Shell basic commands

shell brief introduction

         With all kinds of Linux The graphic degree of the system is constantly improving , Users in the desktop environment , By clicking on 、 Drag and drop and other operations can complete most of the work . However , many Ubuntu Linux Function use shell Command to implement , Than using a graphical interface , Finish faster 、 More direct .

        shell Is a scripting language , The essence is a document , What's in the file is Instructions in a specific format , The system can use a script parser to translate or parse instructions and execute （ It doesn't need to compile ）.shell Both applications It's another scripting language （ Applications analysis Scripting language ）.

shell It's a command line interpreter , Used to protect the kernel shell Script hand shell Batch execution .

command It is the user who sends a control request to the system kernel , The text stream that interacts with it .

shell It's a command line interpreter , Parse user commands into instructions that the operating system can understand , Realize the interaction between users and operating system .

When several commands need to be executed repeatedly , You can put these commands together , Add certain control statements , The editor becomes shell Script file , hand shell Batch execution .

The user types the command text at the command line prompt , Start with Shell Interact

next ,shell Convert the user's commands or keys into instructions that the kernel can understand

Control the operating system to respond , Until the relevant hardware equipment is controlled

then ,shell Pass the output through shell Submit to the user

shell Command parser

The original UNIX shell After years of development , By different agencies 、 For different purposes , Many different types of shell Program . current shell There are mainly several :

• Bourne Shell（ abbreviation sh）： It is Unix One of the first shell Program , It has already become an industrial standard . At present, almost all Linux All systems support it . however Bourne Shell The operation control function of is weak , And it does not support alias and history .
• C Shell（ abbreviation csh）
• Korn Shell（ abbreviation ksh）

• Bourne Again Shell：( abbreviation bash) Ability to provide environment variables to configure users Shell Environmental Science , Support history , Built in arithmetic function , Wildcard expressions are supported , Built in simplification of common commands .

bash In addition to the command editing function sh There are great improvements , It also adds special functions, which greatly facilitates users in Shell Operations on the command line .

• Supplement commands and documents

1. In the use of Shell On command , Many users will often encounter the situation that the command or file name is not fully remembered Bash Shell The command and file name completion function of will help users . After entering the first few characters of a command or file name , Press TAB Key or ESC Key to automatically fill in the remaining string that has not been entered .
2. If there are multiple commands or files with the same prefix ,shell All commands or files with the same prefix will be listed .shell The prompt message given , Help users recall and complete input . Then wait for the user to enter enough characters .
3. Press twice in a row TAB Key or ESC key , Used to command complement ; Press once TAB key , Used to complete the file name .

• Query command history

1. The user is in Shell There is great continuity in the operation under , The command you once entered may need to be used more than once . When users find problems in operation , You need to check the operations you have performed .Bash Save the command sequence that the user has typed in a command history table . Press “ individual ” Key sum “” key , You can query the command history .
2. Bash Shell It also provides history command . This command lists the command history table , From record number 1 Start , All at once .
   history [numberline]

• wait

shell Command format

shell The prompt identifies the beginning of the command line . The user enters a command at the prompt and presses Enter key , Finish submitting instructions to the system .

• Usually shell The command prompt is in the following format ：
• username： user name , Display the account of the currently logged in user
• hostname： Host name , Displays the host name of the login , For example, if far After logging in , Then the login host name will be displayed ;
• direction： Directory name , Displays the current path , When displayed in the root directory as “/”, When displayed in the user's home directory as “~”;

 

Usually a command contains three elements ： Command name 、 Options 、 Parameters . Command name is required , Options and parameters can be optional . The command format is shown in the figure ：

• $：Shell Prompt , If the current user is a superuser , The prompt is “#”, Other users' prompts are “$”;
• Command： Command name ,Shell Order or procedure , Case sensitive
• Options： Command options , Used to change the type of command execution action , from “-” guide , You can have multiple options at the same time ;
• Argument： Command parameter , Point out the object or target of the command , Some commands allow multiple arguments
• The three elements of a command are separated by spaces ;
• If you write multiple commands on one line , Use semicolon （;） Separate the commands ;
• If a command cannot be written on one line , Use a backslash at the end of the line （\） Indicate that the order is not over .

The Conduit

Pipes can connect a series of commands , It means that the output of the first command will be used as the input of the second command , Pass it through the pipe to the second command , The output of the second command will be used as the input of the third command , And so on . It's like using “|” The symbol is connected into a pipe .

Of the above operations , With the help of pipes “|”, take ls The output of is directly used as wc Command input . Using pipes can skillfully combine some commands , Get the effect that a single command cannot achieve . For example, use the above command combination , Get is /usr/bin Number of files in the directory .

Input / Output redirection

Input / Output redirection is a change shell Command or program default standard input / Output target , Redirect to a new goal .

linux The default standard input in is defined as keyboard , Standard output is defined as terminal window .

The user can change the input or output for the current operation , Force the input or output source of a specific command to be an external file .

Order replacement

Command substitution is to take the output of one command as an argument to another command . The command format is as follows .

• among , command command2 The output of will be used as a command command1 Parameters of . We need to pay attention to , The single quotation mark of the command substitution is ESC Key below ` key
• pwd The command is used to display the absolute path of the current directory . On the command line above , Use the command substitution character , take pwd The running result of is as ls Arguments to the command . Final , The result of command execution is to display the file contents of the current directory .
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                （ Now just know how to use it , Later, we will practice in depth ）

frequently-used shell command ：

man ( Get online help )

Generally, the online help information includes the following sections ：

• NAME： Name of the command
• SYNOPSIS： The syntax of the command
• DESCRIPTION： General description and purpose of the command
• OPTIONS： Describe all the parameters or options of the command
• SEE ALSO： List the functions directly related to this command in the online help page Other commands that can be similar
• BUGS： Explain any known problems or defects in the command or its output
• EXAMPLES： Common usage examples
• AUTHORS： The author of the online help page and the command

passwd

Use alone passwd command , It means to change the password of the current user . The following command example is used to modify the user's own password .

su

su The command is used to temporarily change the user's identity , Have permissions of other users . Ordinary users can use su The command temporarily has the authority of super user ; Super users can also use ordinary user identity to complete some operations . When you need to give up your current user identity , have access to exit Command switch back .su The general syntax format of the command is ：

Options “-c” It means that it ends after executing a command ;-m Indicates that the environment variables remain unchanged ;- Indicates when the user identity is changed , Use the user's environment at the same time . 

Use alone su command , The default is to convert to super user root.

echo

echo The command is used in standard output —— The display shows a paragraph of text , It usually serves as a reminder .echo The general syntax format of the command is ：

Options -n Does not wrap after output text . The prompt string can be quoted , Or you can skip it . 

 date

date The command is used to display and set the system date and time .

Options -s In accordance with the said datestr Date display format setting date ; Use alone date command , Used to display the current date in the system clock .

 df

df The command is used to view disk space usage . Checking disk space is something users should often do , Because no one wants to see roots or /var Partitions fill up inadvertently , In order to clean up in time .df The general format of the command is ：

  among , Parameters Filesystem Represents a physical file system . The meaning of each option is shown in the table .

   -l ： List only local file systems      

 du

du command , Lists the number of disk blocks used by directories and files , Each piece accounts for 512 Bytes .

Common parameters ：

• -a ： List only the number of free files
• -h ： List disk usage （KB）
• -s ： List the total free space （KB）