Creating directory hierarchies in bash

The BASH 'brace expansion' feature can be used to create whole directory trees using a single command. From the man page for BASH:

Brace Expansion
       Brace expansion is a mechanism by which arbitrary strings may be gener‐
       ated.   This  mechanism is similar to pathname expansion, but the file‐
       names generated need not exist.  Patterns to be brace expanded take the
       form of an optional preamble, followed by either a series of comma-sep‐
       arated strings or a sequence expression between a pair of braces,  fol‐
       lowed  by  an  optional  postscript.   The preamble is prefixed to each
       string contained within the braces, and the postscript is then appended
       to each resulting string, expanding left to right.

       Brace  expansions  may  be nested.  The results of each expanded string
       are not sorted;  left  to  right  order  is  preserved.   For  example,
       a{d,c,b}e expands into `ade ace abe'.

       A  sequence  expression takes the form {x..y}, where x and y are either
       integers or single characters.  When integers are supplied, the expres‐
       sion  expands  to each number between x and y, inclusive.  When charac‐
       ters are supplied, the expression expands  to  each  character  lexico‐
       graphically between x and y, inclusive.  Note that both x and y must be
       of the same type.

       Brace expansion is performed before any other expansions, and any char‐
       acters  special to other expansions are preserved in the result.  It is
       strictly textual.  Bash does not apply any syntactic interpretation  to
       the context of the expansion or the text between the braces.

       A  correctly-formed  brace  expansion must contain unquoted opening and
       closing braces, and at least one unquoted comma  or  a  valid  sequence
       expression.   Any incorrectly formed brace expansion is left unchanged.
       A { or , may be quoted with a backslash to prevent its being considered
       part  of  a brace expression.  To avoid conflicts with parameter expan‐
       sion, the string ${ is not considered eligible for brace expansion.

       This construct is typically used as shorthand when the common prefix of
       the strings to be generated is longer than in the above example:

              mkdir /usr/local/src/bash/{old,new,dist,bugs}
       or
              chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

       Brace  expansion  introduces  a  slight incompatibility with historical
       versions of sh.  sh does not treat opening or closing braces  specially
       when  they  appear as part of a word, and preserves them in the output.
       Bash removes braces from words as a  consequence  of  brace  expansion.
       For  example,  a word entered to sh as file{1,2} appears identically in
       the output.  The same word is output as file1 file2 after expansion  by
       bash.   If strict compatibility with sh is desired, start bash with the
       +B option or disable brace expansion with the +B option to the set com‐
       mand (see SHELL BUILTIN COMMANDS below).

So, for example:

$ mkdir -p root/{1/{1.1,1.2,1.3},2,3/{3.1,3.2/{3.2.1,3.2.2}}}

$ tree
.
`-- root
    |-- 1
    |   |-- 1.1
    |   |-- 1.2
    |   `-- 1.3
    |-- 2
    `-- 3
        |-- 3.1
        `-- 3.2
            |-- 3.2.1
            `-- 3.2.2

11 directories, 0 files

The '-p' option to mkdir makes it create the parent directories if they do not exist.