What is the difference between <EOF and <'EOF' in shell heredocs?

Question

I'm writing a .spec file for a module for linux build system and came across a small issue and wanted to share it.

For writing a script file :

cat <<EOF > /path/to/somewhere/script
#blah blah
EOF
chmod +x script

When the script ran on the target there were errors pointing to the location of the script as it were in the host system.Basically $0 was wrong.

Fixed it by changing the first line like this after seeing some sample code online:

cat <<'EOF' > /path/to/somewhere/script
#blah blah
EOF
chmod +x script

Wondering what's the difference and what made it work the second time.

Answer 1

The difference is that in this version:

<<EOF
...
EOF

the ... functions roughly as a double-quoted string, performing parameter-expansions and command-substitutions and so on (specifically, in your case, replacing $0 with the value of $0), whereas in this version:

<<'EOF'
...
EOF

the ... functions roughly as a single-quoted string, and no such expansions are performed.

(See §3.6.6 "Here Documents" in the Bash Reference Manual.)

Answer 2

The difference is whether the dollars and back quotes in the body of the here document are interpreted as the here document is expanded.

$ XYZ=pqr
$ cat <<EOF
> echo $XYZ
> EOF
echo pqr
$ cat <<'EOF'
> echo $XYZ
> EOF
echo $XYZ
$

You can try similar tricks with $(...).