Question

I ended up writing a quick little script for this in Python, but I was wondering if there was a utility you could feed text into which would prepend each line with some text — in my specific case, a timestamp. Ideally, the use would be something like:

cat somefile.txt | prepend-timestamp 

(Before you answer sed, I tried this:

cat somefile.txt | sed 's/^/`date`/' 

But that only evaluates the date command once when sed is executed, so the same timestamp is incorrectly prepended to each line.)

Answer 1

Could try using awk:

<command> | awk '{ print strftime('%Y-%m-%d %H:%M:%S'), $0; fflush(); }' 

You may need to make sure that <command> produces line buffered output, i.e. it flushes its output stream after each line; the timestamp awk adds will be the time that the end of the line appeared on its input pipe.

If awk shows errors, then try gawk instead.