Common shell programming knowledge

Common shell programming tips-general Linux technology-Linux programming and kernel information. For more information, see the following. 1. system environment variables after the user logs on to the system:



$ HOME user's own directory

$ PATH: the directory to be searched during Command Execution

$ TZ Time Zone

$ MAILCHECK the number of seconds to check whether a new letter exists

$ PS1 prompt number in the Command Column

$ PS2 indicates the prompt number when Shell requests to re-input when the command has not been completed

$ MANPATH man command search path



Ii. Special variables:



$0 execution name of the program

$ N the nth parameter value of this program, n = 1 .. 9

$ * All parameters of this program

$ # Number of parameters of this program

$ PID of the program

$! PID used to execute the previous command

$? The Return Value of the previous command.



3. Variable in shell:



* Any string

? Any character

[Abc] One of the three: a, B, and c

[A-n] any character from a to n



4. Special characters



\ B Return

\ C is often used to print a row without line breaks.

\ F form feed

\ R press ENTER

\ T tabulation

\ V vertical tabulation

\ Backslash itself



V. Determining file attributes



Format:-operator filename

-If the file-e exists, 1 is returned; otherwise, 0 is returned.

-If the r file is readable, 1 is returned; otherwise, 0 is returned.

-W file write return 1, otherwise return 0

-1 is returned for executable-x Files; otherwise, 0 is returned.

-O: if the file belongs to the user, 1 is returned; otherwise, 0 is returned.

-If the length of the z file is 0, 1 is returned; otherwise, 0 is returned.

-If the f file is a normal file, 1 is returned; otherwise, 0 is returned.

-If the d file is a directory file, 1 is returned; otherwise, 0 is returned.



6. Test the string



String 1 = string 2 true when two strings are equal

String 1! = String 2: true when two strings are not equal

-N string: true when the string length is greater than 0

-Z string: true when the string length is 0

String is true when the string is not empty



7. Test the relationship between two integers



Number 1-eq Number 2 equal to true

The number 1-ne number 2 is not true.

Digit 1-gt digit 2 digit 1 greater than digit 2 true

Number 1-ge number 2 Number 1 greater than or equal to number 2 true

Number 1-lt number 2 Number 1 less than number 2 true

Number 1-le number 2 Number 1 less than or equal to number 2 true



8. Logic Testing



-A and

-O or

! Non



Introduction to shell special characters

====================================

Special characters in shell include:



1. $ dollar sign

2. \ backslash

3. 'quotation marks

4. Double quotation marks

5. <,> ,*,?, [,]



The following is a one-to-one description.



1. $ symbol



1. echo $? Displays the exit status of the previous command.

2. echo "$? "Same effect

3. echo '$? 'Is $?

4. echo \ $? $?

5. echo "\ $? "Show $?



As you may have seen, the $ symbol has special meaning in double quotation marks. Double quotation marks do not work for the $ symbol, but single quotation marks can block the special meaning of special characters so that they can be displayed as characters themselves, the backslash can also remove the special meanings of special characters, so that special characters do not have special meanings.



Ii. \ backslash



The backslash is used to block the special meaning of a special character so that it is the original character.

A = 1234

Echo \ $ A is displayed as $ A. If no value is added, 1234 is displayed.

Echo \ 'displayed'

Echo \ "is displayed as double quotation marks

Echo \ is displayed \



Iii. 'quotation marks



The function of anti-quotation marks is to replace commands and execute strings in the anti-quotation marks as commands. We often use shell programming to assign the execution result of system commands to a variable.



A = 'date'

Echo $ A shows the time string instead of the date.

For example, the content of file A is as follows:

ABCDEFG

1234456

Abcdefg



B = 'cat A | grep 234 '? # Retrieve the row containing string 234 in file

Echo $ B is displayed as 1234456

Echo "$ B" will show why?

Echo "\ $ B" will show why? Try it by yourself



Iv. Double quotation marks



There are some special characters in the system. To avoid referencing these special characters, these special characters are often caused by double quotation marks or single quotation marks, so that they do not have special meanings.

However, some special characters still have special meanings in quotation marks, which are caused by double quotation marks and do not work. The first four special characters listed in this article are in double quotation marks or special characters. In order to make it have no special meaning, one is to use single quotation marks, and the other is to use \ backslash to make it useless.



For example, we want to output these special characters as they are.



Echo """

Echo "$"

Echo "\"

Echo "'"

The above is not the expected result, because double quotation marks do not work for them, you can only output the prototype of these special characters

Echo '"'

Echo '$'

Echo '\'

Echo '''

Or

Echo "\""

Echo "\ $"

Echo "\\"

Echo "\'"

It is displayed as "$ \'



5. Other special characters



We have noticed that apart from the first four special characters, I put all the other special characters in one piece. This is because the first four special characters still have special meanings in double quotation marks, so I will explain them separately, if you want to output the prototype of these special characters, you can use double quotation marks or single quotation marks to make them lose their special meaning.

<,> ,*,?, [,] It has special meanings for shell, but you can use double quotation marks to input these prototypes.



Have you noticed that all special characters have no special meaning in single quotes? If you want to output the original form of special characters but cannot remember that these special characters cannot output the original form in double quotes, we recommend that you use single quotes.



Introduction to conditional test statements today



I. if condition statements



Format:

If condition expression

Then # execute the following statement when the condition is true

Command list

Else # execute the following statement when false

Command list

Fi



If statements can also be nested



If condition expression 1

Then

If condition expression 2

Then

Command list

Else

If condition expression 3

Then

Command list

Else

Command list

Fi

Fi

Else

Command list

Fi



You can perform multi-layer nesting. An if statement must be associated with a fi, indicating that the condition at the layer ends. Otherwise, a syntax error may occur. The preceding example is as follows:

Here we first talk about the command test used in a Condition Statement, which indicates whether the condition after test is true?



If test-f "$1"

Then

Lpr $1

Else

If test-d "$1"

Then

Cd $1

Lpr $1

Else

Echo "$1 is not a file or directory"

Fi

Fi



The above example can also be changed to the following:



If test-f "$1"

Then

Lpr $1

Elif test-d "$1" # elif is the same as else if

Then

(Cd $1; lpr $1)

Else

Echo "$1 is not a file or directory"

Fi ??????



Do you understand the above examples?

If we save this example as prfile

Chmod + x prfile

Execute the program

./Prfile aaa



This example is to check whether your input parameter is a file. If it is a file, print it. If it is a directory, convert it to the directory before printing. If it is not a file or a directory, a prompt is given.



Ii. Multi-condition test statement case



Format:

Case string in

Mode) command list ;;

Mode) command list ;;

....

Esac



The multi-Condition Statement ends with esac starting with case. Multiple condition lists are available in the middle. The function is to test whether the string matches the mode in it. If yes, execute the command list in it, the pattern can also be a "*" sign, indicating any string. The last character in each pattern must be the heart. Double quotation marks are used to end. Otherwise, a syntax error occurs.



The following is an example:



Case $1 in

*. C)

Cc $1

;;

*. Txt)

Lpr $1

;;

*)

Echo "unknown type"

Esac



If you save the preceding content in the abc file



Chmod + x abc

Execute./abc a. c to compile the file a. c.

Execute./abc readme.txt and the file will be passed through the printer

If I change the preceding content, do you know the execution result?



Case $1 in

*)

Cc $1

;;

*. Txt)

Lpr $1

;;

*. C)

Echo "unknown type"

Esac



Today we will introduce loop statements



1. while Loop



While Command Format



While condition table

Do

Command table

Done



Execution Process



Shell first executes the condition table. If the exit status of the last statement in the condition table is zero, execute the command table in the environment. After the execution, check the condition table, if the exit status is zero, the execution continues until the exit status of the last statement in the condition table is non-zero. if the exit status is zero, the condition is True.



For example, if the content of the shell file is as follows:



Sum = 0

I = 0

While true # true is the system keyword indicating true

Do

I = 'expr $ I + 1'

Sum = 'expr $ Sum + $ I'

If [$ I = "100"]

Then

Break;

Fi

Done

Echo $ I $ Sum

In the end, this program shows 100 5050. The operation of this program is to add 1 to 100.



Next, let's change this program.



Sum = 0

I = 0

While [$ I! = "100"]

Do

I = 'expr $ I + 1'

Sum = 'expr $ Sum + $ I'

Done

Echo $ I $ Sum



The Operation Result of the modified program is the same as above, but the program is more concise than above.



In this loop, until can also be used as the test condition, which is the opposite of the while test condition, that is, when the condition is false, the statement in the loop will continue to be executed, otherwise, exit the loop body. This example is also used below.



Sum = 0

I = 0

Until [$ I = "100"]

Do

I = 'expr $ I + 1'

Sum = 'expr $ Sum + $ I'

Done

Echo $ I $ Sum



When I is not equal to 100, the loop is a loop when the condition is false. Otherwise, the loop is exited. The first example is a loop when I is not equal to 100, that is, a loop when the test condition is true.



II. for Loop



Command Format:

For variable in Name List

Do

Command list

Done



The name list here is a list of strings separated by spaces. shell extracts a string from the name table each time it executes the for loop and assigns it to the loop variable as the variable value.

When writing a for statement, you can also omit the in name list section, which means that the current location parameter is used to replace the name list.

The following is an example.

For example, there are two directories in your computer, one is aa, and the other is bb. There are five identical files in these two directories, but one or more files in one of the directories have just been modified. Now I forget the files I just modified, so I am relying on the reseller to raise a fund for Ke yunnai, the reef in the xianxian region? Elliptical? Lazy? The procedure is as follows:



For File in a1 a2 a3 a4 a5

Do

Diff aa/$ File bb/$ File

Done



The following is an example without a name list.



For File

Do

Echo $ Filw

Done



The file content is saved in a. sh and the command line is as follows when we execute this shell program:

A. sh a1 a2 a3 a4 a5

The execution result is as follows:

A1

A2

A3

A4

A5

In this example, we can see that the command line parameters are read one by one.



Iii. Loop Control statements



The break command does not execute the statements in the current cycle following the break.

The continue command is used by the program to ignore the following statements in the current cycle and start execution from the loop header.



I. Command combination:



There are two methods in parentheses and curly shells to combine commands: parentheses and curly brackets. parentheses enable shell to create a sub-shell to read and execute the name command. no matter where the left and right brackets appear in the command line, shell considers them as special combinations. it indicates the original meaning of parentheses or curly brackets only when they are enclosed and referenced in double quotes. for example:



Echo a (B)

Syntax errors may occur. to output a (B) strings, you must enclose them.

Echo "a (B )"

Or echo a "(" B ")"

In this way, it can be correctly explained by shell.

What is the function of using combined commands?



2. use parentheses to combine commands



The combination command of parentheses can create sub-processes to run the combined program. It is very useful to create sub-processes, because the sub-shell operations in the Combined Command do not affect the values of various variables of the current shell.

For example:

The sub-process changes the working directory when executing the combined command, and runs a series of commands in the new working directory. After the command is executed, it does not have to return to the original working directory, because the change of the sub-process working directory does not affect the current working directory.



After a sub-process is created, the current environment is also passed to the sub-shell. The variables output to the environment using export in the current shell are equally valid in the sub-shell.



The curly braces can also be used to combine commands. The left and right curly braces contain special meanings only when they appear as the first word of a command.



Unlike parentheses, curly braces do not create subshells, but are read and executed by the current shell. sometimes you want to use the sequential output of a group of commands as the input of another group of commands. It is very convenient to use curly brackets. whether the parentheses are used or not, the exit status is equal to the exit status of the last enclosed command.



3. commands that can be executed in the Current shell



When using shell, you must understand the commands that can be executed in the current shell. The commands that cannot be executed in the Current shell include:



Break case cd continue

Echo eval exec exit

Export for if read

Readonly return set shift

Test times trap umask

Until wait while