CODEWITHSUNDEEP
shellheredoc
todd1215
todd1215

Reputation: 413

heredoc's, redirects and file descriptors

I know there are lots of posts in here about heredocs and also redirects and file descriptors, but I can't seem to find anything related to what I want to do.

I want to open a file descriptor to a file and then write a heredoc to that file descriptor.

Here's what I have using bash shell:

exec 3>/tmp/testfile.txt
cat <<EOF>>3
write to fd using heredoc
EOF

I check my file and its empty. I'm assuming its empty because the reference to my file descriptor in the heredoc goes to the bit bucket. I tried using an ampersand & like this EOF>>&3 but that throws an error in the shell.

How can I get this accomplished?

Upvotes: 3

Views: 1876

Answers (3)

todd1215
todd1215

Reputation: 413

OK here's the answer:

exec 3>/tmp/testfile.txt
cat <<EOF 1>&3
this is a redirect to fd 3 via a heredoc
EOF

Upvotes: 2

Jonathan Leffler
Jonathan Leffler

Reputation: 753835

The Bash manual for append redirection says:

The general format for appending output is:

[n]>>word

You appended to a file 3. However, in practice, you don't need the >> operator; you need to send the standard output of cat to descriptor 3:

cat <<EOF 1>&3
write to fd using heredoc
EOF

This won't truncate the file, so it achieves 'append' anyway.

Upvotes: 0

larsks
larsks

Reputation: 311615

If you write:

echo something >> 3

This will create a filed called 3. If you want to refer to a file descriptor, you need to use the &3 syntax. If you want to append to a file using a file descriptor, you can do this:

exec 3>>/tmp/testfile.txt
cat <<EOF >&3
This is a test.
EOF

That is, you decide append vs. overwrite when you open the file descriptor with the exec statement. If you did this instead:

exec 3>/tmp/testfile.txt

That would overwrite the file rather than appending to it.

Upvotes: 4

Related Questions