Shell one liner to prepend to a file

Question

This still uses a temp file, but at least it is on one line:

echo "text" | cat - yourfile > /tmp/out && mv /tmp/out yourfile

Credit: BASH: Prepend A Text / Lines To a File

echo '0a
your text here
.
w' | ed some_file

ed is the Standard Editor! http://www.gnu.org/fun/jokes/ed.msg.html

The hack below was a quick off-the-cuff answer which worked and received lots of upvotes. Then, as the question became more popular and more time passed, outraged people started reporting that it sorta worked but weird things could happen, or it just didn't work at all, so it was furiously downvoted for a time. Such fun.

The solution exploits the exact implementation of file descriptors on your system and, because implementation varies significantly between nixes, it's success is entirely system dependent, definitively non-portable, and should not be relied upon for anything even vaguely important.

Now, with all that out of the way the answer was:

---

Creating another file descriptor for the file (exec 3<> yourfile) thence writing to that (>&3) seems to overcome the read/write on same file dilemma. Works for me on 600K files with awk. However trying the same trick using 'cat' fails.

Passing the prependage as a variable to awk (-v TEXT="$text") overcomes the literal quotes problem which prevents doing this trick with 'sed'.

#!/bin/bash
text="Hello world
What's up?"

exec 3<> yourfile && awk -v TEXT="$text" 'BEGIN {print TEXT}{print}' yourfile >&3

John Mee: your method is not guaranteed to work, and will probably fail if you prepend more than 4096 byte of stuff (at least that's what happens with gnu awk, but I suppose other implementations will have similar constraints). Not only will it fail in that case, but it will enter an endless loop where it will read its own output, thereby making the file grow until all the available space is filled.

Try it for yourself:

exec 3<>myfile && awk 'BEGIN{for(i=1;i<=1100;i++)print i}{print}' myfile >&3

(warning: kill it after a while or it will fill the filesystem)

Moreover, it's very dangerous to edit files that way, and it's very bad advice, as if something happens while the file is being edited (crash, disk full) you're almost guaranteed to be left with the file in an inconsistent state.

Not possible without a temp file, but here goes a oneliner

{ echo foo; cat oldfile; } > newfile && mv newfile oldfile

You can use other tools such as ed or perl to do it without temp files.

If you need this on computers you control, install the package "moreutils" and use "sponge". Then you can do:

cat header myfile | sponge myfile

It may be worth noting that it often is a good idea to safely generate the temporary file using a utility like mktemp, at least if the script will ever be executed with root privileges. You could for example do the following (again in bash):

(tmpfile=`mktemp` && { echo "prepended text" | cat - yourfile > $tmpfile && mv $tmpfile yourfile; } )