Testing variables

String-based
if  [  "$somevar"  =  "hello"  ];  then
if  [  "$somevar"  !=  "hello"  ];  then
Numeric-based
if  [  "$num"  -lt  42  ];  then
if  [  "$num"  -ge  42  ];  then
File-based
if  [  -r  "$fname"  ];  then
if  [  "$fname"  -nt  "file.txt"  ];  then
Many more, for info:  man  test