UNIX for Dummies Questions & Answers

i need to replace '/' forward slash with \/(backward slash follwed by a forward slash) using sed command when the forward slash occurs as a first character in a file..

Tried something like this but doesn't seem to work.

Code:

find $1 -print0 | xargs -0 sed -i -e 's/^\//\\\//g'

Can someone help on this?

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Assuming GNU sed (given your example usage):

Code:

sed -i '1s|^/|\/|' infile

Thanks for that.. can someone tell if i could change the script to exclude the last line alone from this replace.

To "exclude the last line alone", you mean change all lines except the last one?

Actually, radoulov's sed didn't work for me, strangely.

Code:

sed '$!s|^/|\\\/|' file

That's because the replacement text in radoulov's solution is using an implementation-defined sequence. Since the slash is no longer the regular expression delimiter, preceding it with a backslash is undefined (as is a backslash before any non-special character).

In the replacement text of a sed substitute command:
My guess is that scottn's sed treated the \/ as a / and replaced each / found with an identical /. However, since it is an implementation-defined situation, a sed implementation is free to yell at you, shake its fist at you, and refuse to process any data ... if it were so inclined.

The \\\/ in scottn's solution exhibits the same shortcoming as radoulov's: an undefined \/ sequence. \\\/ may yield the desired result with some sed implementations, but it is still depending on unspecified behavior.

A portable solution could use s|^/|\\/| or (the uglier, less readable) s/^\//\\\//.

Regards,
Alister

It replaces last line also

line is like this

Code:

end;
/

I dont want the last forward slash to be replaced

Ah, OK. I got it!

When radoulov's solution didn't work, I did the first thing I almost always do: Switch to different quotes - to double quotes in this case.

When it still didn't work, I did what I usually do second: Threw another slash at it (now we have sed "s|/|\\/"), which still didn't work. So I threw another slash at it(!) and bingo!

The difference was between ' and ".

Code:

$ sed "s|^/|\\/|" file1
/3file
/3fileX
/file1
/tmp/A/a
/tmp/A/a/1.txt
/tmp/A/A.tar
$ sed 's|^/|\\/|' file1
\/3file
\/3fileX
\/file1
\/tmp/A/a
\/tmp/A/a/1.txt
\/tmp/A/A.tar