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This article mainly describes how SSH remote execution tasks, the article is very detailed, has a certain reference value, interested friends must read!

remotely execute commands

If we want to check disk usage on a host, do we have to log on to the target host to execute df? Of course not, we can use the ssh command to execute df on a remote host and display the results directly. The whole process is like executing a command locally:

$ ssh nick@xxx.xxx.xxx.xxx "df -h"

So how do you execute multiple commands at once? In fact, it is also very simple, using semicolons to separate different commands is OK:

$ ssh nick@xxx.xxx.xxx.xxx "pwd; cat hello.txt"

The first command returns the result: /home/nick

This means that the current directory when executing the command in this way is the home directory of the logged in user.

The second command returns the contents of the hello.txt file.

Note that it is best to enclose more than one command in quotes, otherwise all but the first command is executed locally on some systems.

Execute commands that require interaction

Sometimes we need to execute commands remotely that are interoperable.

$ ssh nick@xxx.xxx.xxx.xxx "sudo ls /root"$ ssh nick@xxx.xxx.xxx.xxx "top"

Although these two commands fail for different reasons, they have one thing in common: they both require user interaction (TTY required). So they fail for the same reason:

By default, when you perform an ssh connection without commands, you are assigned a TTY. Because you should want to run a shell session at this point.

However, when you execute commands on a remote host via ssh, no TTY is assigned to the remote session. At this point ssh immediately exits the remote host, so commands that require interaction are terminated.

Fortunately, we can explicitly tell ssh that we need a TTY remote shell to interact with via the-t argument!

After adding the-t argument, ssh stays logged in until you exit the command that requires interaction.

To summarize, let's look at the official interpretation of the-t parameter:

"Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty. "

Well, what's even more impressive is that we can specify multiple-t parameters!

Execute multiline commands

Sometimes we may need to write a few lines of simple logic, which is no problem, ssh can easily do it!

You can start with a single quote or double quote, write a few lines, and end with the same quote.

So what if you need to use quotation marks in a command?

In fact, there is a common rule for similar situations, which is to mix single and double quotes. The same rule applies here:

What happens when we reference variables in commands?

Notice that the last line in the image above does not print the nick we expected. This is a bit weird because if the variable is not explained, the output should be $name. But there's nothing out here.

For writing references to variables, you can ensure that variables are interpreted correctly by:

Notice that we specified the-c parameter for bash in the command above.

Remote Execution Script

For scenes where complex functions are to be completed, if only a few commands can be executed, it is simply weak. We may need to write lengthy shell scripts to accomplish a mission! At this point SSH is still a good helper (haha, the previous content is only an appetizer ah!).

Execute local scripts

We create a script file test.sh locally with the following content:

lspwd

Then run the following command:

$ ssh nick@xxx.xxx.xxx.xxx < test.sh

By redirecting stdin, the local script test.sh is executed on the remote server.

Next we want to pass an argument to the script www.example.com. To verify the argument passed in, add two lines to the end of the test.sh file: test.sh

echo $0echo $1

Then try the following command:

$ ssh nick@xxx.xxx.xxx.xxx < test.sh helloworld$ ssh nick@xxx.xxx.xxx.xxx < "test.sh helloworld"

The following figure shows the results of the implementation:

It seems that none of the above methods can pass parameters to scripts.

To execute scripts with arguments in this case (executing local scripts remotely), you need to specify the-s argument for bash:

$ ssh nick@xxx.xxx.xxx.xxx 'bash -s' < test.sh helloworld

In the last two lines of the image above, the output is "bash" and "helloworld" for $0 and $1, respectively.

Executing scripts on remote servers

In addition to executing a local script, there is also a scenario where the script file is stored on a remote server and we need to execute it remotely!

There is a script test.sh in the home directory of user nick on the remote server. The document reads as follows:

lspwd

Execute the following command:

$ ssh nick@xxx.xxx.xxx.xxx "/home/nick/test.sh"

Note that you need to specify the absolute path of the script!

Let's also try passing parameters for scripts. Add two lines to the end of the test.sh file on the remote host:

echo $0echo $1

Then try the following command:

$ ssh nick@xxx.xxx.xxx.xxx /home/nick/test.sh helloworld

Great, the last two lines "/home/nick/test.sh" and "helloworld" correspond to $0 and $1 respectively.

That's all for SSH How to Perform Tasks Remotely. Thanks for reading! Hope to share the content to help everyone, more relevant knowledge, welcome to pay attention to the industry information channel!

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