Unix: How can I prepend output to a file?

This works by creating an output file:

Let's say we have the initial contents on file.txt

echo "first line" > file.txt echo "second line" >> file.txt 

So, file.txt is our 'bottom' text file. Now prepend into a new 'output' file

echo "add new first line" | cat - file.txt > output.txt # <--- Just this command 

Now, output has the contents the way we want. If you need your old name:

mv output.txt file.txt cat file.txt 

The only simple and safe way to modify an input file using bash tools, is to use a temp file, eg. sed -i uses a temp file behind the scenes (but to be robust sed needs more).

Some of the methods used have a subtle "can break things" trap, when, rather than running your command on the real data file, you run it on a symbolic link (to the file you intend to modify). Unless catered for correctly, this can break the link and convert it into a real file which receives the mods and leaves the original real file without the intended mods and without the symlink (no error exit-code results)

To avoid this with sed, you need to use the --follow-symlinks option.
For other methods, just be aware that it needs to follow symlinks (when you act on such a link)
Using a temp file, then rm temp file works only if "file" is not a symlink.

One safe way is to use sponge from package moreutils

Unlike a shell redirect, sponge soaks up all its input before opening the output file. This allows for constructing pipelines that read from and write to the same file.

sponge is a good general way to handle this type of situation.

Here is an example, using sponge

hbu=~/'Documents/Homebrew Updates.txt' { date "+%Y-%m-%d at %H:%M"; cat "$hbu"; } | sponge "$hbu" 

Simplest way IMO would be to use echo and cat:

echo "Prepend" | cat - inputfile > outputfile 

Or for your example basically replace the tee -a ~/Documents/Homebrew\ Updates.txt with cat - ~/Documents/Homebrew\ Updates.txt > ~/Documents/Homebrew\ Updates.txt

Edit: As stated by hasturkun this won't work, try:

echo "Prepend" | cat - file | tee file 

But this isn't the most efficient way of doing it any more...

Similar to the accepted answer, however if you are coming here because you want to prepend to the first line - rather than prepend an entirely new line - then use this command.

sed -i "1 s/^/string_replacement/" some_file 

The -i flag will do a replacement within the file (rather than creating a new file).
Then the 1 will only do the replacement on line 1.

Finally, the s command is used which has the following syntax s/find/replacement/flags.
In our case we don't need any flags. The ^ is called a caret and it is used to represent the very start of a string.

Try this http://www.unix.com/shell-programming-scripting/42200-add-text-beginning-file.html There is no direct operator or command AFAIK.You use echo, cat, and mv to get the effect.

{ date; brew update |tee /dev/tty; cat updates.txt; } >updates.txt.new mv updates.txt.new updates.txt 

I've no idea why you want to do this. It's pretty standard that logs like this have later entries appearing, well, later in the file.