How to remove the milliseconds from timestamps with sed?

Question

My input file is as follows:

12/13/2011,07:14:13.724,12/13/2011 07:14:13.724,231.56.3.245,LasVegas,US

I wish to get the following:

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Answer 1

sed 's/\(:[0-9][0-9]\)\.[0-9]\{3\}/\1/g' input_file.csv > output.csv

You were almost there. In classic sed, you have to use backslashes in front of parentheses and braces to make them into metacharacters. Some versions of sed may have a mechanism to invert operations, so that the braces and parentheses are metacharacters by default, but that's not reliable across platforms.

Also (strong recommendation): use single quotes around the sed command. Otherwise, the shell gets a crack at interpreting those backslashes (and any $ signs, etc) before sed sees it. Usually, this confuses the coder (and especially the maintaining coder). In fact, use single quotes around arguments to programs whenever you can. Don't get paranoid about it - if you need to interpolate a variable, do so. But single-quoting is generally easier to code, and ultimately easier to understand.

I chose to work on just one time unit; you were working on three. Ultimately, given systematically formed input data, there is no difference in the result - but there is a (small) difference in the readability of the script.