Replacing text with special character in linux shell script?

Question

I am currently working on a project porting a lot of C programs onto a new Linux system. My goal is to automate most of the simple repetitive errors if possible.

Right now I am dealing with a lot of invalid assignments of NULL, which should be '\\0' on that specific line(s). Example:

attribute = NULL;

should be

attribute = '\0';

I have a list of all lines where this error occurs. So now all I need to do is create a script that changes NULL to \\0. This is the line of code I have tried, with different variations:

sed -i "${1}s/NULL/\\0/" ${2}

${1} being the line the error is occuring and ${2} being the filename.

Edit:

Instead of replacing NULL with \\0, it is being replaced as 'NULL'. I need a command that will replace NULL with the characters \\ and 0, not the special character.

Answer 1

$ cat file
attribute = NULL;

$ sed 's/NULL/'\''\\0'\''/' file
attribute = '\0';

You might want to add word boundaries so you don't replace strings with NULL in them. You must be using GNU sed for that usage of -i so:

$ cat file
attribute = NULL;
FOONULLBAR

$ sed 's/NULL/'\''\\0'\''/' file
attribute = '\0';
FOO'\0'BAR

$ sed 's/\<NULL\>/'\''\\0'\''/' file
attribute = '\0';
FOONULLBAR

You may want to also consider defining a variable NUL = '\\0' in a common header and using that in place of NULL instead of hard-coding \\0 everywhere.

To only do a substitution on a specific line number that's stored in a variable then:

$ lineNr=1; sed "$lineNr"'s/NULL/'\''\\0'\''/' file
attribute = '\0';
FOONULLBAR

$ lineNr=2; sed "$lineNr"'s/NULL/'\''\\0'\''/' file
attribute = NULL;
FOO'\0'BAR

or if you don't mind doubling up on backslashes and know you won't have anything else in your sed command that the shell could interpret/expand (as is true for this specific case):

$ lineNr=1; sed "${lineNr}s/NULL/'\\\\0'/" file
attribute = '\0';
FOONULLBAR

$ lineNr=2; sed "${lineNr}s/NULL/'\\\\0'/" file
attribute = NULL;
FOO'\0'BAR