Latest high-performance remote session management tool-screen

Latest high-performance remote session management tool-screen
GuideLinux is a multi-user, multi-process operating system. We often use ssh for remote linux operations. We can log on to an inaccessible terminal, but each terminal is a link variable. Do you often have a headache for long-running tasks, such as system backup and ftp transmission. Generally, we open a remote terminal window for each such task because it takes too long to execute. You must wait for the task to complete. During this period, you cannot close the window or disconnect the connection. Otherwise, the task will be killed and everything will be abandoned.I. analyze the cause of unexpected terminal program shutdownIn Linux/Unix, there are several related concepts:

Process group: A collection of one or more processes. Each process group has a unique process group ID, that is, the ID of the process leader process.
Session: a set of one or more process groups. It has a unique session leader ). The ID of the first process.

A single control terminal (controlling terminal) is available during the session ). The first process connecting to the control terminal is called the control process ). The process currently interacts with the terminal is called the foreground process group. Other process groups are called background process groups.

According to POSIX.1, the default action of SIGHUP is to terminate the program. When the terminal interface detects that the network connection is disconnected, it sends the hanging signal to the control process (the first process in the session ). If the first process of the session ends, the signal is sent to the foreground Process Group of the session. When a process exits and an orphan process group is generated, if any process in the orphan process group is in the STOP state, the SIGHUP and SIGCONT signals are sent to all processes in the process group.
Therefore, when the network is disconnected or the terminal window is closed, the control process receives the SIGHUP signal and exits, causing other processes to exit during the session period.

Session instance

Open two SSH terminal windows and run one of the top commands.

[root@tivf09 root]# top

In another terminal window, find the top process ID is 5180, and its parent process ID is 5128, that is, log on to shell.

[root@tivf09 root]# ps -ef|grep toproot      5180  5128  0 01:03 pts/0    00:00:02 toproot      5857  3672  0 01:12 pts/2    00:00:00 grep top

Use the pstree command to see the relationship more clearly:

[root@tivf09 root]# pstree -H 5180|grep top|-sshd-+-sshd---bash---top

Run the ps-xj command. You can see that the login shell (PID 5128) and top are in the same session period. shell is the first process in the session period and the PGID of the Process Group is 5128, the pgid of the top process group is 5180, which is the foreground process group.

[root@tivf09 root]# ps -xj|grep 5128 5126  5128  5128  5128 pts/0     5180 S        0   0:00 -bash 5128  5180  5180  5128 pts/0     5180 S        0   0:50 top 3672 18095 18094  3672 pts/2    18094 S        0   0:00 grep 5128

Close the first SSH window. In the other window, you can see that the top is also killed.

[root@tivf09 root]# ps -ef|grep 5128root     18699  3672  0 04:35 pts/2    00:00:00 grep 5128

Ii. Solution: use toolsSolution 1: Use the nohup command

If we can ignore the SIGHUP signal, closing the window will not affect the running of the program. The nohup command can achieve this purpose. If the program's standard output/standard error is a terminal, nohup redirects it to the nohup. out file by default. It is worth noting that the nohup command only causes the program to ignore the SIGHUP signal and also requires the flag& Put it in the background for running.

nohup <command> [argument…] &

Although nohup is easy to use, it is still relatively "simple". It can handle simple commands and is troublesome for complicated tasks that require human-computer interaction.

Solution 2: Use the utility screen

We can use a more powerful utility screen, which can be downloaded and installed from the official website of GNU screen to solve this problem.

[root@linuxprobe ~]# rpm -qa|grep screenxscreensaver-4.18-5.rhel4.11screen-4.0.2-5

Screen tool usage

To put it simply, Screen is a window manager that can reuse a physical terminal among multiple processes. Screen has the concept of session. You can create multiple screen Windows in one screen session, just like operating a real telnet/SSH connection window in each screen window. There are several ways to create a new window in screen:

1. Directly type the screen command in the command line.

[root@tivf06 ~]# screen

Screen creates a full Screen window for executing shell. You can execute any shell program, as in the ssh window. In this window, type exit to exit the window. If this is the only window of the screen Session, the screen session will exit; otherwise, the screen will automatically switch to the previous window.

2. The Screen command is followed by the program you want to execute.

[root@tivf06 ~]# screen vi test.c

Screen creates a single window session for executing vi test. c. Exit vi and exit the window/session.

3. Create a screen session in both of the preceding methods. We can also create a new window in an existing screen session. In the current screen window, type C-a c, that is, Ctrl + a, and then press c. screen generates a new window in the session and switches to the window.

Screen also has more advanced features. You can temporarily disconnect (detach) the screen session without interrupting the running of the program in the screen window, and re-Connect (attach) The session at a later time to re-control the program running in each window. For example, open a screen window to edit the/tmp/abc file:

[root@tivf06 ~]# screen vi /tmp/abc

After that, we want to temporarily exit and do something else, such as going out for a walk, then type C-a d in the screen window, and Screen will display the detached prompt:
Temporarily interrupt session

After half an hour, I came back and found the screen session:

[root@tivf06 ~]# screen -lsThere is a screen on:        16582.pts-1.tivf06      (Detached)1 Socket in /tmp/screens/S-root.

Reconnect session:

[root@tivf06 ~]# screen -r 16582

Let's see what happened. It's great. Everything is there. Continue.

You may notice that a special key combination C-a is used to send commands to screen. This is because the information we typed on the keyboard is directly sent to the current screen window, you must use other methods to send a command to the screen window manager, by default, screen receives commands starting with C-. This command form is called key binding in screen, and C-a is called command character ).

You can use "C-? "To view all key bindings. common key bindings include:

C-? Show all key binding information C-a w show all windows List C-a switch to the previously displayed window C-a c create a new shell window and switch to this window c-a n switch to the next window C-a p switch to the previous window (relative to C-a n) c-a 0 .. 9 switch to window 0 .. 9C-a a sends C-a to the current window C-a d temporarily disconnects the screen Session C-a k to kill the current window C-a [enters the copy/rollback Mode

Screen common options

Use the key to bind C-? The default Command key is C-a, and the Escape Character C-a (literal ^ a) is:

Because screen regards C-a as the beginning of the screen command, if you want the screen window to receive the C-a character, enter C-. Screen also allows you to use the-e Option to set your own command characters and escape characters. The format is:

-Exy x is the command character and y is the character of the escape command character

The screen session started by the following command specifies that the command character is C-t, and the escape C-t character is t, through C-t? Command to see the change.

[root@tivf18 root]# screen -e^tt

Custom command characters and escape characters

Other common command options include:

-C file: Use the configuration file instead of the default $ HOME /. screenrc-d |-D [pid. tty. host] does not enable a new screen session, but disconnects other running screen sessions-h num specifies that the historical rollback buffer size is num row-list |-ls to list existing screen sessions, the format is pid. tty. host-d-m starts a session in disconnected mode-r sessionowner/[pid. tty. host] reconnects to a disconnected session. In multi-user mode, you must specify the sessionowner to connect to screen sessions of other users, setuid-root permission-S sessionname is required to specify a name for the session when the screen session is created-v displays the screen version information-wipe [match] Same-list, but deletes sessions that cannot be connected

In the following example, two screen sessions in the detached state are displayed. You can use screen-r <screen_pid> to reconnect to the screen:

[root@tivf18 root]# screen –lsThere are screens on:        8736.pts-1.tivf18       (Detached)        8462.pts-0.tivf18       (Detached)2 Sockets in /root/.screen.[root@tivf18 root]# screen –r 8736

If one of the sessions is unexpectedly terminated for some reason, screen-list displays the session as dead. Run the screen-wipe command to clear the session:

[root@tivf18 root]# kill -9 8462[root@tivf18 root]# screen -ls  There are screens on:        8736.pts-1.tivf18       (Detached)        8462.pts-0.tivf18       (Dead ???)Remove dead screens with 'screen -wipe'.2 Sockets in /root/.screen.[root@tivf18 root]# screen -wipeThere are screens on:        8736.pts-1.tivf18       (Detached)        8462.pts-0.tivf18       (Removed)1 socket wiped out.1 Socket in /root/.screen.[root@tivf18 root]# screen -ls  There is a screen on:        8736.pts-1.tivf18       (Detached)1 Socket in /root/.screen.[root@tivf18 root]#

The-d-m option is an interesting partner. They start a session in disconnected mode. You can connect to the session as needed later. Sometimes this is a very useful function. For example, we can use it to debug background programs. This option is more commonly used:-dmS sessionname

Start an initial disconnected screen session:

[root@tivf06 tianq]# screen -dmS mygdb gdb execlp_test

Connect to this session:

[root@tivf06 tianq]# screen -r mygdb

Manage your remote sessions

Let's take a look at how to use screen to solve the SIGHUP problem. For example, we want to transfer a large file over ftp. If you follow the old method, log on to the system via SSH and run the ftp command to start transmission .. If the network speed is okay, congratulations, you don't have to wait too long. If the network is not good, wait honestly. You can only disconnect the SSH connection after the transmission is complete. Let's try using screen.

Log on to the system through SSH and Type screen in the command line.

[root@tivf18 root]# screen

In the screen shell window, enter the ftp command, log on, and start transmission. Don't you want to wait? OK. In the window, type C-a d:
Manage your remote sessions

Then .. Log out of SSH? As you do, just don't kill the screen session.

Is it convenient? Furthermore, we can use screen to manage your remote sessions and save all your work content. Do you have to open many windows every time you log on to the system, and then open them again every day? Let screen help you "save". You only need to open an ssh window and create a screen window. When you exit, C-a d will "save" your work, after the next login, you can directly use screen-r <screen_pid>.

It is better to give a name for each window so that you can remember it. Use C-a A to name the window. Use C-a w to view the names of these windows. The names may appear in different positions.

Use putty:

Use telnet:

More Screen Functions

Screen provides rich and powerful customization functions. You can configure the files/etc/screenrc and $ HOME/by default in Screen /. specify more in screenrc, such as setting screen options, customizing binding keys, setting screen session self-start Windows, enabling multi-user mode, and customizing user access permission control. If you want to, you can also specify the screen configuration file.

Taking the multi-user function as an example, screen runs in single-user mode by default. You need to specify multiuser on in the configuration file to enable multi-user mode, through acl * (acladd, acldel, aclchg ...) command, You can flexibly configure other users to access your screen session. For more information about the configuration file, see the man page of screen.

From: http://www.ibm.com/developerworks/cn/linux/l-cn-screen/

Address: http://www.linuxprobe.com/screen-remote-use.html