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Unix: How can I prepend output to a file?

Specifically, I'm using a combination of >> and tee in a custom alias to store new Homebrew updates in a text file, as well as output on screen:

alias bu="echo `date "+%Y-%m-%d at %H:%M"` \
    >> ~/Documents/Homebrew\ Updates.txt && \
    brew update | tee -a ~/Documents/Homebrew\ Updates.txt"

Question: What if I wish to prepend this output in my textfile, i.e. placed at the beginning of the file as opposed to appending it to the end?

Edit1: As someone reported in the answers below, the use of temp files might be a good approach, which at least helped me partially:

targetLog="~/Documents/Homebrew\ Updates.txt"
alias bu="(brew update | cat - $targetLog \
> /tmp/out1 && mv /tmp/out1 $targetLog \
&& echo `date "+%Y-%m-%d at %H:%M":%S` | \
cat - $targetLog > /tmp/out2 \
&& mv /tmp/out2 $targetLog)"

But the problem is the output to STDOUT (previously made possible by tee), which I'm not sure can be incorporated in this tempfile approach …?

Solution

  • sed will happily do that for you, using -i to edit in place, eg.

    sed -i -e "1i `date "+%Y-%m-%d at %H:%M"`" some_file