Here document and input redirection with -

Question

What does the following command do? In particular I don't understand the role of - in input redirection.

 cat <<-EOF | command $argument
    first option


    second option
    EOF

Is it mandatory to have the - in the input redirection?

Answer 1

From man bash:

If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.

$ cat <<-EOF
    foo
    bar
    EOF
foo
bar

Answer 2

In context to your question regarding - in Here document, I will like to add something in Chris answer above

cat - filename <<-EOF
    foo
    bar
    EOF

Here if you notice in above code; there are 2 -. Both have different purpose 1) -EOF which truncates the leading tabs in the delimiter.

2) cat - filename : which basically prints first stdin content (till delimiter is found) + filename content. cat specifically provides this - to have control over the order of stdin and file content. In above case changing the command to cat filename - will print filename and the stdin content (till delimiter is found)