What does effect does a trailing number have on the body of an awk script?

For awk, 1 mean true, and when an expression is true, awk print the current line by default.

Examples :

• awk '1' /etc/passwd will print the whole file
• awk '0' /etc/passwd will not print the whole file

If you're good in arithmetic (or you have imagination or programming skills), there's many tricks you can do with this behaviour.

"The '1' at the end of whole Awk one-liner prints out the modified line (it's syntactic sugar for just "print" (that itself is syntactic sugar for "print $0"))".

-- catonmat

You got your answer to your specific question BUT your posted script is unsafe. The first argument for printf is a format string, not data. You can sometimes use it to print data but only when it's hard-coded in your script, not when you're reading it from input. Look what happens if your input happens to contain a printf formatting string:

$ cat fileonetwo%sthreefour$ awk '/two/ { printf $1; next; } 1' fileoneawk: cmd. line:1: (FILENAME=file2 FNR=2) fatal: not enough arguments to satisfy format string        `two%s'            ^ ran out for this one

The correct way to write the script is:

$ awk '/two/ { printf "%s",$1; next; } 1' fileonetwo%sthreefour