How to understand "cat > file_name << blah" command?

Question

In following command cat takes the content of here-doc and redirects it to file named conf:

cat > conf << EOF
var1="cat"
var2="dog"
var3="hamster"
EOF

How to understand the order of commands here? Does bash first processes everything else(here-doc part) and as a final step it looks the > conf part?

Solution 1

Here-Document is a kind of shell redirection, so the shell will perform it as normal redirection, from beginning to the end (or from left to right, or order of appearance). This's defined by POSIX:

If more than one redirection operator is specified with a command, the order of evaluation is from beginning to end.

---

In your command, cat will perform > conf first, open and truncate conf file for writing, then reading data from Here-Document.

Using strace, you can verify it:

$ strace -f -e trace=open,dup2 sh -c 'cat > conf << EOF
var1="cat"
var2="dog"
var3="hamster"
EOF
'
...
open("conf", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
dup2(3, 1)                              = 1
dup2(3, 0)                              = 0
...

Solution 2

Well, let's find out:

unset file
cat >"$file" <<EOF
this is not in ${file=./myfile}
EOF

---

bash: : No such file or directory

Dang. I guess it must be doing the >"$file" part first then. But what if...?

unset file
<<EOF cat >"$file"
this is in ${file=./myfile}
EOF

...no error...?

cat ./myfile

---

this is in ./myfile

As it appears, order matters.

Comments

[comeback4you]

I see. So basically what happens in case of cat > file_name << blah is that before executing cat the stdout of shell is connected to file named file_name and then stdin of shell is connected to here-doc?

[cuonglm]

@Martin: Yes. You can see open was called before dup2.