str=" \t  \n1234 dd 56 \t\n "

trim_str=`expr match "$str" '[ \t\n]*\(.*[^ \t\n]\)[ \t\n]*$'`

echo "'$trim_str'"  #’1234 dd 56’

[ \t\n]* for leading white spaces

\(.*[^ \t\n]\) for substring in between with non-whitespace ending

[ \t\n]*$ for trailing white spaces

expr match "$string" '\($substring\)'

Extracts $substring at beginning of $string, where $substring is a regular expression.

expr "$string" : '\($substring\)'

Extracts $substring at beginning of $string, where $substring is a regular expression.