Say I have a file with 2 rows, named file.txt:

Row 1: 80 is a nice number
Row 2: 80 is a cool number
Row 3: 80 is a piquant number
Row 4: 80 is a sexy number

I did sed -i 's/80/8080/g', but by mistake I ran it twice and the result in all rows was 80808080 instead of just 8080.

I believe it is most likely impossible in the common technology but I still want to ask if maybe, there is a sophisticated way to make sed do it's effect only once on this file. I mean, some kind of "lock" that will avoid happening of such mistakes like running this sed more than one time one the file, until I remove tat lock. This might (and I emphasis the word "might") be helpful when dealing with much longer rows, especially in large amounts.

It's not really a problem and I should avoid such a mistake and that's it, but I am curious to know if someone try to develop some technical "defense mechanism" from such sed mistakes.

Note: I believe a similar solution could be helpful to awk users as well.

  • 10
    don't use -i until you're sure? – Jeff Schaller May 10 '17 at 18:53
  • 1
    aren't there 4 lines/rows in the sample file? – Jeff Schaller May 10 '17 at 18:53
  • 4
    don't use /g unless you want every match in the line changed – Jeff Schaller May 10 '17 at 18:53
  • 1
    This is a training / experience problem, not a technical problem. If you don't want to run sed twice, then be careful enough to not run it twice. – John May 10 '17 at 19:12
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    You could write the regex so that it doesn't match the result of its own substitution - e.g. using word boundaries s/\b80\b/8080/ – steeldriver May 10 '17 at 19:13

Just don't use -i for in-place editing, especially if you're trying something out or developing some script. Instead, always write to a temporary file. Then check the file to see if it's ok, and possibly replace the original file with it.

The -i option is non-standard and is horribly non-portable between sed implementations.

The portable way to change a file using sed is

sed 'expression' file >newfile && mv newfile file

awk does not have the same issue. None of GNU awk, mawk or nawk supports in-place editing (which that isn't really something one often wants to do with awk anyway), so the problem of accidentally running the same awk script over a file twice (and getting twice the "effect" of editing the file) very seldom arises.


This isn't very sophisticated, but it is a sort of a defense mechanism, when substituting a string with a string that contains itself is to add the previous and/or next characters to prevent duplicate substitution.

Per your example above I recommend something like one (or both) of the following:

  • sed -i 's/ 80 / 8080 /g'
  • sed -i 's/^80 /8080 /g'

Also, be careful with -i :)

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