作业控制和日志（bg、fg、tail）

When using the terminal, you might want to run a long-term command without locking up the shell. Linux supports Job Control, which allows you to send processes to the background and switch them back to the foreground. In addition, you will learn how to monitor logs to check what is happening on the system.

Running Commands in the Background: & and jobs

To start a command directly in the background (releasing control of the terminal immediately), append the & (ampersand) character to the end of the command:

sleep 1000 &

This command returns a job number (e.g., [1]) and a PID.

Listing Jobs: jobs

The jobs command lists the active processes associated with the current shell session that are running in the background or suspended:

jobs

Moving Processes: fg, bg, and Ctrl+Z

If a command is running in the foreground and blocking the terminal, you can:

1. Press Ctrl + Z to suspend (pause) the current process.
2. Use the bg (background) command to restart the suspended process in the background:
   Bash

   复制

   bg %1   # Restarts job 1 in the background

3. Use the fg (foreground) command to bring a background process back to the foreground:
   Bash

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   fg %1   # Brings job 1 to the foreground

Monitoring Logs: tail and journalctl

System and application logs record important events. Often, you need to observe these files in real-time as they are being written.

Viewing the End of a File: tail

The tail command displays the last lines of a file (default is 10 lines):

tail /var/log/syslog
tail -n 20 /var/log/syslog    # Shows the last 20 lines

To monitor a file in real-time (for example, to see log messages generated while testing an application), use the -f (follow) option:

tail -f logs/server.log

Try it yourself

Exercise 1: List active jobs

Exercise 2: Bring a job to the foreground

Exercise 3: Check the last lines of a log