A variable may be assigned to by a statement of the form


If value is not given, the variable is assigned the null string.  All  values  undergo  tilde  expansion,
parameter  and  variable  expansion,  command  substitution, arithmetic expansion, and quote removal (see
EXPANSION below).  If the variable has  its  integer  attribute  set,  then  value  is  evaluated  as  an
arithmetic  expression even if the $((...)) expansion is not used (see Arithmetic Expansion below).  Word
splitting is not performed, with the exception of "$@"  as  explained  below  under  Special  Parameters.
Pathname  expansion  is  not performed.  Assignment statements may also appear as arguments to the alias,
declare, typeset, export, readonly, and local builtin commands.

In the context where an assignment statement is assigning a value to a shell variable or array index, the
+=  operator  can  be used to append to or add to the variable's previous value.  When += is applied to a
variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and
added  to the variable's current value, which is also evaluated.  When += is applied to an array variable
using compound assignment (see Arrays below), the variable's value is not unset (as it is when using  =),
and  new  values  are  appended to the array beginning at one greater than the array's maximum index (for
indexed arrays) or added as additional key-value pairs in  an  associative  array.   When  applied  to  a
string-valued variable, value is expanded and appended to the variable's value.