if

if condition1
then commands1
[ elif condition2
    then commands2 ]
.
.
.
[ else commands3 ]
fi

If condition1 is met, do commands1; otherwise, if condition2 is met, do commands2; if neither is met, do commands3. Conditions are usually specified with the test and [[ ]] commands. See test and [[ ]] for a full list of conditions, and see additional Examples under : and exit.

Examples

Insert a 0 before numbers less than 10:

if [ $counter -lt 10 ]
then number=0$counter
else number=$counter
fi

Make a directory if it doesn't exist:

if [ ! -d $dir ]; then
   mkdir $dir
   chmod 775 $dir
fi