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I am trying to use sed to search for the occurrence of an exact string that contains / in it. Below is the approach I am trying, but without luck.

input_string="/path /path1 /path2"
search_string="/path"
input_string=echo $input_string | sed "s#\b$search_string\b##g"

But when I run these sequence of commands, I am still input_string to contain the value "/path /path1 /path2" instead of "/path1 /path2"

I even tried input_string=echo $input_string | sed "s#\<$search_string\>##g" but without any luck

I am using RHEL6.5 and running this command from shell.

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    What shell is this? AFAIK an assignment like input_string=echo blah isn't legal (you need a command substitution like $( ... ) or backticks. Aside from that, I suspect the issue is that / is a non-word character, so that there is no word boundary before it - do you get the desired result if you remove the initial \b? FYI you may be able to do what you want more simply using shell parameter substitution. – steeldriver Dec 30 '16 at 14:29
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    I suppose input_string is set with input_string=$( ), beside there is no leading blank in your test case. – Archemar Dec 30 '16 at 14:30
  • No shell handy, but I assume that with "...\b..." the shell will already try to parse that as an escape sequence, so sed doesn't even see it. Do you mind trying if "...\\b..." works better? – Ulrich Schwarz Dec 30 '16 at 14:46
  • echo ${input_string##$search_string} will strip leading $search_string (but not first space, hence I didn't post as answer). – Archemar Dec 30 '16 at 14:51
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    @Archemar how about ${input_string##$search_string }? – steeldriver Dec 30 '16 at 15:11
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The problem here is the / in the pattern in $search_string.

Since / is not an alphanumeric character, it's not considered a "word". This means that the pattern \</path\> would not match /path since there is no word boundary before the /.

Change the search string to /path\> (i.e. s##$search_string\>##g) instead, or use /path[^0-9], or, with these specific input data and search string, you could simply use s#$search_string ##g as the substitution.

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short answer

use this command to get what you want :

input_string=$(echo $input_string | sed "s#$search_string\b##g")

Long answer

Why sed isn't replacing

\b means a word boundary, and here is it's definition :

The metacharacter \b is an anchor like the caret and the dollar sign. It matches at a position that is called a "word boundary". This match is zero-length.

There are three different positions that qualify as word boundaries:

  • Before the first character in the string, if the first character is a word character.
  • After the last character in the string, if the last character is a word character.
  • Between two characters in the string, where one is a word character and the other is not a word character.

Found it here. So, the first character of your search_string is a /, and it seems that sed can't find a word boundary because of the first rule (because the / isn't a word character).

Why the result isn't stored in input_string

you need to first execute the two commands (echo and sed), then store the result in the variable input_string. To do this, put the two commands inside $(). This will make them execute and return the result to your variable (see the command in the short answer).

I know it's not a complete answer, but that's all I got.

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Exact word matching in sed is tricky, especially if you want to write a portable solution, or if your definition of a "word" contains special characters other than _ (as in your situation). The best way I have found to do this is to consider all 4 cases where a word might appear in a line of input:

  1. At the beginning
  2. At the end
  3. At both the beginning and the end (occupying the entire line)
  4. In the middle somewhere

If your definition of a word is simply a sequence of non-whitespace characters, then the following solution should work for you (tested):

_s='[ ]' # contains a space and a literal tab character (CTRL-V + TAB) input_string=`echo "$input_string" | sed \ -e "s|^$search_string\\($_s\\)|\\1|g" \ -e "s|\\($_s\\)$search_string$|\\1|g" \ -e "s|^$search_string$||g" \ -e "s|\\($_s\\)$search_string\\($_s\\)|\\1\\2|g"`

Note that I have chosen | as the delimiter instead of #. This is because # can show up in file names (for example, Emacs auto-save files) while | is reserved, at least on Linux. Also note that the output will contain the whitespace character(s) surrounding each instance of $search_string. If you would rather omit the whitespace, you can use this instead (tested):

_s='[ ]' # contains a space and a literal tab character (CTRL-V + TAB) input_string=`echo "$input_string" | sed \ -e "s|^$_s*$search_string$_s\\{1,\\}||g" \ -e "s|$_s\\{1,\\}$search_string$_s*$||g" \ -e "s|^$search_string$||g" \ -e "s|\\($_s\\)$search_string$_s\\{1,\\}|\\1|g"`

That last expression looks a little different, and for good reason -- you don't want to remove whitespace on both sides of $search_string once you've gotten this far in processing the input line, because in doing so, you would remove whitespace between unmatched words on either side of $search_string, which would smoosh them together.

Final notes:

  • You might be able to get away with omitting some of the g modifiers in the sed expressions, but it doesn't hurt to leave them in.
  • It probably goes without saying, but if your $search_string is a path, it cannot contain any space.
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