sed variable substitution NOT to match any lines?

[A. Daniel King_1]

Hi, folks,

I've got a script that defines a string, say:

x=abc123

and then modifies a file based upon a one-line sed script:

sed "1,/$x/ d" infile > outfile

...

Here is the kicker: How do I define $x so that the sed script matches NO lines? I want, under certain circumstances, for the input file to exactly match the output file.

This would be an easy extra line or two to implement, but sed probably has some very easy way to do this internally.

Thanks in advance.

[Anthony deRito]

Some of these may help....

http://www.student.northpark.edu/pemente/sed/sed1line.txt

[Jose Mosquera]

Hi,

Pls try this:

cat infile|sed 's/$x/ d/g' > outfile

This replace string contened in $x (abc123) by " d" string (space an letter d), note that quotes must be single.

Rgds.

[James R. Ferguson · Accepted solution]

Hi:

I don't think you can get there from here!

Assuming that you *only* change the value of $x then the best you can do is only delete the first line from the output string:

Consider:

# x=
# sed "1,/$x/d" /etc/hosts > /tmp/hosts
# diff /etc/hosts /tmp/hosts

...you will see that only the first line has been deleted.

Regards!

...JRF...

[Sridhar Bhaskarla]

I second JRF.

You will need to go with a small if statement to verify the value of 'x' and based on it use the sed statement.

-Sri

[H.Merijn Brand (procura]

To match exactly, you will need anchors

sed "1,/^$x\$/d" infile >outfile

or in perl

perl -ne "m{^$x\$} or print" infile >outfile

or in grep

grep -v "^$x\$" infile >outfile

[A. Daniel King_1]

I ended up doing:

x="1,/abc123/ d"

and sometimes

x=":"

sed "$x" infile > outfile

Thanks for the feedback, folks.

[Thomas M. Williams_1]

Whenever you use a variable substitution in a command like sed you need to quote it with single ticks. It appears you are trying to delete lines containing the value of $x.

To delete lines:
sed '/'$x'/d' infile > outfile

To substitute all lines:
sed '1,$s/'$x'/whatever/'

Hope this helps.