What does EOF do?

Hi

I saw this script and I'm not sure why or how it works:

cat >> test << STOP
line 1
line 2
line 3
STOP

or equally

cat >> test << EOF
line 1
line 2
line 3
EOF

Is this an OS issue or a bash issue?

Can anyone help explain this to me.

Thanks
Dan

That is known as a "Here Tag". Basically it tells the shell that you are going to enter a multi-line string until "Here". You could call it anything you want, not just EOF or STOP.

The <<HERE part tells the shell that you're going to enter multi-lines until the HERE tag. Thus the shell waits until it sees HERE to consider the rest of the stuff above it as input for cat.

Some rules about the Here tags:
1. The tag can be any string, uppercase or lowercase, though most people use uppercase by convention.
2. The tag will not be considered as a Here tag if there are other words in that line. In this case, it will merely be considered part of the string. The tag should be by itself on a separate line, to be considered a tag.
3. The tag should have no leading or trailing spaces in that line to be considered a tag. Otherwise it will be considered as part of the string.
Here's another chunk to consider:

Note that in the above, we have the word HERE showing up 3 times. The first two times, it occurs with other words or leading spaces and hence it is considered as part of the string.

Many scripting languages (perl, PHP, awk, ruby etc.) support the concept of the "Here tag", so it isn't unique to bash alone.

Hey thanks for that. Much appreciated.

All of it makes sense. Although why does the following work?

cat <<EOF>> filename

how is that order isn't important, I mean I could have :

cat >> filename <<EOF

and this would still work, how does bash know where to start from?

Thanks for your help with this.
Dan

That's because when you enter the command, bash parses the first line and sees that it needs to run cat and redirect the output to a file called filename. It also sees that the input is going to come from STDIN (i.e. the terminal) and that it should read input until it sees your "here" tag. The bash parser can accept input/output redirection operators in any order really.