ..., shell

Keep these variables, and do not allow users to define them in a way that allows them to be defined. Same variables, reference them with the $ symbol.

How to use quotation marks in 3.shell

The shell uses quotation marks (single quotation marks/double quotes) and backslashes ("\") to mask some special characters to the shell interpreter

. The inverted quotation mark (") has a special meaning for the shell.

Such as:

Abc= "How is You" (Bash/pdksh)

Set abc = "How Is You" (TCSH)

This command line assigns a string of three words to the variable ABC as a whole.

Abc1= ' @LOGNAME, how is you! ' (Bash/pdksh)

Set abc1= ' $LOGNAME, how is you! ' (TCSH)

Abc2= "$LOGNAME, How is you!" (Bash/pdksh)

Set abc2= "$LOGNAME, How is you!" (TCSH)

The logname variable is a shell variable that holds the current user name, assuming that his current value is: Wang. Execute two Lives

After the order, the content of ABC1 is logname, how is you!. And ABC2 's content is, Wang, how is you!.

Like single quotes, backslashes can also block all special characters. But he can only block one character at a time.

Masks a set of characters.

The function of anti-quotation marks is different from the above three kinds of symbols. He does not have the ability to block special characters. But through him.

Pass the result of a command to another command.

Such as:

contents= ' ls ' (bash/pdksh)

Set contents = ' ls ' (tcsh)

Test command in a 4.shell program

In Bash/pdksh, the command test is used to evaluate the value of a conditional expression. They are often in conditional statements and loops

The statement is used to determine whether certain conditions are met.

Syntax format for the test command:

Test expression

Or

[Expression]

In the test command, you can use many of the shell's internal operators. These operators are described below:

(1) string operator for evaluating string expressions

Test Command | Meaning

-----------------------------------------

STR1 = str2 | Returns True when Str1 and str2 are in the same phase

str1! = str2| Returns True when STR1 is different from str2

Str | Returns True when STR is not a null character

-N-Str | Returns True when the length of STR is greater than 0 o'clock

-Z Str | Returns True when the length of STR is 0 o'clock

-----------------------------------------

(2) Integer operators have functions similar to the character operators. It's just that their operation is for integers.

Test-expression | Meaning

---------------------------------------------

Int1-eq int2| returns True when Int1 equals Int2

Int1-ge int2| Returns True when INT1 is greater than/equal to Int2

Int1-le int2| Returns True when Int1 is less than/equal to Int2

INT1-GT int2| Returns True when Int1 is greater than int2

Int1-ne int2| Returns True when Int1 is not equal to Int2

-----------------------------------------

(3) Operators for file operations, they can check: whether the file exists, file type, etc.

Test-expression | Meaning

------------------------------------------------

-D File | Returns True when file is a directory

-F File | Returns True when file is a normal

-R File | Returns True when file is a read

-S File | Returns True when the filename is longer than 0 o'clock

-W File | Returns True when file is a writable

-X File | Returns True when file is an executable file

------------------------------------------------

(4) The Shell's logical operator is used to modify/concatenate an expression containing integers, strings, and file operators

Test-expression | Meaning

----------------------------------------------------------

! Expr | Returns True when the value of expr is false

Expr1-a expr2| returns True when the EXPR1,EXPR2 value is True

Expr1-o expr2| Returns True when the value of EXPR1,EXPR2 has at least one true

-----------------------------------------------------------

Attention:

The TCSH shell does not use the test command, but the expression in tcsh can also assume the same function. tcsh

Expressions that support expressions in C are the same. Typically used in the IF and while commands.

TCSH-expression | Meaning

-------------------------------------------------------

Int1 <= Int2 | Returns True when INT1 is less than/equal to Int2

Int1 >= Int2 | Returns True when INT1 is greater than/equal to Int2

Int1 < Int2 | Returns True when INT1 is less than int2

Int1 > Int2 | Returns True when INT1 is greater than int2

STR1 = = STR2 | Returns True when STR1 is the same as str2

Str1! = str2 | Returns True when STR1 is different from str2

-R File | Returns True when file is a readable

-W File | Returns True when file is a writable

-X File | Returns True when file is an executable file

-E File | Returns True when file exists

-O File | Returns True when the file owner is the current user

-Z File | Returns True when file length is 0 o'clock

-F File | Returns True when file is a normal

-D File | Returns True when file is a directory

EXP1 | | Exp2 | Returns True when the value of EXP1 and EXP2 is at least one true

EXP1 && Exp2 | Returns True when the value of EXP1 and EXP2 is the same as true

! Exp | returns TRUE when EXP has a value of false

5. Conditional statements

As with other high-level language programs, the branch and loop control structures are often used in complex shell programs.

Bash,pdksh and tcsh each have two different forms of conditional statements: The IF statement and the case statement.

(1) If statement

Syntax format:

Bash/pdksh usage:

if [expression1]

Then

Commands1

elif [Expression2]

Commands2

Else

Commands3

If

TCSH usage:

if (expression1) then

Commands1

else if (expression2) then

Commands2

Else

Commands3

endif

Meaning: When the expression1 condition is true, the shell executes the COMMANDS1 command after then;

When the condition of expression1 is false and the condition of expression2 is true, the shell executes

Commands2 command, when the condition value of expression1 and Expressin2 is false, the shell executes

The COMMANDS3 command. The IF statement ends with his anti-write fi.

(2) Case statement

The case statement requires the shell to compare a string s with a set of string patterns P1,p2,..., pn, when S and

When a pattern pi wants to match, it executes the corresponding part of the program/command. Character in Shell Case statement

A pattern can contain wildcards such as *.

Syntax format:

Bash/pdksh usage:

Case string1 in

STR1)

COMMANDS1;;

STR2)

COMMANDS2;;

*)

COMMANDS3;;

Esac

TCSH usage:

Switch (string1)

Case STR1:

Statements1

Breaksw

Case STR2:

Statements2

Breaksw

Default

Statements3

Breaksw

Endsw

Meaning: The shell compares string string1 to string patterns str1 and str2 respectively. If string1 and str1

Match, then

The shell executes the COMMANDS1 command/statement; if String11 and str2 match, the Shell executes COMMANDS2

The command/

Statement. Otherwise the shell executes the COMMANDS3 program/command. Each branch of the program/command is

With a total of two

Semicolon (;;) End.

6. Looping statements

The loop statement is used when certain actions need to be repeated.

(1) For statement

You know that in many programming languages the for statement is the most common. In the shell is no exception. The For statement requires the shell

Will contain

A set of commands in this statement executes a certain number of times consecutively.

Syntax format:

Bash/pdksh

Usage 1:

For var1 in list

Do

Commands

Done

Meaning: In this for statement, which corresponds to each value in the list, the shell executes a commands represented by a

Group command.

In each execution of the entire loop, the variable var1 will take the different values in the list.

Usage 2:

For var1

Do

Setatements

Done

Meaning: In this for statement, the shell performs a statements representation for each item in the variable var1, respectively.

A set of

command. When using this form of statement, the shell thinks that the VAR1 variable contains all the positional variables, and the position

Variables in

The value of the command line parameter that holds the program. That is, he is equivalent to the following form:

For var1 in "[email protected]"

Do

Statements

Done

TCSH usage:

There is no for this word in tcsh, and the same function as the For statement is the foreach statement

foreach Name (list)

Commands

End

Example:

for file; Bash/pdksh

Do

TR A-Z a-z< $file >file.caps

Done

#; tcsh

foreach file ($ *)

TR A-Z a-z< $file > $file. Caps

End

(2) While statement

The while statement is another circular statement provided by the shell. The while statement specifies an expression and a set of commands. This

A

Statement causes the shell to repeatedly execute a set of commands until the value of the expression is false.

Syntax format:

while expression; bash

Do

Statements

Done

while (expression); tcsh

Statements

End

Example:

count=1; bash

While [-N "$ *"] * * *

Do

echo "This is a parameter number $count $"

Shift

Count= ' expr $count + 1 '

Done

Set count = 1; tcsh

while ("$ *"!) = "")

echo "This is a parameter number $count $"

Shift

Set count = ' expr $count + 1 '

End

The function of the shift command in a statement is to pass all command-line arguments in turn.

(3) until statement

The until and while statements have similar syntax formatting and functionality, unlike the value of expression in the while

True,

The shell executes the command group, and the shell executes the set of commands when the value of expression is false in until.

Syntax format:

Until expression

Do

Commands

Done

Example:

Count=1

Until [-Z "$ *"] * * *

echo "This is a parameter number $count $"

Shift

Count= ' expr $count + 1 '

Done

Note the above example with a * * number. Expression in while:-n string, he means when string does not

is empty

String, the value of the expression is true; the expression in until:-Z string, his meaning is when the string

is empty

String, the value of the expression is true. This shows that the two programs set the conditional expression exactly the opposite.

(4) Shift statement

Both bash and TCSH support the shift command. Shift places the command-line arguments that are stored in the position variable, and then left-handed

. For example

The current value of the position variable is:

$1=file1 $2=file2 $3=file3

Once the shift command is executed, the value of the position variable is:

$1=file2 $2=file3

You can also specify the number of times the position variable is transferred in the shift command, such as:

Shift N

Example:

While ["$"]

Do

If ["$" = "-i"] Then

Infile= "$"

Shift 2

else if ["$" = "-O"] Then

Outfile= "$"

Shift 2

Else

echo "Rogram $ does not recognize option $"

Fi

Done

TR A-Z a-z< $infile > $outfile

(5) SELECT statement

The SELECT statement is a unique looping statement provided by Pdksh. He is different from the Loop statement described earlier. He doesn't

Is

Evaluates a conditional expression repeatedly and determines whether to execute a set of commands based on the value of the expression. The function of Select is

Auto-

Generates a simple text menu.

Syntax format:

Select menu [In List_of_items]

Do

Commands

Done

Meaning: When executing a SELECT statement, Pdksh creates a single member of each column in the List_of_items, respectively.

A menu

Options. List_of_items can be a variable that contains multiple options, or it can be directly listed in the program.

A set of options

If the statement does not provide a list_of_items,select statement, the position variable is used as the

List_of_items.

Example:

Select MenuItem in Pick1 Pick2 Pick3

Do

echo "Is you sure-want to pick $menuitem"

Read res; receives input from the user and stores the input values in a specific variable.

if [$res = "Y"-o $res = "Y"]

Then

break; To exit a looping statement such as While,for,select

Fi

Done

(6) Repeat statement

The repeat statement is a unique loop statement provided by TCSH. Use the repeat command to require the shell to execute a command

Must

The number of times.

Syntax format:

Repeat Count command

such as

foreach num ($ *)

Repeat $num echo-n "*"

echo ""

End

Functions in 7.shell

The shell allows users to define their own functions. Functions are an important structure in a high-level language. A function in the shell is in C or

Person Other

functions defined in the language. The main advantage of using a function is that it has a

Facilitate the organization

The entire program. In bash, a function has the following syntax:

FName () {

Shell Comands

}

Once you have defined the functions, you need to call them in your program. The format of the calling function in bash:

fname [Parm1 parm2 parm3 ...]

When you call a function, you can pass any number of arguments to the function. The function considers these parameters to be the command-line arguments that hold his

Number of

Position variable.

Example:

This program defines 4 functions:

Upper (): Converts the letters in the file passed to him to uppercase, and holds the file with an. Out at the end of the same name

In.

Lower (): Converts the letters in the file passed to him into lowercase, and stores the file with the. Out at the end of the same name

In.

Print (): Outputs the contents of the file passed to him.

Usage_error (): The Help information for the output program.

The main module of the program is a case condition statement, which determines the function that the program will complete according to the first parameter in the command line.

, and call the corresponding

function to complete this function.

Upper () {

Shift

For I

Do

TR a-a A-z<$!>$1.out

RM $

MV $1.out $

Shift

Done }

Lower () {

Shift

For I

Do

TR A-Z a-z<$1>$1.out

RM $

MV $1.out $

Shift

Done }

Print () {

Shift

For I

Do

LPR $

Shift

Done }

Usage_error () {

echo "$ syntax is $"

echo ""

echo "where option is one of the following"

echo "P--to Print frame Files"

echo "U--to Save As uppercase"

echo "l--to Save As lowercase";}

Case $ in

P | -p) Print [email protected];;

u | -u) Upper [email protected];;

l | -l) Lower [email protected];;

*) Usage_error;;

Esac

----------------------------------------------------------

Summarize

The use of shell programming is an important means to improve the efficiency of system management, learn the shell and understand the basic command of the system

and management

How to use the tool is equally important!

Report:

Commands that are commonly used in A.bash

Command | Meaning

-----------------------------------------------------------

alias | set command Alias

Bg | A suspended process is executed in the background

CD | Change the user's current directory

Exit | Terminate a Shell

Export | The variable and its current value, which is used as the argument for this command, are available in the child process of the currently running shell

See

FC | Edit the current command line History list

FG | Let a suspended process execute in the foreground

Help | shows you helpful information for bash internal commands

History | Displays a certain number of command lines that have been recently entered

Kill | Terminates a process

PWD | Displays the user's current working directory

Unalias | Remove command-line aliases

-------------------------------------------------

System variables commonly used in B.bash

Variables | Meaning

-------------------------------------------------

Editor,fcedit | Default text editor for the FC command of Bash

Histfile | Specifies the name of the most recently entered command line file

Histsize | Specifies the size of the command line history file

HOME | The current user's host directory

Oldpwd | The previous directory used by the user

Path | Specifies the paths that bash searches for when searching for executable files

PS1 | Display the first level hint symbol in the command line environment

PS2 | Show second level hint symbol in command line environment

PWD | User's current working directory

SECONDS | Run time (in seconds) of the currently running bash process

This article is from the "lake and Laughter" blog, please make sure to keep this source http://xuexuhui.blog.51cto.com/9647696/1653576

Shell day-to-day app win