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This article introduces the relevant knowledge of "what are the matters needing attention in the use of Linux shell". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Shell is the user interface of the system, which provides an interface for users to interact with the kernel. It receives the command entered by the user and sends it to the kernel for execution. It is the interpreter program between the Linux kernel and the user, and now Linux usually refers to the / bin/bash interpreter responsible for translating to the kernel and communicating user / program instructions.

I. the difference between standard input and parameters

This problem must be the most confusing, specifically, it is not clear when to use pipe characters | and file redirection >

For example, I now have a shell script connect.sh that automatically connects to broadband, which is stored in my home directory:

$where connect.sh / home/fdl/bin/connect.sh

What should I do if I want to delete this script and want to type less? I've tried this before:

$where connect.sh | rm

In fact, this is wrong, and the right thing to do should be like this:

$rm $(where connect.sh)

The former attempts to connect the result of where to the standard input of rm, while the latter attempts to pass in the result as a command-line argument.

Standard input is a command such as scanf or readline in a programming language; parameters refer to the array of args characters passed in by the program's main function.

Pipe characters and redirects take data as standard input to the program, while $(cmd) reads the data output from the cmd command as parameters, as explained in the previous drawing:

Input redirection means that when the program wants to read data, it will read it to files [0], so as long as we point files [0] to a file, then the program will read data from that file, not from the keyboard.

Similarly, output redirection is to point files [1] to a file, so that the output of the program is not written to the monitor, but to that file.

Pipe characters are actually the same, connecting the output stream of one process to the input stream of another process into a "pipeline" in which the data is passed:

The underlying principles of Linux processes, threads, and file descriptors

Using the example just now, the rm command source code certainly does not accept standard input, but accepts command-line arguments and deletes the corresponding file. For comparison, the cat command accepts both standard input and command-line arguments:

$cat filename... file text... $cat echo 'hello world' | cat hello world

If the command can block the terminal, it means that the command accepts standard input, otherwise it does not. For example, if you only run the cat command without any parameters, the terminal will block, waiting for you to enter a string and echo the same string.

Second, running programs in the background

Let's say you log in remotely to the server and run a Django web program:

$python manager.py runserver 0.0.0.0 Listening on 0.0.0.0 purl 8080...

Now you can test the Django service with the server's IP address, but the terminal is blocked and you don't respond to anything unless you type Ctrl-C or Ctrl-/ to terminate the python process.

You can add an & symbol after the command so that the command line does not block and respond to commands you enter later, but if you log out of the server, you will not be able to access the web page.

If you want to be able to access the web service after exiting the server, you should wrap the command like this (cmd &):

$(python manager.py runserver 0.0.0.0 &) Listening on 0.0.0.0 purl 8080... $logout

The underlying principle goes like this:

Each command line terminal is a shell process, and the program you execute in this terminal is actually a child process separated by the shell process. Normally, the shell process will block and wait for the child process to exit before re-receiving the new commands you enter. Add the & sign just so that the shell process is no longer blocked and can continue to respond to your new commands. But in any case, if you close the shell command line port, all child processes attached to it will exit.

When (cmd &) runs the command like this, it hangs the cmd command under the name of a systemd system daemon and recognizes systemd as the father, so that when you exit the current terminal, it will have no effect on the previous cmd command.

Similarly, there is a common practice of running in the background:

$nohup some_cmd &

The nohup command works in a similar way, but through my tests, the form (cmd &) is more stable.

The difference between single quotation marks and double quotation marks

There are slight differences between different shell behaviors, but one thing is certain: for the symbols $, (,), strings enclosed by single quotes will not be escaped, and strings enclosed by double quotes will be escaped.

The behavior of shell can be tested. Using the set-x command, the echo of shell command will be enabled. You can observe exactly what commands shell is executing through echo:

It can be seen that echo and (cmd) ", the results are similar, but there are still differences. Note that the result of double quotation mark escape will automatically increase single quotation marks, while the former will not.

That is, if the parameter string read by $contains spaces, it should be enclosed in double quotation marks, otherwise an error will occur.

4. Sudo cannot find the command

Sometimes we ordinary users can use commands that add permissions with sudo and then report an error command not found:

$connect.sh network-manager: Permission denied $sudo connect.sh sudo: command not found

The reason is that the script connect.sh exists only in the user's environment variables:

$where connect.sh / home/fdl/bin/connect.sh

When using sudo, the system thinks that the root user is executing the command, so it searches for the root user's environment variables, and this script is certainly not found in the root environment variables directory.

The workaround is to use the path to the script file, not just the script name:

This is the end of the content of $sudo / home/fdl/bin/connect.sh "what are the precautions for using Linux shell". Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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