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I am trying to use ed to edit the first line of a large file, but I have to wait for ed to read all lines. Is there any way I could stop ed from reading the whole file and start editing immediately, with the current line being the first line?

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ed or ex can't do this. Unix utils expect to write to a file, not go in and fiddle with bytes of the file, or the file allocation table definitions. sed '1d' file > newFile will be about as fast as you can get without writing your own C program. Also, ed and ex (at least the ones I have worked with) are limited to the size of the file they can work on, often times by space available in /tmp (or /var/tmp) or in ex for the internal setting of directory. (You know ex is the steam version of vi|vim? Do :set all to find the value for directory). Good luck. – shellter Jul 12 '12 at 3:10
Thank you for your explanation. – abcd Jul 13 '12 at 11:06

As noted by shellter ed cannot do this. Most editing operations require that the file is rewritten, only replacements can be done in-place, in that case you could use hexedit(1).

The least memory hungry way is to use sed operations, e.g. to replace August with June on the first line:

sed '1s/August/June/' FILE > NEWFILE
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Thanks. I am aware of the solution using sed. I particularly find the -i flag useful in order to avoid creating temp files. I was looking for an ed solution, but I will give hexedit a try. Thank you. – abcd Jul 13 '12 at 11:12
@sina, -i does create a temporary file, but sed handles it transparently in the background. See info sed and search for -i. – Thor Jul 13 '12 at 11:38
This is bad news. But it does make sense considering shelter's comment and your explanations. Thanks for pointing it out. – abcd Jul 14 '12 at 9:41

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