Moderator's Comments:

I edited the threads title to correct the typo ("substite"-> "substitute", "charather" -> "character").

Please note that the -i switch is NOT part of the standard sed and is only understood by GNU-sed. In addition i strongly suggest NOT to use it even if it is available:

Code:

sed -i '....'  /path/to/file                              # not recommended

if sed '....' /path/to/file > /path/to/file.tmp ; then    # recommended way
     mv  /path/to/file.tmp  /path/to/file
     chown <correct ownership> /path/to/file              # optional
     chmod <correct filemode> /path/to/file               # optional
else
     <error handling procedure here>
fi

Basically sed always relies on a second file for output because it cannot work how it does work (that is: as a pipeline) in any other way. The -i switch does not change that at all, it just makes sed do it "behind the curtain". In fact a second (temporary) file is created and then moved over the original file.

The problem with this is that: the inode number (and all the other metadata) of the file is changed, of course. If you do it manually as i suggested this is obvious because you obviously have a new file with which you replace the old one. With the -i switch you seem to have the same file - but you don't! So, if you rely on the file ownership or the filemode staying constant but have a directory with sticky bits set or if you rely that the inode number stays constant you are going to be disappointed.

The even bigger problem (IMHO) is that a temporary file gets created but - unlike with the standard way of doing things - you have no control over the process. In case of an error the temporary file might stay, wasting space. Or it may be created in a filesystem where you don't expect it and this filesystem may be too small. Or....

As a last concern i'd like to offer: it is not portable. The way i sketched out above will work everywhere. The method using the -i swithc will only work on select systems. This usually will not pop up for years but the incompatibility will raise its ugly head in the moment you need that least - and it will inadvertently bite you in the behind.

(Sad truth: this will happen anyway sooner or later. If you can minimise the opportunities it can happen it is likely to hurt a bit less - and i for my part am pretty particular when it comes to parts of me i like to use for sitting.)

I hope this helps.

bakunin
I hope this helps.