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What is a pipeline command? Judging from the name, it should be divided into 'pipeline' and 'command'.

pipeline

Let's look at pipelines first. This is how I understand pipelines. Pipelines are used to establish a connection between two commands, that is, the standard output of the previous command is the standard input of the next command. This process is achieved through pipelines.

The pipe uses the symbol '|'. It should be noted that the pipe processes the standard output of the previous command. The standard error output of the previous command will be ignored. In other words, the output information of the previous command that is executed correctly will be used as the input of the next command. If the previous command is executed incorrectly, the error information printed will not be used as the input of the next command. This is the role of the pipe, see the following example:

# ls –a /etc | grep ^s

This example is used to search for files in /etc that begin with s. The results of searching for files in /etc are piped as the standard input of the grep command.

Order

After talking about pipes, let's look at commands. The commands mentioned here are mainly for the commands following the pipe (|). As we said above, the function of the pipe is to use the standard output of the previous command as the standard input (stdin) of the next command, so the next command is required to be able to read data from the standard input device. So not all commands supported by the Linux system can be used as pipe commands.

Like the case introduced in this article, we cannot use the cp command directly after the pipeline. The following command is wrong:

# find /a –name login* | cp –t /b //Error

Because cp is not a pipe command, it cannot accept data from standard input (stdin).

The commands that accept data from standard input are:

Select command: cut , grep

Sorting statistics commands: sort , uniq , wc

Bidirectional redirection: tee

Character conversion commands: tr , col , join , paste , expand

Split command: split

Parameter substitution: xargs

The above commands are pipeline commands that can accept data from standard input. I will not introduce these commands in this article. I will introduce the usage of these commands in subsequent articles.

Summarize

Here is a summary of the pipeline commands:

1. The pipeline command will only process standard output and ignore standard error output

2. The pipeline command must be able to accept the data from the previous command as standard input and continue processing.

This is the end of the introduction to the concept of pipeline commands. I hope this article will be helpful to you.