UNIX for Dummies Questions & Answers

Well, there is a basic technique you might want to learn: escaping. That means: in some programs (here: sed, but the same goes for the shell too) certain characters (here: the "/") have a special meaning. We need a way to have them treated as normal characters, with their special meaning removed, sometimes.

If you want to use "/" as part of the search- and/or replacement string you will have to solve this, because changing i.e. "bla/blub" to "something/else" is not going to work this way. sed simply won't know what is part of the command and what is part of the strings to work on:

Code:

sed 's/bla/blub/something/else/g'

Fortunately there is a device - in fact two of them - you can use. The first is to change seds special character: every character following the "s" command will be used, the only requirement is you have to use it consistently. To use "/" as a delimiter is just convention, not part of the syntax. The following two lines are equivalent and will both work:

Code:

sed 's_bla/blub_something/else_g'
sed 's!bla/blub!something/else!g'

The problem with this is that it shifts the problem only around: now you replaced "/" with another character you will have to worry about. Here is the definitive solution for that, more work, but final: you prepend the character in question with a "\". This will tell sed to not interpret the following character but take it literally:

Code:

sed 's/bla\/blub/something\/else/g'

Notice, that this works with "\" too - a "\\" will be interpreted as a literal backslash! You can even automatically "sedify" your variables before feeding them to sed. Try the following:

Code:

search="bla/blub"
replace="some/thing"

# naive way - will NOT work:

sed 's/'"${search}"'/'"${replace}"'/' /some/file > /some/otherfile

# but this works:

search="$(echo $search | sed 's/\//\\&/')"
replace="$(echo $replace | sed 's/\//\\&/')"
sed 's/'"${search}"'/'"${replace}"'/' /some/file > /some/otherfile

The application of this is left as an exercise to the interested reader. ;-))

I hope this helps.

bakunin

sed is probably failing because $Var_name contains newlines.

Hi bakunin, that was an neat and clear explanation, really appreciate your help.

But am not that good in sed, so some how i have manged to understand whats the sed is doing in ur command.

But, still am getting error that it cannot be parsed to sed. .

Based on the If loop,

Code:

Var_name="This is first line.<br />This is second line.<br />This is third line."

sed: Function s/searchstring/This is first line.<br />This is second line.<br />This is third line./g cannot be parsed.

Modification as per your suggestion :

Code:

search="search_string"
Var_name="This is first line.<br />This is second line.<br />This is third line."
search="$(echo $search | sed 's/\//\\&/')"
replace="$(echo "$Var_name" | sed 's/\//\\&/')"
sed 's/'"${search}"'/'"${replace}"'/g' | sed 's/search/replace/g' > tmp_file

Please let me know what mistake am doing ?

---------- Post updated at 08:18 AM ---------- Previous update was at 07:22 AM ----------

Thanks Scrutinizer, That helps and fixed the issue as well perfectly. But am not able to find your post online, i got the mail, but am not able to find it here. I really want to thank you.
It may sound little weird, but am not able to see your reply.

I really appreciate the help from Scott and Scrutinizer.

Was it a private message? They're at top-right.

Hi CarloM,

Thanks for your reply, no i chked over there as well, its not visible.

Anyway, am posting the response from Scrutinizer here,


*******************
This is probably failing because $Var_name contains forward slashes. Try using s|...|| instead of s/.../../
******************

This solves the problem, thanks to Scrutinizer for giving this.