[Shell command collection system management] Linux terminates or sends a signal to the process skill command user guide

Directory title

  • Description
    • grammar format
    • parameter description
    • Error condition
  • Notes
  • Bottom layer implementation
  • Example
    • example one
    • Example two
    • Example three
    • Example four
    • Example five
    • Example six
    • Example seven
  • Implemented in c language
  • Conclusion

Shell Command Column: Full Analysis of Linux Shell Command

description

In Linux, the skill command is used to terminate or send a signal to a process. It allows users to select processes to terminate based on their name or process ID (PID), and can choose to send different signals to achieve different actions.

Using the skill command, users can perform the following operations:

  1. Terminate process: By specifying the name or PID of the process, you can use the skill command to terminate the specified process. This is useful if you need to forcefully close an unresponsive process or terminate a process that is hogging too many resources.

  2. Sending Signals: In addition to terminating the process, the skill command can also send different signals to the process. These signals can be used to implement different actions such as reloading configuration files, restarting processes, etc.

  3. Select process: The skill command allows the user to select the process to operate based on the process name or PID. This allows users to operate specific processes according to their needs without affecting other processes.

  4. Batch operations: The skill command also supports batch operations, and users can terminate or send signals to multiple processes at one time. This is useful for situations where multiple processes need to be manipulated simultaneously.

All in all, the skill command is a powerful tool that helps users manage and control processes. It provides the functions of terminating processes and sending signals, and can select the process to be operated according to the name or PID of the process, and supports batch operations. This allows users to manage processes in the system more flexibly.

Grammar format

skill [options] [arguments]

parameter description

  • -: Specifies the signal to send. Commonly used signals include KILL, STOP, CONT, etc.
  • -u : Specify the user name to operate.
  • -c : Specify the name of the process to operate.
  • -p : Specifies the process ID to be operated.
  • -f: Ignore the permission check of the process.
  • -n: Do not send a signal to the process, only display what will be done.
  • -V: Display the version information of the command.

Error

  • If the specified process ID or process name does not exist, an error message will be displayed.
  • If you do not have sufficient permissions to terminate or send a signal to a process, a permissions error will be displayed.
  • If the specified signal is not available or not supported, an error message will be displayed.

Note that specific error messages may vary by OS version and configuration. When using the skill command, be careful and ensure that the operation on the process complies with the security and stability requirements of the system.

Notes

There are a few caveats to be aware of when using the skill command in the Linux shell:

  1. Choose the signal carefully: choose the appropriate signal to send to the process according to your needs. Different signals have different effects, for example, the KILL signal will forcibly terminate the process, and the HUP signal will reload the configuration file. Make sure to understand the meaning and impact of each signal to avoid terminating or manipulating processes unexpectedly.

  2. Confirm Process ID or Name: Before executing the skill command, make sure you know exactly the ID or name of the process you want to operate on. Incorrectly selecting a process may result in unexpected termination of other processes or inability to operate the target process.

  3. Pay attention to permissions: some operations may require root or specific user permissions to perform. Before using the skill command, make sure you have sufficient privileges to terminate or send a signal to the target process. Otherwise, you may receive a permission error or an error message that the operation cannot be performed.

  4. Use force termination with caution: Terminating a process with the KILL signal is a forceful operation that may result in data loss or system instability. Before using the KILL signal, make sure you have tried other signals or methods, and make sure you understand the potential impact of terminating a process.

  5. Pay attention to batch operations: When using the skill command for batch operations, pay special attention to selecting the correct process ID or name. Terminating multiple processes at once may cause system instability or service interruption.

  6. Understand system services: When operating system services, take special care not to kill critical system processes or services. Terminating the wrong process may cause the system to crash or not function properly.

  7. Use the -n option with caution: Use the -n option to preview what will be done without actually sending a signal to the process. When using this option, make sure to double check what you are about to do to avoid terminating or manipulating processes unexpectedly.

In conclusion, when using the skill command, exercise caution and make sure you understand the meaning and impact of each parameter and option. Before operating critical processes, it is best to back up data or make other necessary preparations to avoid irreversible losses or system failures.

Bottom layer implementation

The underlying implementation of the skill command in the Linux shell involves the following steps:

  1. Parsing command parameters: When the user enters the skill command in the shell, the shell will parse the command parameters, including the signal to be sent, the process ID or name to be operated, and so on.

  2. Obtain process information: According to the process ID or name provided by the user, the system will obtain the information of the target process through the process table or other methods, such as process ID, parent process ID, process status, etc.

  3. Send signal: According to the signal specified by the user, the operating system will send the corresponding signal to the target process. This involves the process management mechanism of the underlying operating system, such as sending signals through the system call kill().

  4. Signal processing: The process that receives the signal will perform corresponding processing according to its signal processing mechanism. For example, for the SIGKILL signal, the process will terminate immediately, while for other signals, the process can choose to ignore, terminate or execute a custom signal processing function.

It should be noted that the skill command itself is not a low-level system call, but a command-line tool in user space. It implements the functions of sending signals and operating processes by calling the underlying system calls. The underlying system calls are responsible for interacting with the operating system kernel to terminate processes and send signals.

In Linux, process management is implemented through the process control block (Process Control Block, PCB) and process table provided by the kernel. The operating system kernel will send a corresponding signal to the target process through a system call according to the user’s operation request, thereby realizing the function of the skill command.

In short, the bottom layer of the skill command realizes the operation of the process through steps such as parsing command parameters, obtaining process information, sending signals and signal processing. This involves the underlying system calls and the process management mechanism of the operating system kernel.

Example

Example 1

skill -KILL 1234

This command will terminate the process with process ID 1234 using the signal KILL.

Example 2

skill -STOP process_name

This command will suspend the process with the name process_name using the signal STOP.

Example three

skill -CONT 5678

This command will resume the process with process ID 5678 using the signal CONT.

Example four

skill -HUP httpd

This command will restart the process named httpd with the signal HUP.

Example five

skill -TERM -u username

This command will terminate all processes belonging to the username username with the signal TERM.

Example six

skill -USR1 9876

This command will send a custom signal to the process with process ID 9876 using signal USR1.

Example seven

skill -STOP -c process_name

This command will suspend all processes with the name process_name using the signal STOP.

Implemented in c language

The following is a sample code written in C language, which implements the function similar to the skill command, which can terminate the specified process or send a signal to the process. The code uses the Linux system call kill() to send signals.

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>

int main(int argc, char *argv[]) {<!-- -->
    // check command line arguments
    if (argc < 3) {<!-- -->
        printf("How to use: ./skill <signal> <pid>\\
");
        return 1;
    }

    // Get signal name and process ID
    char *signal_name = argv[1];
    int pid = atoi(argv[2]);

    // parse the signal name
    int signal_num = 0;
    if (strcmp(signal_name, "KILL") == 0) {<!-- -->
        signal_num = SIGKILL;
    } else if (strcmp(signal_name, "STOP") == 0) {<!-- -->
        signal_num = SIGSTOP;
    } else if (strcmp(signal_name, "CONT") == 0) {<!-- -->
        signal_num = SIGCONT;
    } else {<!-- -->
        printf("Invalid signal name\\
");
        return 1;
    }

    // send signal to process
    int result = kill(pid, signal_num);
    if (result == -1) {<!-- -->
        printf("Send signal failed\\
");
        return 1;
    }

    printf("Successfully send signal to process %d\\
", pid);

    return 0;
}

In this sample code, the signal name and target process ID to be sent are obtained through command line parameters. Then, the corresponding signal number is parsed out according to the signal name. Finally, use the kill() system call to send a signal to the target process. If the signal is sent successfully, a successful prompt message will be output; otherwise, a failed prompt message will be output.

Please note that this is just a simple sample code, which only implements some of the functions of the skill command. The actual skill command has more options and functions, such as batch operations, selecting processes by username, etc. More code and logic is required to fully implement a program that does exactly the same thing as the skill command.

Conclusion

During our exploration, we have gained a deep understanding of the powerful functions and wide applications of Shell commands. However, learning these techniques is just the beginning. The real power comes in how you incorporate them into your daily work to increase efficiency and productivity.

Psychology tells us that learning is a process of continuous and active participation. So, I encourage you not only to read and understand these commands, but also to practice them. Try creating your own commands and gradually master shell programming as part of your daily routine.

At the same time, please remember that sharing is a very important part of the learning process. If you find this blog helpful to you, please like it and leave a comment. Sharing your own problems or interesting experiences when using Shell commands can help more people learn from them.
In addition, I also welcome you to bookmark this blog and come back to check it at any time. Because review and repeated practice are also the key to consolidating knowledge and improving skills.

Finally, please remember: everyone can become an expert in Shell programming through continuous learning and practice. I look forward to seeing you make even more progress on this journey!

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