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, e.g. in a single -e type of command I can run;

$ echo "Today is XX" | sed -e "s/XX/$(date +%F)/"
Today is 2012-10-11

However I can only specify the sed aspects in a file (and then point the process at the file), E.g. a file called replacements.sed might contain;

s/XX/Thursday/

So obviously;

$ echo "Today is XX" | sed -f replacements.sed
Today is Thursday

If I want to use an environment variable or shell value, though, I can't find a way to make it expand, e.g. if replacements.txt contains;

s/XX/$(date +%F)/

Then;

$ echo "Today is XX" | sed -f replacements.sed
Today is $(date +%F)

Including double quotes in the text of the file just prints the double quotes.

Does anyone know a way to be able to use variables in a sed file?

Answer 1

This might work for you (GNU sed):

cat <<\! > replacements.sed
/XX/{s//'"$(date +%F)"'/;s/.*/echo '&'/e}
!
echo "Today is XX" | sed -f replacements.sed

If you don't have GNU sed, try:

cat <<\! > replacements.sed
/XX/{
    s//'"$(date +%F)"'/
    s/.*/echo '&'/
}
!
echo "Today is XX" | sed -f replacements.sed | sh

Answer 2

AFAIK, it's not possible. Your best bet will be :

INPUT FILE

aaa
bbb
ccc

SH SCRIPT

#!/bin/sh

STRING="${1//\//\\/}"   # using parameter expansion to prevent / collisions

shift

sed "
s/aaa/$STRING/
" "$@"

COMMAND LINE

./sed.sh "fo/obar" <file path>

OUTPUT

fo/obar
bbb
ccc

Answer 3

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.)