How do I trim lines read from standard input on bash?

You can use sed to trim it.

sed 's/^ *//;s/ *$//'

You can test it really easily on a command line by doing:

echo -n "  12 s3c  " | sed 's/^ *//;s/ *$//' && echo c

$ trim () { read -r line; echo "$line"; }
$ echo "   aa   bb   cc   " | trim
aa   bb   cc
$ a=$(echo "   aa   bb   cc   " | trim)
$ echo "..$a.."
..aa   bb   cc..

To make it work for multi-line input, just add a while loop:

trim () { while read -r line; do echo "$line"; done; }

Using sed with only one substitution:

sed 's/^\s*\(.*[^ \t]\)\(\s\+\)*$/\1/'