using vars as numbers in sed [duplicate]

StackOverflow https://stackoverflow.com/questions/23659042

  •  22-07-2023
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Question

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

Was it helpful?

Solution

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
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