Two ways: using ‘expr’ or ${#variable}. Example:

zia@lappy:~$ string=supercalifragilisticexpialidocious
zia@lappy:~$ echo ${#string}
34
zia@lappy:~$ expr length $string
34
zia@lappy:~$ expr “$string” : ‘.*’
34
zia@lappy:~$ expr match “$string” ‘.*’
34

Source: http://www.tldp.org/LDP/abs/html/string-manipulation.html