How to use pipes | in Linux?

In Linux, the pipeline character "|" is used to use the output of the previous command as the input of the next command, realizing the processing of data in series by multiple commands. 1. The basic usage is to connect two commands, such as ps aux | grep "ssh", which means passing the result of ps aux to grep for filtering; 2. Multi-level pipelines can connect multiple commands in series to form a data pipeline, such as cat data.txt | sort | uniq | wc -l, which complete reading, sort, deduplication and statistics in turn; 3. The pipeline is often used in combination with grep, sort, uniq, awk, wc and other commands, such as counting the number of processes or analyzing the frequency of historical commands; 4. Note when using: the pipeline only processes standard output, and the error output needs to be merged with 2>&1. The failure of the command will affect subsequent execution. The nesting should not be too deep and can be combined with xargs to improve flexibility.

In Linux, | (pipeline character) is a very practical tool that allows you to directly use the output of one command as input to another command. Simply put, it is "the result of the previous command is handed over to the next command to handle." This mechanism allows multiple commands to be connected in series to complete more complex tasks.

1. Basic usage: connect two commands

The easiest way to use is to connect two commands with | . for example:

 ps aux | grep "ssh"

The meaning of this command is: first run ps aux to view all running processes, and then pipe the result to grep "ssh" to filter out the lines containing "ssh".

Common scenarios:

• Filter keywords when viewing logs
• Statistics the number of files or lines
• Sorting, deduplication and other operations on command results

2. Multi-level pipeline: concatenate multiple commands

You can string multiple commands together to form a "data processing pipeline". For example:

 cat data.txt | sort | uniq | wc -l

The purpose of this command is:

• Read the data.txt file content
• Sort by alphabetical order
• Remove duplicate rows
• Finally count how many rows are left

Tip: Not all commands can be connected in series like this. Make sure that the output format of the previous command is the input format that the subsequent command can accept.

3. Use it in conjunction with common commands

Pipelines are often used with the following commands to combine them with very powerful functions:

• grep : filter specific content
• sort : sort
• uniq : Go to the heavy
• awk / cut : Extract fields
• wc : count row count, word count, etc.
• less / more : View on page

For example, check the number of processes running in the current system:

 ps aux | wc -l

For example, find out the first few commands that are most commonly used (combined with history):

 history | awk '{print $2}' | sort | uniq -c | sort -nr | head -n 10

4. Precautions and tips

Although the pipeline is very useful, there are several points to note:

• The pipeline only processes standard output (stdout). If you need to handle error messages (stderr), use 2>&1 to merge the output.
• If the command execution fails, the subsequent command may not work as expected.
• Do not over-neck, otherwise it will reduce readability.
• It can be used with xargs to achieve more flexible operations.

Basically that's it. Pipelines are one of the most basic and attractive features in the Linux command line. Mastering its use will greatly increase your efficiency in daily operation and maintenance or development.

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