Here document in Shell

1. Glossary:

The following is Wikipedia's explanation:

Here document^{[1], also known as heredoc, hereis, here-string or here-script, is a shell (such as SH, CSH, KSh, Bash, powershell, and zsh) in the command line) and define a string method in the program language. It can save the line breaks, dashes, and other blank characters in the text. Some languages allow variable replacement and command replacement in strings.}

The most common syntax of the here document is<<Keep up with an identifier, starting from the next line is the text to be referenced, and then close it with the same identifier on a separate line. In Unix shell, the here document is usually used to provide input content for commands.

It seems complicated. The purpose of the here document is to deliver a series of commands that need to be input from the keyboard, simulating manual input, one line at a time to interactive applications or commands.

The format is:

$ tr a-z A-Z <<END_TEXT > one two three > uno dos tres > END_TEXT ONE TWO THREE UNO DOS TRES

END_TEXTUsed as an identifier. It specifies the start and end of the here document,ONE
TWO THREEAndUNO DOS TRESYes after executiontr.

2. Example

$ cat << EOF > Working dir $PWD > EOF Working dir /home/user

EOF calibrated the start and end of here. This example is equivalent to the input of cat working dir $ PWD, and the output is the same. Do you understand?
Dir $ PWD) is the input of the command (CAT) before the here document!

Note: The here document requires that the input data, especially the string separator "limitstring" used as the end mark, be located at the starting position of a separate line.