Error while matching End of line with $ in Sed command

I am currently applying the below command in csh through a shell script.

sed -i "s/cb $i$/cb $i $cb/" */callback_events

Where:

• $i - Value of Variable i
• $cb - Value of Variable cb
• $ - To match the end of line

However, I am getting the following error with the above command.

Variable name must contain alphanumeric characters.

My current shell is /bin/csh.

Solution 1:

You must escape the character $ when it is to stand for itself inside a string in doublequotes; otherwise the shell will think that it introduces a variable expansion, and will be unhappy if the character after it is not alphabetic or one of the special variables.

sed -i "s/cb $i"\$"/cb $i $cb/" */callback_events

There are two levels of character interpretation here.

1. First the shell reads the command and applies its rules. One of the rules is that inside doublequotes $ introduces variable expansion.

2. After the shell has finished the command looks like this:

    sed -i s/cb <value-of-i>$/cb <value-of-i> <value-of-cb/ dir1/callback_events dir2/callback_events...
   

   Note that the quotes are gone, $i and $cb are replaced with their values, and \$ became just $. Also */callback_events got replaced with a list of files.

3. This is then passed to sed, which applies its rules. One of those rules is that a $ at the end of the search pattern means end-of-line.