How to find line with least characters

Here's a variant of an awk solution for printing the first found minimum line:

awk '
  NR==1 || length<len {len=length; line=$0}
  END {print line}
'

which can simply be extended by one condition to print all minimum lines:

awk '
  length==len {line=line ORS $0}
  NR==1 || length<len {len=length; line=$0}
  END {print line}'
'

With sqlite3:

sqlite3 <<EOT
CREATE TABLE file(line);
.import "data.txt" file
SELECT line FROM file ORDER BY length(line) LIMIT 1;
EOT

A Perl way. Note that if there are many lines of the same, shortest length, this approach will only print one of them:

perl -lne '$m//=$_; $m=$_ if length()<length($m); END{print $m if $.}' file

perl -lne : -n means "read the input file line by line", -l causes trailing newlines to be removed from each input line and a newline to be added to each print call; and -e is the script that will be applied to each line.
$m//=$_ : set $m to the current line ($_) unless $m is defined. The //= operator is available since Perl 5.10.0.
$m=$_ if length()<length($m) : if the length of the current value of $m is greater than the length of the current line, save the current line ($_) as $m.
END{print $m if $.} : once all lines have been processed, print the current value of $m, the shortest line. The if $. ensures that this only happens when the line number ($.) is defined, avoiding printing an empty line for blank input.

Alternatively, since your file is small enough to fit in memory, you can do:

perl -e '@K=sort{length($a) <=> length($b)}<>; print "$K[0]"' file

@K=sort{length($a) <=> length($b)}<> : <> here is an array whose elements are the lines of the file. The sort will sort them according to their length and the sorted lines are saved as array @K.
print "$K[0]" : print the first element of array @K: the shortest line.

If you want to print all shortest lines, you can use

perl -e '@K=sort{length($a) <=> length($b)}<>; 
         print grep {length($_)==length($K[0])}@K; ' file

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