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Here Documents

Most shells, including Bash support here documents which enable you to embed text as part of a command using the << operator and some text to use as a delimiter.

In the following example, text is passed to the tr command using a here document and END_TEXT as the delimiting identifier which specifies the start and end of the here document.

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

The output lines produced from tr after execution are ONE TWO THREE and UNO DOS TRES.

A common use of here documents is to add text to a file. By default variables and also commands in backticks are evaluated:

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

This can be disabled by setting the label in the command line in single or double quotes:

$ cat << "EOF"
> Working dir $PWD
> EOF
Working dir $PWD

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