The basics before you learn shell scripting

Reprinted from: http://www.92csz.com/study/linux/12.htm

The basics before you learn Shell scripting

A shell script is essential for everyday Linux system management, and if you don't write a shell script, you're not a qualified administrator. At present, many organizations in the recruitment of Linux system administrators, Shell script writing is a must-test project. Some units even use the ability to write shell scripts to measure the rich experience of this Linux system administrator. The author said that the purpose of this is only one, that is to let you take the shell script seriously, from the beginning to grasp the basic knowledge firmly, and then to continue to practice, as long as your shell script good, I believe your Linux job path will be much easier. In this chapter, I will not introduce the shell script in detail, but just take you into the world of shell script, if you are interested then please go to the Internet to download the relevant materials or to buy books in the bookstore.

Before you learn a shell script, you need to know a lot about the shell, which is the basis for writing shell scripts, so I hope you can master it skillfully.

" What is Shell "

Simple understanding, is the system with the computer hardware to use intermediate media, it is only a tool of the system. In fact, there is a layer of stuff between the shell and the computer hardware that is the system kernel. For example, if computer hardware is compared to a person's body, and the system kernel is the human brain, as for the shell, it is more appropriate to compare it to a person's facial features. Back to the computer, the user is not directly facing the computer hardware but the shell, the user tells the shell, and then the shell again to the system kernel, and then the kernel to control the computer hardware to perform various operations.

The default installation of the Linux release version (Redhat/centos) system by the author is called Bash, the Bourne Again shell, which is an enhanced version of the SH (Bourne shell). Bourn Shell is the first to line up a shell, the founder is called Steven Bourne, in order to commemorate him so called Bourn Shell, the detection of SH. So what's the feature of this bash?

1) Record command history

The commands we've knocked on, Linux will be recorded, and presets can record 1000 historical commands. These commands are saved in the. bash_history file in the user's home directory. One thing you need to know is that commands that run in the current shell are saved to the. bash_history file only when the user exits the current shell normally.

There is an interesting character associated with the command history that is "!" The There are several applications commonly used: (1)!! (Two consecutive "!" ), which represents the execution of the previous instruction, (2)!n (n here is a number), indicating that the nth instruction in the history of the command is executed, such as "!100" means the 100th command in the history of the Execution command; (3)! string (string greater than or equal to 1), such as!ta, Represents the last instruction in the command history to begin with TA.

2) command and file name completion

At the beginning of this tutorial, I introduced this feature, remember? The right thing to do is to press the TAB key, which can help you to complete a command, or to help you complete a path or a file name. Press the TAB key two times consecutively, and the system will list all the instructions or filenames.

3) Aliases

There has been an introduction to Alias, which is one of the features of bash. We can alias a commonly used and very long instruction by aliases a simple and easy-to-remember instruction. If you don't want to use it, you can also use Unalias to remove the alias feature. Directly hitting alias will see the current system preset alias:

See, the system preset alias instructions are just a few, you can also customize the command alias you want. Alias syntax is simple, alias [command alias]=[' specific Command '].

4) wildcard characters

Under Bash, you can use * to match 0 or more characters, and match one character with a.

5) input/output from directional

Input redirection is used to change the input of the command, and output redirection is used to change the output of the command. Output redirection is more common, and it is often used to enter the results of a command into a file, rather than on the screen. The command to enter the redirect is <, the command to output redirection is, and the error redirect 2>, and the Append redirect >>, which will be described in more detail later.

6) Pipe character

The pipe symbol "|" has been mentioned before, which is the result of running the previous command to the following command.

7) Operation control.

When you run a process, you can pause it (press CTRL + Z), then use the FG command to restore it, use the BG command to get him to run in the background, or you can make it stop (press CTRL + C).

" variables "

In the previous section, I introduced the environment variable path, which is a variable of the shell preset, usually the shell preset variables are uppercase. Variable, the simple point is to use a simpler string instead of some special meaning settings and data. Taking path, this path replaces the absolute path setting for all common commands. Because of the variable path, we no longer have to enter the global path when we run a command, just hit the command name. You can use the echo command to display the value of a variable.

Besides path, HOME, logname, what are the system preset environment variables?

All system variables for system presets are listed using the ENV command. However, users who log on are different. These environment variables also have different values. The environment variables for the account root are currently displayed. The following is a brief introduction to the common environment variables:

path determines to which directories the shell will look for commands or programs

Home Current User Home directory

Histsize Number of historical records

LogName The current user's login name

Hostname refers to the name of the host

Shell Front user shell type

Lang language-related environment variables, multiple languages can modify this environment variable

Mail storage directory for the current user of mail

PWD current Directory

The env command displays variables that are only environment variables, and there are many more variables in the system preset, and you can use the SET command to display all the variables of the system preset.

Confined to space, the author in the above example did not put all the results shown. Set can not only display the system preset variables, but also can be displayed with user-defined variables. User-defined variables? Yes, the user can also define the variables themselves.

Although you can customize the variable, the variable can only be used in the current shell, do not believe you login to a shell to try?

Using the Bash command to open a shell again, the previously set myname variable no longer exists, exiting the current shell back to the original Shell,myname variable is still there. What if the variable you want to set is always in effect? There are two types of situations:

1) You can use this variable if you want all users in the system to log in

You need to add "export myname=aming" to the last line of the/etc/profile file and then run "source/etc/profile" to take effect. At this point you can run the Bash command again or su-test the account directly.

2) Just want the current user to use the variable

You need to add "export myname=aming" to the last line of the. bashrc file in the user's home directory and then run "source. BASHRC" to take effect. When you log in to the test account again, the myname variable does not take effect. The function of the source command above is to say that the configuration refresh is currently set, that is, it can take effect without logging off or logging on.

The author uses "Myname=aming" in the above example to set the variable myname, so what are the rules for setting custom variables under Linux?

A. The format of the variable is "a=b", where a is the variable name, B is the contents of the variable, there can be no space on either side of the equal sign;

B. Variable names can only consist of English, numerals, and underscores, and cannot begin with a number;

C. When the contents of a variable have special characters (such as spaces), you need to add single quotation marks;

In one case, you need to note that the variable content itself is enclosed in single quotes, which requires double quotes.

D. If the contents of the variable need to use other commands to run the result, you can use the back quotation marks;

E. Variable content can accumulate the contents of other variables, need to add double quotation marks;

If you accidentally add double quotes to single quotes, you won't get the results you want.

As you can see from the above examples, the difference between single and double quotes is that double quotation marks do not cancel the function of the special characters that appear inside ($), and the special characters in single quotation marks all lose their function.

In the previous example, I used the bash command several times, and if you run bash in the current shell, you will enter a new shell, which is the Shell's child shell, so you might want to use the Pstree command to see it.

Pstree This command will print all the processes in the Linux system through a tree structure. Limited to the length of the author not all listed, you can directly enter Pstree view. After setting a variable in the parent shell, the variable will not take effect after entering the child shell, and if you want this variable to take effect in the child shell, the Export command is used, which I used before.

Export is actually a declaration of the meaning of this variable, so that the shell's child shell also know that the value of the variable ABC is 123. If no variable name is added to the export, it declares all variables.

The last side, along with our custom variables, are declared.

How do I set a variable in front of you, and what if I want to cancel a variable? Just enter "unset variable name".

With unset ABC, the Echo $abc will no longer output any content.

" configuration files for system environment variables and personal environment variables "

It says a lot about the variables of the system, so where are these variables stored in the Linux system, and why do users automatically have these variables when they log on to the shell?

/etc/profile : This file presets several important variables, such as path, USER, LOGNAME, MAIL, INPUTRC, HOSTNAME, Histsize, UMAs, and so on.

/ETC/BASHRC : This file is mainly preset umask and PS1. This PS1 is the string of characters we have in front of the command, such as the author's Linux system PS1 is [[email protected] ~]#, you might as well look at the value of PS1.

\u is the user, \h hostname, \w is the current directory, \$ is that ' # ', if the average user is displayed as ' $ '

In addition to the two system-level profiles, there are several such hidden files under each user's home directory:

. Bash_profile : Defines the file name of the user's personalization path and environment variable. Each user can use the file to enter shell information that is specific to their own use, which is performed only once when the user logs on.

. BASHRC : This file contains bash information dedicated to your shell, which is read when you log on and every time you open a new shell. For example, you can write user-defined alias or custom variables to this file.

. Bash_history : Records the command history.

. Bash_logout : When you exit the shell, the file is executed. You can put some cleanup work into this file.

" Special symbols in theLinux shell "

While you are learning Linux, you may have been exposed to a special symbol, such as "*", which is a wildcard symbol that represents 0 or more characters or numbers. The following is the author of the special characters used to say.

1. * : Represents 0 or more characters or numbers.

Test can have no characters, or multiple characters, or it can be matched.

2.? : represents only one arbitrary character

Whether it is a number or a letter, as long as it can be matched.

3. # : This symbol in Linux indicates the meaning of the comment description, that is, "#" after the content of Linux ignored.

Insert "#" at the beginning or middle of the command, and Linux will ignore it. This symbol is used in a lot of shell scripts.

4. \ : de-ideographic character, revert the following special symbol (for example "*") to normal characters.

5. | : The pipe character, which has been said many times before, is the function of throwing the result of the preceding command to the command following the symbol. The following commands, which are mentioned here, are not available for all commands, and are commonly used for document operations such as Cat, less, head, tail, grep, cut, sort, WC, uniq, tee, tr, split, SED, awk, and so on, where G Rep, sed, awk are the tools that must be mastered for regular expressions, which are described in detail in the following sections.

6. $: In addition to the identifier used in front of the variable, there is also a magical, is and '! ' Use it together.

The '!$ ' represents the last variable in the previous hit (perhaps called a variable, which is the last thing in the previous command), such as the last command, which is test.txt, then entering!$ under the current command represents Test.txt.

1 ) grep : Filtering one or more characters will be described in detail in subsequent content.

2) Cut : Intercepts a field

Syntax: cut-d "separator character" [-CF] n here n is the number

-D: followed by delimited characters, delimited by double quotation marks

-C: followed by the first few characters

-F: The next section is followed by the number of blocks

-D followed by the delimiter character, where a colon is used as the split character, and-F 1 is the first paragraph, and the space between-F and 1 is optional.

The-C can be either 1 digits n or an interval n1-n2, or multiple digits n1,n2,n3

3) Sort : Used for sorting

Syntax: sort [-t delimiter] [-KN1,N2] [-NRU] Here's N1 < N2

-T delimiter: function with the-D one meaning of cut

-N: Sorting with pure numbers

-R: Reverse Sort

-U: To repeat

-KN1,N2: Sorted by N1 interval to N2 interval, can only write-kn1, sort N1 field

4) WC : Count the number of lines, characters, and words of the document, and the commonly used options are:

-L: Count rows

-M: Statistics of characters

-W: Number of statistical words

5 ) uniq : To repeat the line, the author used only one of the options:

-C: Count the number of repeated rows and write the number of lines in front

There is a point to note that before the uniq, you need to sort and then uniq, otherwise you will not get what you want, the author of the above experiment has been sorted so omitted that step.

6 ) Tee : followed by the file name, similar to redirect ">", but the specific gravity of a function, the file is written in the following file, while also displayed on the screen.

7 ) TR : Replacement character, commonly used to handle special symbols appearing in a document, such as ^m symbols appearing in a DOS document. There are two common options:

-D: Delete a character, followed by the character you want to delete

-S: Remove repeated characters

The most common is to capitalize the lowercase: tr ' [A-z] ' [A-z] '

Of course, replacing a character is perfectly possible.

However, the substitution, deletion and repetition are all aimed at one character and have some limitations. If it is not useful for a string, so I suggest simply understand the TR can be, you will learn more in the future to implement the string manipulation tools.

8 ) Split : Cutting documents, common options:

-B: Divide the document by size in bytes

Format as above, the following passwd is the prefix of the filename after the split, the file name is Passwdaa, Passwdab, Passwdac ...

-L: Split document by number of rows

6. ; : Semicolon. Usually we are in a row to hit a command, and then enter the run, so you want to run two or two more commands in a row? You need to add a ";" between the commands. The

7. ~ : User's home directory, if Root is/root, ordinary user is/home/username

8. & : If you want to put a command in the background, you need to add this symbol. This is typically used when the command runs for a very long time.

Jobs can be used to view tasks performed in the background in the current shell. The FG can be transferred to the foreground for execution. The Sleep command here is the meaning of sleep, followed by a number, in seconds, in the shell script of the usual term loop.

At this point, you press CTRL +z to pause it, and then enter BG to go back into the background.

In the case of multitasking, if you want to move the task to the foreground, FG is followed by the task number and the task number can be obtained using the jobs command.

9. >>, 2>, 2>> : The above-mentioned directional symbols > and >> denote the meaning of substitution and addition, then there are two symbols which are 2> and 2>> here. Indicate error redirection and error append redirect respectively, when we run a command error, the error message will be output to the current screen, if you want to redirect to a text, then use 2> or 2>>.

[] : brackets, the middle is a combination of characters, representing any one of the intermediate characters

&& | |

A semicolon has just been mentioned above for the delimiter between multiple commands. There are also two special symbols that can be used in the middle of multiple commands: "&&" and "| |". Here are a list of the following:

1) Command1; Command2

2) Command1 && Command2

3) Command1 | | Command2

Use ";" , the Command2 is executed regardless of whether the Command1 executes successfully, and when "&&" is used, only command2 executes after Command1 execution succeeds, otherwise command2 does not execute; use "| |" , Command1 execution succeeds Command2 not execute, otherwise go to execute Command2, in short Command1 and Command2 always have a command to execute.

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The basics before you learn shell scripting