Shell Programming and Scripting

Your method is simplest than mine. Just replace space by the only character possible ('\n') and at the end, replace all '\n' by space:

Code:

echo "Mary walks at the park every day with her children" |  sed ":A;s/\n/\n/${NB};tB;s/$/\n/;tA;:B;s/\n/ /g"

Regards.

The whole concept of adding a random number of spaces to the end of a string doesn't make any sense to me.

I can see wanting to pad a field with trailing spaces to create a fields that contains exactly X characters:
printf '%-*.*s\n' $X $X "$field_contents"

I can see wanting to pad a field with leading spaces to create a field that contains exactly X characters:
printf '%*.*s\n' $X $X "$field_contents"

I can see wanting to pad a field with trailing spaces to create a field that is at least X characters (but not truncate the field if it is longer):
printf '%-*s\n' $X "$field_contents"

I can see wanting to pdd a field with leading spaces to create a field that is at least X characters (but not truncate the field if it is longer):
printf '%*s\n' $X "$field_contents"

Why do you want to add a random number of space???

Still there is an academical interest if this can be done with sed.
Apart from the s/x/y/num that must go from left to right (so can hardly be used on x$,
there is x\{num\}, a special case of x\{min,max\}.
Modified the previous idea, and for Unix sed replaced some ; by newline:

Code:

NB=14
echo "Mary walks at the park every day with her children" | sed '
:A
/ \{'"$NB"'\}$/b
s/$/ /;bA'

NB instead of newlines one can use several '-e line':

Code:

echo "Mary walks at the park every day with her children" | sed -e ':A' -e '/ \{'"$NB"'\}$/b' -e 's/$/ /;bA'

lol...
by moment I really complicate my life