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Passing UNIX commands inside sed

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# 1  
Old 02-28-2009
Question Passing UNIX commands inside sed
Hi,

In sed, is it possible to match patterns by directly executing UNIX commands inside sed?

For e.g. -
sed "s/`cat file.txt | cut -d "|" -f2`/replace_string"

Will the above command work?

My expectation is to search for the second field in file.txt (file delimited by | ) and replace those with replace_string.

I know I could store the output of cat file.txt | cut -d "|" -f2 in a variable and pass it to sed. But I want to know if direct command substitution is possible

Any help is greatly appreciated.

-Thanks
# 2  
Old 02-28-2009
Yes, you can directly substitute commands into strings this way, though there may be quoting issues for strings in strings in strings.
# 3  
Old 02-28-2009
Quote:
Originally Posted by devanathann
Hi,

In sed, is it possible to match patterns by directly executing UNIX commands inside sed?

For e.g. -
sed "s/`cat file.txt | cut -d "|" -f2`/replace_string"

Will the above command work?

Yes, so long as there is only one line in file.txt.
Quote:
My expectation is to search for the second field in file.txt (file delimited by | ) and replace those with replace_string.

I know I could store the output of cat file.txt | cut -d "|" -f2 in a variable and pass it to sed. But I want to know if direct command substitution is possible

By using a variable, you can eliminate two external commands, cat and cut:

Code:
IFS='|' read a b c < file.txt
sed "s/$b/replace_string/"

# 4  
Old 02-28-2009
Quote:
Originally Posted by devanathann
Hi,

In sed, is it possible to match patterns by directly executing UNIX commands inside sed?

For e.g. -
sed "s/`cat file.txt | cut -d "|" -f2`/replace_string"

Will the above command work?
No, it should not work that way, to replace the second column in a pipe delimited file with a shell variable in sed you can do something like this:

Code:
sed "s/\([^|]*\)|[^|*]|\(.*\)/\1|$var|\2/" file > newfile
mv newfile file

With awk:

Code:
awk -F"|" '{$2=s}1' s=$var OFS="|" file > newfile
mv newfile file

Regards
# 5  
Old 02-28-2009
Quote:
Originally Posted by Franklin52
No, it should not work that way

Why not? It works for me (after terminating the s command):

Code:
$ cat ~/txt
a|b|c|d
$ printf "%s\n" abc vbn |
  sed "s/`cut -d "|" -f2 ~/txt`/replace_string/"
areplace_stringc
vreplace_stringn

Last edited by cfajohnson; 02-28-2009 at 08:38 PM.. Reason: Removed UUOC
# 6  
Old 02-28-2009
Quote:
Originally Posted by cfajohnson

Why not? It works for me (after terminating the s command):

Code:
$ cat ~/txt
a|b|c|d
$ printf "%s\n" abc vbn |
  sed "s/`cat ~/txt | cut -d "|" -f2`/replace_string/"
areplace_stringc
vreplace_stringn

Quote:
My expectation is to search for the second field in file.txt (file delimited by | ) and replace those with replace_string.
The file has more then one line and the output should be:

Code:
a|replace string|c|d

Regards
# 7  
Old 02-28-2009
Quote:
Originally Posted by Franklin52
The file has more then one line

As I pointed out, it would only work if the file containing the search term contained only one line. The script doesn't make any sense if it has more than one line.
Quote:
and the output should be:

Code:
a|replace string|c|d

Regards

No, it shouldn't. The file containing "a|b|c|d" is where the search string is coming from, not the file that sed is operating on.
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