Ubuntu – Look through a file and print text from specific lines

command linetext processing

I have a file with data that I save. Now I would like to print my results into a new file.

For instance, let's take this example, randomlog.log:

Link encap:Ethernet HWaddr 08:00:00:00:00:67
inet addr:10.10.10.10 Bcast:10.10.10.10 Mask:255.255.255.0
inet6 addr: fe80::casf:sdfg:23ra:dg12/64 Scope:Link

How can I take only data from the 12th to the 20th character of the first line and then the 4th to 8th characters of the 3rd line? Output would look something like this: 

Ethernet
t6 ad

Is this possible?
I want to set the line and from position to this position.

Best Answer

Here's a sed approach:

$ sed -nE '1s/.{11}(.{8}).*/\1/p; 3s/.{3}(.{4}).*/\1/p' file  
Ethernet
t6 a

Explanation

The -n suppresses normal output (normal is to print every input line) so that it only prints when told to. The -E enables extended regular expressions.

The sed script has two commands, both using the substitution operator (s/original/replacement/). The 1s/.{11}(.{8}).*/\1/p will only run on the 1st line (that's what the 1s does), and will match the 1st 11 characters of the line (.{11}), then it captures the next 8 ((.{8}), the parentheses are a "capture group") and then everything else until the end of the line (.*). All this is replaced with whatever was in the capture group (\1; if there were a second capture group, it would be \2 etc.). Finally, the p at the end (s/foo/bar/p) causes the line to be printed after the substitution has been made. This results in only the target 8 characters being output.

The second command is the same general idea except that it will only run on the 3rd line (3s) and will keep the 4 characters starting from the 4th.

You could also do the same thing with perl:

$ perl -ne 'if($.==1){s/.{11}(.{8}).*/\1/}
            elsif($.==3){s/.{3}(.{4}).*/\1/}
            else{next}; print; ' file 
Ethernet
t6 a

Explanation

The -ne means "read the input file line by line and apply the script given by -e to each line. The script is the same basic idea as before. The $. variable holds the current line number so we check if the line number is either 1 or 3 and, if so, run the substitution, else skip. Therefore the print will only be run for those two lines since all others will be skipped.

Of course, this is Perl, so TIMTOWTDI:

$ perl -F"" -lane '$. == 1 && print @F[11..19]; $.==3 && print @F[3..6]' file 
Ethernet 
t6 a

Explanation

Here, the -a means "split each input line on the character given by -F and save as the array @F. Since the character given is empty, this will save each character of the input line as an element in @F. Then, we print elements 11-19 (arrays start counting at 0) for the 1st line and 3-7 for the 3rd.

Related Question