Sed - replace value in file with regex match in another file

Question

I am trying to code a bash script in a build process where we only have a few tools (like grep, sed, awk) and I am trying to replace a value in an ini file with a value from a regular expression match in another.

So, I am matching something like "^export ADDRESS=VALUE" in file export_vars.h and putting VALUE into an ini file called config.ini in a line with "ADDRESS=[REPLACE]". So, I am trying to replace [REPLACE] with VALUE with one command in bash.

I have come across that sed can take an entire file and insert it into another with a command like

sed -i -e "/[REPLACE]/r export_vars.h" config.ini

I need to somehow refine this command to only read the pattern match from export_vars.h. Does anyone know how to do this?

Just to clarify, you wish for the line in export_vars.h (e.g. export ADDRESS=VALUE) to replace a line already in config.ini (e.g. ADDRESS=VALUE)? I assume the thing you are trying to match here is ADDRESS and you wish to replace VALUE in this example? — Kind Stranger
– Kind Stranger, Commented Apr 20, 2017 at 13:25
Yes - or just replace the [REPLACE] in the ini file with VALUE. I guess either would work. — Kevin Hirst
– Kevin Hirst, Commented Apr 20, 2017 at 13:27

Ed Morton · Accepted Answer · 2017-04-20 13:34:51Z

1

sed is for simple substitutions on individual lines, that is all. You need to be looking at awk for what you're trying to do. Something like:

awk '
BEGIN { FS=OFS="=" }
NR==FNR { 
    if ( $1 == "export ADDRESS" ) {
        value = $2
    }
    next
}
{ sub(/\[REPLACE\]/,value); print }
' export_vars.h config.ini

Untested, of course, since you didn't provide testable sample input/output.

answered Apr 20, 2017 at 13:34

Ed Morton

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8 Comments

Kevin Hirst Over a year ago

Thanks, that worked perfectly. Need to learn awk more, apparently, because we have other tasks like this.

Ed Morton Over a year ago

You're welcome. To learn awk, read the book Effective Awk Programming, 4th Edition, by Arnold Robbins. All other books are obsolete, and all online tutorials are buggy and/or misleading. If you are considering doing anything in sed that uses constructs other than s, g, and p (with -n) then you are using the wrong tool. Sed is for s/old/new/, that is all, and even then has caveats (e.g. it doesn't understand strings). All of the other arcane sed constructs became obsolete in the mid-1970s when awk was invented and are useful today only for mental exercise, never for production software.

Ed Morton Over a year ago

I've never heard of it but from the title 2 things spring to mind: 1) You do NOT need any book to tell you how to use sed properly (s/old/new/ - done!), 2) it has the word Hacks in it - who in their right mind would want to learn how to write hacks instead of idiomatically correct, elegant code? Oh I see, it was written by the guy with the terrible awk "tutorial" at thegeekstuff - no, avoid that like the plague.

RomanPerekhrest Over a year ago

@EdMorton, right, and I didn't find it at any online book-shop ... He wrote I have been adding 1’s and 0 ‘s for more than 17 years in the IT industry ...

Ed Morton Over a year ago

I wonder what answer he gets when he adds 1 plus 0 :-). Take a look at his tutorial 3. Awk RS Example: Record Separator variable. He says Awk RS defines a line - no it doesn't, it separates records. He shows RS="\n\n" to read blank-line separated records - no you don't you use RS="" and RS="\n\n" would be gawk-specific anyway due to multi-char RS. I've provided corrections to the examples on his site in the past but he doesn't listen.

|

James Brown · Accepted Answer · 2017-04-20 17:30:28Z

1

Another in awk:

$ awk '/ADDRESS/{if(a!="")$0=a;else a=$NF}NR>FNR' export_vars.h config.ini
ADDRESS=VALUE

Explained:

$ awk '
/ADDRESS/ {                 # when ADDRESS is found in record
    if(a!="") $0=a          # if a is set (from first file), use it
    else a=$NF }            # otherwise set a with the last field
NR>FNR                      # print all record of the last file
' export_vars.h config.ini  # mind the order

This solution does not tolerate space around = since $0 is replaced with $NF from the other file.

answered Apr 20, 2017 at 17:30

James Brown

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