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Question

I'm parsing a template using linux command line using pipes which has some 1-indexed pseudo-variables, I need 0-indexing. Bascially:

...{1}...{7}...

to become

...{0}...{6}...

I'd like to use one of grep , sed or awk in this order of preference. I guess grep can't do that but I'm not sure. Is such arithmetic operation even possible using any of these?

• numbers used in the file are in range 0-9 , so ignore problems like 23 becoming 12
• no other numbers in the file, so you can even ignore the {}
• I can do that using python, ruby, whatever but I prefer not to so stick to standard command line utils
• other commend line utils usable with pipes with regex that I don't know are fine too

EDIT : Reworded first bullet point to clarify

Answer 1

If the input allows it, you may be able to get away with simply:

tr 123456789 012345678

Note that this will replace all instances of any digit, so may not be suitable. (For example, 12 becomes 01 . It's not clear from the question if you have to deal with 2 digit values.)

If you do need to handle multi-digit numbers, you could do:

perl -pe 's/\{(\d+)\}/sprintf( "{%d}", $1-1)/ge'

Answer 2

You can use Perl.

perl -pe 's/(?<={)\d+(?=})/$&-1/ge' file.txt

If you are sure you can ignore {...} , then use

perl -pe 's/\d+/$&-1/ge' file.txt

And if index is always just one-digit number, then go with shorter one

perl -pe 's/\d/$&-1/ge' file.txt

Answer 3

With gawk version 4, you can write:

gawk '
    {
        n = split($0, a, /[0-9]/, seps)
        for (i=1; i<n; i++)
            printf("%s%d", a[i], seps[i]-1)
        print a[n]
    }
'

Older awks can use

awk '
    {
        while (match(substr($0,idx+1), /[0-9]/)) {
            idx += RSTART
            $0 = substr($0,1, idx-1) (substr($0,idx,1) - 1) substr($0,idx+1)
        }
        print 
    }
'

Both less elegant than the Perl one-liners.