using vars as numbers in sed [duplicate] - regex

Question

This question already has answers here:
sed substitution with Bash variables
(6 answers)
Closed 8 years ago.
Having some difficulty in getting a sed | grep pipe to work when using vars as numbers.
In the string below the '3,5p' works fine, but when substituting the numbers for vars I get the error
sed: -e expression #1, char 4: extra characters after command
working=$(sed -n '3,5p' ${myFile} | grep -n "string" |cut -f1 -d: )
notWorking=$(sed -n '${LINESTART},${LINEEND}p' ${myFile} | grep -n "string" |cut -f1 -d: )
I would also be interested in any advice how I could change command so the line number returned is replaced with $string2 in the file myFile
thanks
Art

Answer 1

You need the variables to be expanded by sed. For that, you have to enclose the expression within double quotes:
sed -n "${LINESTART},${LINEEND}p" ${myFile}
^ ^
instead of
sed -n '${LINESTART},${LINEEND}p' ${myFile}
As you are checking for the line number in $myFile where string is found, it line is in between $LINESTART and $LINEEND, you can do:
awk 'NR>=start && NR<=end && /string/ {print NR}' start=$LINESTART end=$LINEEND ${myFile}
Suppose you want to replace a string just if it appears in specific lines. You can use this:
sed -i.bak "$LINESTART,$LINEEND s/FIND/REPLACE/' file
-i.bak makes a backup of the file and does an in-place edit: file will contain the modified file, while file.bak will be the backup.
Test
$ cat a
hello
this is
something
i want changed
end
but this is not to be changed
$ sed -i.bak '3,5 s/changed/NEW/' a
$ cat a
hello
this is
something
i want NEW <---- "changed" got replaced
end
but this is not to be changed <---- this "changed" did not