Using variables in sed -f (where sed script is in a file rather than inline)

Question

We have a process which can use a file containing sed commands to alter piped input.

I need to replace a placeholder in the input with a variable value,

Answer 1

As others have said, you can't use variables in a sed script, but you might be able to "fake" it using extra leading input that gets added to your hold buffer. For example:

[ghoti@pc ~/tmp]$ cat scr.sed 
1{;h;d;};/^--$/g
[ghoti@pc ~/tmp]$ sed -f scr.sed <(date '+%Y-%m-%d'; printf 'foo\n--\nbar\n')
foo
2012-10-10
bar
[ghoti@pc ~/tmp]$ 

In this example, I'm using process redirection to get input into sed. The "important" data is generated by printf. You could cat a file instead, or run some other program. The "variable" is produced by the date command, and becomes the first line of input to the script.

The sed script takes the first line, puts it in sed's hold buffer, then deletes the line. Then for any subsequent line, if it matches a double dash (our "macro replacement"), it substitutes the contents of the hold buffer. And prints, because that's sed's default action.

Hold buffers (g, G, h, H and x commands) represent "advanced" sed programming. But once you understand how they work, they open up new dimensions of sed fu.

Note: This solution only helps you replace entire lines. Replacing substrings within lines may be possible using the hold buffer, but I can't imagine a way to do it.

(Another note: I'm doing this in FreeBSD, which uses a different sed from what you'll find in Linux. This may work in GNU sed, or it may not; I haven't tested.)