How do multiple blocks work in AWK?

Question

I came across this one-liner script fu for getting rid of newline characters in a fixed width text file. The idea is to change a file full of entries like:

>IGHV1-18*01
CAGGTTCAGCTGGTGCAGTCTGGAGCTGAGGTGAAGAAGCCTGGGGCCTCAGTGAAG
GTCTCCTGCAAGGCTTCTGGTTACACCTTTACCAGCTATGGTATCAGC
TGGGTGCGACAGGCCCCTGGACAAGGGCTTGAGTGGATGGGATGGATCAGCGCTTAC
AATGGTAACACAAACTATGCACAGAAGCTCCAGGGCAGAGTCACCATGACCACA
GACACATCCACGAGCACAGCCTACATGGAGCTGAGGAGCCTGAGATCTGACGACACGGCC
GTGTATTACTGTGCGAGAGA

to

>IGHV1-18*01
CAGGTTCAGCTGGTGCAGTCTGGAGCTGAGGTGAAGAAGCCTGGGGCCTCAGTGAAGGTCTCCTGCAAGGCTTCTGGTTACACCTTTACCAGCTATGGTATCAGCTGGGTGCGACAGGCCCCTGGACAAGGGCTTGAGTGGATGGGATGGATCAGCGCTTACAATGGTAACACAAACTATGCACAGAAGCTCCAGGGCAGAGTCACCATGACCACAGACACATCCACGAGCACAGCCTACATGGAGCTGAGGAGCCTGAGATCTGACGACACGGCCGTGTATTACTGTGCGAGAGA

I am not very experienced with AWK so I figured it would be a good learning experience to try and decipher it. However, I am having difficulties. Specifically about multiple blocks coming after each other, is the first block an implicit for-loop?

awk '/^>/ {printf("\n%s\n",$0);next; } { printf("%s",$0);}  END {printf("\n");}' < file.fa

Answer 1

Kinda, yes. Only it's not implicit. The format is actually:

/foo/{something}

Which is the same as

if(/foo/){something}

In other words, if the current line matches foo (in your example, if it matches >), then print a newline, the current line and another newline.

The next ensures that if the 1st block is executed, the script skips the rest of the blocks and moves on to the next line. The oneliner could also be written like this:

awk '{
        if(/^>/){
            printf("\n%s\n",$0);
        }
        else{ 
            printf("%s",$0);
        }
        END {
                printf("\n");
        }' < file.fa

Finally, since the simple print call of awk adds a newline, you could use a slightly simpler version of the above:

awk '/^>/{print "\n"$0;next;}{printf("%s",$0);} END{print}' file.fa

Answer 2

awk will read, line by line (you might consider as a block, but it is a line, ending by line-feed or CR)

let's break that code

awk '/^>/ {printf("\n%s\n",$0);next; } { printf("%s",$0);}  END {printf("\n");}'

As you can see in man awk, awk programs are in the form of /pattern/ { actions}, thus the program turns into :

• /^>/ {printf("\n%s\n",$0);next; }

  • for lines that begin with > ( /^>/ )
  • print the line surrounded by \n ( printf("\n%s\n",$0) )
  • fetch the next line ( next ), do not go to the next awk command.

• { printf("%s",$0);}

  • for all patterns ( pattern clause is empty )
  • print line without a newline character ( printf("%s",$0); )

• END {printf("\n");}

  • after the end of the file ( or files) ( END )
  • print a newline ( printf "\n" ; )