The userdel command is used to delete a specified user and files related to the user. The full English name is "user delete". In fact, the userdel command actually modifies the system's user account files /etc/passwd, /etc/shadow and /etc/group files. This coincides with the Linux system's idea of â€‹â€‹"all operations are files".

It is worth noting that, but if there is a process running related to the user to be deleted, the userdel command usually does not delete a user account. If you really must delete, you can terminate the user process first, and then execute the userdel command to delete. But the userdel command also provides a parameter to face this situation, that is, the "-f" option.
<h3><strong>Syntax</strong>:</h3>
<pre>userdel [parameter] [user name]</pre>
<h5>Command parameters</h5>
<table class="has-fixed-layout">
<tbody>
<tr>
<td>-f</td>
<td>Forcibly delete user accounts</td>
</tr>
<tr>
<td>-r</td>
<td>Delete the user's home directory and any files in it</td>
</tr>
<tr>
<td>-h</td>
<td>Display the help information of the command</td>
</tr>
</tbody>
</table>
<h5>Example</h5>
Delete users, but do not delete their home directories and files:
<pre class="wp-block-preformatted">[root@linuxstar ~]# userdel linuxstar</pre>
Delete users, and delete their home directories and files:
<pre class="wp-block-preformatted">[root@linuxstar ~]# userdel -r linuxstar</pre>
Forcibly delete users:
<pre class="wp-block-preformatted">[root@linuxstar ~]# userdel -f linuxstar</pre>

userdel command-delete user


        
<p>The English spelling of bc is "Binary Calculator", which is a calculator language that supports arbitrary precision interactive execution. Bash has built-in support for the four arithmetic operations of integers, but does not support floating-point operations. The bc command can easily perform floating-point operations. Of course, integer operations are no longer a problem.</p>



<p><strong>Syntax</strong>: bc [options]</p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-i</td><td>Force into interactive mode</td></tr><tr><td>-l</td><td>Define the standard math library used </td></tr><tr><td>-w</td><td>Define the standard math library used </td></tr><tr><td>-q</td><td>Print normal GNU bc environment information </td></tr></tbody></table>



<h5>Example</h5>



<p>Arithmetic operations advanced operations bc command It can perform floating-point operations and some advanced functions:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# echo "1.212*3" | bc 3.636</pre>



<p>Set decimal precision (numerical range):</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# echo "scale=2;3/8" | bc .37</pre>



<p>Calculate the square and square root: </p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# echo "10^10" | bc 10000000000 [root@linuxstar ~]# echo "sqrt(100)" | bc 10</pre>
              

bc command-floating point operations


        
<p>rmmod is short for "remove module", and the rmmod command is used to delete a module. Execute the rmmod command to delete unnecessary modules. The core of the Linux operating system has modular features, so when compiling the core, you don't need to put all the functions into the core, you can compile these functions into individual modules, and then load them separately when needed.</p>



<p><strong>Syntax</strong>: rmmod [parameter] [module name]</p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-a</td><td>Delete all modules that are not currently needed</td></tr><tr><td>-s</td><td>Output information to syslog resident service instead of terminal interface</td></tr><tr><td>-v</td><td>Show detailed information about command execution</td></tr><tr><td>-f</td><td>Force removal of the module, using this option is more dangerous</td></tr><tr><td>-w</td><td>Waiting, know that the module can be removed before removing the module</td></tr></tbody></table>



<h5>Example</h5>



<p>Uninstall the module pppoe and display the execution information:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# rmmod -v pppoe</pre>



<p>Uninstall the bridge module:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# rmmod bridge</pre>



<p>Uninstall the bridge module and write error information to syslog:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# rmmod -s bridge</pre>



<p>Isolate the bridge module being used, knowing that it is not being used:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# rmmod -w bridge</pre>



<p>Delete the bridge module in use (-f can delete the module in use, it is very dangerous, use it with caution):</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# rmmod -f bridge</pre>



<p></p>
              

rmmod command-delete module


        
<p>The dirs command is used to display the stack records in the Shell directory. When no option is added, all directories in the current Shell command stack are displayed. When displaying, the left side is the newly added directory.</p>



<p><strong>Syntax</strong>: dirs [parameter]  </p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-c</td><td>Delete all records in the directory stack</td></tr><tr><td>-l</td><td>Display the directories in the stack in full format. When displaying the user's home directory, list the full path instead of "~"</td></tr><tr><td>-n</td><td>Display the nth directory from the right (n counts from 0)</td></tr><tr><td>+n</td><td>Display the nth directory from the left (n counts from 0) </td></tr><tr><td>-p</td><td>List multiple records in the stack with one record per line</td></tr><tr><td>-v</td><td>List all records in the stack with one record per line, and add a serial number to each line (numbering starting from 0)</td></tr></tbody></table>



<h5>Example</h5>



<p>Display the records in the Shell directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs</pre>



<p>Display the contents of the Shell directory stack, one record per line:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs -p </pre>



<p>Display the contents of the Shell directory stack, one record per line, plus the serial number:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs -v</pre>



<p>Delete records in the Shell directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs -c </pre>
              

dirs command-Display records in the Shell directory stack


        
<p>The hash command is responsible for displaying and clearing the hash table that the system prefers to query when the command is running.</p>



<p>When the hash command is executed without specifying parameters or flags, the hash command reports the contents of the path name list to standard output. This report contains the path name of the command in the current shell environment found by the previous hash command call. It also contains those commands that are called and found by the normal command search process.</p>



<p><strong>Syntax</strong>: hash [parameter] [directory]</p>



<p>Common parameters:ï»¿</p>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-d</td><td>Clear records in the hash table</td></tr><tr><td>-l</td><td>Show commands in hash table</td></tr><tr><td>-p&lt;command&gt;</td><td>Add the command with the full path to the hash table</td></tr><tr><td>-r</td><td>Clear the records in the hash table</td></tr><tr><td>-t</td><td>Display the full path of the command in the hash table</td></tr></tbody></table>



<h5>Example</h5>



<p>Display the commands in the hash table:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# hash -l </pre>



<p>Delete the command in the hash table:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# hash -r </pre>



<p>Add commands to the hash table:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# hash -p /usr/sbin/adduser myadduser </pre>



<p>Clear records in the hash table:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# hash -d</pre>



<p>Display the full path of the command in the hash table:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# hash -t</pre>
              

hash command-display and clear the hash table queried when the command is running


        
<p>The shopt command can set the switch values â€‹â€‹of the variables that control the behavior of the Shell to achieve the purpose of controlling the behavior of the Shell. When executing the command shopt without any parameters, those characteristics and their states controlled by shopt will be listed.</p>



<p><strong>Syntax</strong>: shopt [parameter]  </p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-s</td><td>Without options, all open options are displayed. If followed by the corresponding option, turn on this option</td></tr><tr><td>-u</td><td>Without options, all unopened options are displayed. If followed by the corresponding option, turn on this option</td></tr><tr><td>-p</td><td>Show all options</td></tr></tbody></table>



<h5>Example</h5>



<p>Show all configuration options:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# shopt -p </pre>



<p>View the open configuration options:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# shopt -s</pre>



<p>View the closed configuration options:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# shopt -u</pre>
              

shopt command-set switches that control shell behavior variables

The EXTENDED_HISTORY environment variable affects the execution of the fc command. If the variable is set to ON, the time is recorded, otherwise the time is not recorded.

<strong>Syntax</strong>: fc [parameter]
<h3>Command parameters:</h3>
<table class="wp-block-table has-fixed-layout">
<tbody>
<tr>
<td>-e&lt;text editor&gt;</td>
<td>Specify the text editor used to edit the command, the default is vi</td>
</tr>
<tr>
<td>-l</td>
<td>List the history commands within the command range of the first and last day. If the command range is not followed, the 16 most recently used history commands will be displayed by default</td>
</tr>
<tr>
<td>-n</td>
<td>Do not display command serial number when displaying historical commands</td>
</tr>
<tr>
<td>-r</td>
<td>Display all historical commands in reverse order</td>
</tr>
<tr>
<td>-s&lt;command name&gt;</td>
<td>Find the specified command from the current position in the history command and execute it</td>
</tr>
</tbody>
</table>
<h5>Example</h5>
Display the history command list (the last 16 history commands are printed by default):
<pre class="wp-block-preformatted">[root@linuxstar ~]# fc -l</pre>
Specify editing commands using the ex text editor:
<pre class="wp-block-preformatted">[root@linuxstar ~]# fc -e ex</pre>
Display the history command list but not the command sequence number:
<pre class="wp-block-preformatted">[root@linuxstar ~]# fc -n</pre>
Display all historical commands in reverse order:
<pre class="wp-block-preformatted">[root@linuxstar ~]# fc -r</pre>
Find the find command from the historical commands and execute:
<pre class="wp-block-preformatted">[root@linuxstar ~]# fc -s find</pre>

fc command-edit and execute historical commands


        
<p>The pushed command is used to add a directory to the stack, add a record to the top of the directory stack, and switch to the directory. If no parameters are added, the two directories at the top of the record stack will swap positions.</p>



<p><strong>Syntax</strong>: pushed [parameter] [directory]</p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-n </td><td>Move the nth directory from the right to the top of the stack (n starts counting from 0) </td></tr><tr><td>+n </td><td>Move the nth directory from the left to the top of the stack (n starts counting from 0) </td></tr></tbody></table>



<h5>Example</h5>



<p>Display Shell's default directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs
/var/log</pre>



<p>Add the directory /etc to the Shell directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# pushed /etc</pre>



<p>Display the contents of the Shell directory stack, one record per line, plus the serial number:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs -v
0 /sbin
1 /etc
2 /var/log</pre>



<p>Move the first record to the top of the stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# pushed -1
/etc /var/log /sbin</pre>
              

pushed command-add records to the shell directory stack


        
<p>Used to delete records from the Shell directory stack. If no option is added, the popd command deletes the record at the top of the Shell directory stack first, and then switches the current directory to the top directory of the new Shell directory stack.</p>



<p><strong>Syntax</strong>: popd [parameter]  </p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-n</td><td>Delete the nth directory from the right (n starts counting from 0)</td></tr><tr><td>+n</td><td>Delete the nth directory from the left (n starts counting from 0)</td></tr></tbody></table>



<h5>Example</h5>



<p>Display the contents of the Shell directory stack, one per line:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# dirs -v
0 /sbin
1 /etc
2 /var/log</pre>



<p style="text-align:left">Remove /sbin from the shell directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# popd
/etc /var/log</pre>



<p>Display the current working directory:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# pwd
etc</pre>



<p>Delete the top directory from the directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# popd
/var/log</pre>



<p>Delete the second directory from the right from the directory stack:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# popd -2</pre>
              

popd command-delete records from the shell directory stack


        
<p>The jobs command is mainly used to display the list of tasks in the system and their running status.</p>



<p>This command can display the task number and its corresponding process number. The task number is from the perspective of ordinary users, and the process number is from the perspective of system administrators. A task can correspond to one or more process numbers.</p>



<p><strong>Syntax</strong>: jobs [parameter] [directory]</p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-l</td><td>Include process number when displaying job list</td></tr><tr><td>-n</td><td>Display the jobs whose status has changed since the last use of jobs</td></tr><tr><td>-p</td><td>When displaying the job list, only the corresponding process number is displayed</td></tr><tr><td>-r</td><td>Show only running jobs</td></tr><tr><td>-s</td><td>Show only stopped jobs</td></tr></tbody></table>



<h5>Example</h5>



<p>Display the current job list:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# jobs </pre>



<p>Display the current job number and the corresponding process number:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# jobs -l</pre>



<p>Show only running jobs:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# jobs -r</pre>



<p>Show only suspended jobs:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# jobs -s</pre>



<p> Display the jobs whose status has changed since the last use of jobs: </p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# jobs -n </pre>
              

jobs command-Display job information of the shell


        
<p>The declare command is used to define shell variables, but the variables defined by declare can only work in the current shell environment, and will be invalid after switching the shell environment.</p>



<p><strong>Syntax</strong>: declare [parameter] [directory]</p>



<h5>Command parameters</h5>



<table class="wp-block-table has-fixed-layout"><tbody><tr><td>-a</td><td>Declare array variables</td></tr><tr><td>-f</td><td>Only show functions</td></tr><tr><td>-F</td><td>Do not display function definitions</td></tr><tr><td>-i</td><td>First calculate the expression and assign the result to the declared variable</td></tr><tr><td>-p</td><td>Display the defined method and value of a given variable. When this option is used, other options will be ignored</td></tr><tr><td>-r</td><td>Define read-only variables</td></tr><tr><td>-x</td><td>Convert the specified Shell variables into environment variables</td></tr></tbody></table>



<h5>Example</h5>



<p>Display the defined variables:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# declare </pre>



<p>Define a new Shell variable:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# declare var1="hello"</pre>



<p>Use "-i" to define a new variable:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# declare -i var2=100+200</pre>



<p> Use "-p" to display the definitions of variables var1 and var2:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# declare -p var1 var2</pre>



<p>Show all environment variables:</p>



<pre class="wp-block-preformatted">[root@linuxstar ~]# declare -x</pre>
              

-f	Forcibly delete user accounts
-r	Delete the user's home directory and any files in it
-h	Display the help information of the command

Syntax:

Command parameters

Example