TCSH Shell Script: substitute a variable using sed inside file

Question

in my tcsh script I have this

sed -i "s/^${y}$//g" $x

How do I get this to work? either I get no change or some error like Illegal Variable Name or Variable name must contain alphanumeric characters. Tried different combinations of "". '' within the sed . Example

sed 's/^'"${y}"'$//g' $x

Answer 1

Expansion of variable names in double-quoted strings is tricky in tcsh. In your command:

sed -i "s/^${y}$//g" $x

the value of the shell or environment variable $y should be expanded correctly, but the shell will try to expand $/ to the value of $/, which as the error message says is an illegal variable name.

Your second command:

sed 's/^'"${y}"'$//g' $x

is almost correct; it's just missing the -i argument to sed.

This should work: . sed 's/^'"${y}"'$//g' $x

The { and } around y aren't needed in this case, so you could write:

sed 's/^'"${y}"'$//g' $x

Or you can do it like this:

sed -i "s/^$y"'$'"//g" $x

This last version switches from double quotes to single quotes, and back to double, just around the $ character. It's a matter of emphasis, whether you want to use single quotes only around the $ that you don't want expanded, or to use double quotes only around $y which you do want expanded.

Even more simply, you can escape a $ character in a double-quoted string by preceding it with \ (I think that may be specific to tcsh; the original csh might not have been able to do that). So I think this is the preferred approach -- assuming you have tcsh and not old-style csh:

sed -i "s/^$y\$//g" $x

I'm assuming that what you want to do is update the file named by $x in place, deleting the string contained in $y at the beginning of each line. That seems to be what you're trying to do, but I can't be certain.