7 SSH command usages under Linux that you don't know

A system administrator may manage multiple servers at the same time, and these servers may be placed in different places. It is obviously not the best way to manage them one by one in person. The remote control method should be the most effective.

There are several remote management tools for Linux systems: telnet, ssh, vnc, etc. Among them, ssh is the most commonly used management method, using ciphertext transmission, which is simple and safe.

Secure ShellThe abbreviation is SSH, formulated by the IETF Network Working Group (Network Working Group), SSH is a security protocol created based on the application layer and transport layer, providing a secure transmission and usage environment for the computer's shell.

Let’s introduce the 7 major uses of SSH.

1. Basic usage

The simplest usage is to enter ssh without parameters and then add the host address, for example:

ssh 192.168.0.116

This form will log in to the host. By default, the current user is used to log in. When connecting for the first time, SSH will confirm the authenticity of the target host. If there is no problem, just enter yes.

If we want to log in to the host with a specified username, there are two methods:

a. Use the -l option

ssh -l alvin 192.168.0.116

b. Use user@hostname format

ssh alvin@192.168.0.116

These two methods, the second one is particularly commonly used.

2. Specify port to log in

The default port number used by SSH is 22. Most modern Linux systems have port 22 open. If you run the ssh program without specifying a port number, it will directly send the request through port 22.

If we don’t want to log in through port 22, then we can use the -p option to specify the port.

ssh 192.168.0.116 -p 1234

Extended topic: How to change the port number?

Just modify /etc/ssh/ssh_config and modify the following line:

Port 22

3. Request compression for all data

Use -C option, all data sent or received via SSH will be compressed and still encrypted.

ssh -C 192.168.0.116

However, this option is more useful when the Internet speed is not very fast. When the Internet speed is fast, using compression will reduce efficiency, so use it according to the situation.

4. Turn on debug mode

For some reasons, we want to trace and debug the SSH connection we established. The -v option parameter provided by SSH is designed for this. It can see where the problem occurred.

[Alvin.Alvin-computer] ➤ ssh -v pi@192.168.0.116
OpenSSH_7.1p2, OpenSSL 1.0.1g 7 Apr 2014
debug1: Reading configuration data /etc/ssh_config
debug1: Connecting to 192.168.0.116 [192.168.0.116] port 22.
debug1: Connection established.
debug1: key_load_public: No such file or directory
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.4p1 Raspbian-10+deb9u4
debug1: match: OpenSSH_7.4p1 Raspbian-10+deb9u4 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 192.168.0.116:22 as 'pi'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received

5. Bind source address

If your client has more than two IP addresses, it is impossible for you to tell which IP is being used to connect to the SSH server. . To solve this situation, we can use the -b option to specify an IP address. This IP will be used as the source address to establish the connection.

[Alvin.Alvin-computer] ➤ ssh -b 192.168.0.105 pi@192.168.0.116
Linux raspberrypi 4.14.71-v7+ #1145 SMP Fri Sep 21 15:38:35 BST 2018 armv7l

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Sun Feb 24 08:52:29 2019 from 192.168.0.105

6. Remote execution of commands

If we want to execute a command on the target host, our usual approach is to log in to the target host first, execute the command, and then exit. Of course it is possible to do this, but it is more troublesome.

If we just want to execute a command remotely, we can just follow the command directly, as follows:

[Alvin.Alvin-computer] ➤ ssh pi@192.168.0.116 ls -l
Desktop
Documents
Downloads
MagPi
Music

7. Mount the remote file system

Another one A great SSH-based tool is called sshfs. sshfs allows you to directly mount the remote host's file system locally. Its usage format is as follows:

sshfs -o idmap=user user@hostname:/home/user ~/Remote

For example:

sshfs -o idmap=user pi@192.168.0.116:/home/pi ~/Pi

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