Following your attempt a little closer than the way Corona688 did it...

This is an awk script, not a shell script; so $ before a variable name references the field named by contents of the variable instead of the contents of the variable AND awk variables are not expanded in quoted strings AND ${#var} and $((expr)) are shell expressions that are not valid awk expressions.

If what you are trying to do is produce 8 character output strings (assuming the length of the input string is never more than 8 characters) with varying length alphabetic and numeric parts in the input, try:

Code:

awk -v LEN=8 '
{	NUMBER = substr($0, match($0, /[[:digit:]]*$/))
	PREFIX = substr($0, 1, PRELEN = (RSTART - 1))
	DIGITS = LEN - PRELEN
	printf "%s%0*d\n", PREFIX, DIGITS, NUMBER
}' test.file

Assuming test.file contains:

Code:

S1
S2  
S12  
S21  
sk1  
sk12  
sk321  
sk1344

as shown in post #18 in this thread, it produces the output:

Code:

S0000001
S2  0000
S12  000
S21  000
sk1  000
sk12  00
sk321  0
sk001344

Remember the input file format I specified in post #10 in this thread. Your sample input file has trailing spaces (violating item #2: Each input string is an alphanumeric string ending in one or more decimal digits.)

If we remove all of the trailing spaces from file.test or change all occurrences of $0 in the above script to $1 (so we just look at the first field instead of the entire line), the output produced is:

Code:

S0000001
S0000002
S0000012
S0000021
sk000001
sk000012
sk000321
sk001344

which I assume is closer to what you were trying to do.

As a learning exercise, can you explain why the awk script Corona688 suggested didn't have a problem with trailing spaces while my script above does have a problem with trailing spaces?

And, no, I'm not a lawyer. But I did like the Perry Mason, Matlock, and Boston Legal TV series. And, in my last job, my boss referred to me as his standards lawyer because he could get me to answer any questions about why we were failing POSIX/UNIX standards conformance tests and directions on how to fix our code (when we had a bug) or how to file a bug report against the test suite (when there was a bug in the test suite or it was assuming behavior above and beyond what the standards require).