Instalation instructions

First, download most recent stable version 
GNU/Linux, Solaris, BSD
Make sure that you have all of core binaries/builtins ([ printf cat grep sed od dd awk) and one of downloaders (wget curl axel fetch). 
(Optional, only if you planing to use your danbooru account to download files) Make sure that one of hashers (digest sha1 sha1sum) installed. 
Run script with sh grab.sh 
Windows
Install cygwin 
While installing make sure you selected wget package. 
Run script in cygwin environment with sh grab.sh 
Modes

There are two basic modes in which script works: 'search' and 'download' 
Search Mode

Search mode activated by default. In search mode you search for specified tags 
$ sh grab.sh 'cirn*'                                                
    [Notice] Downloader found: fetch                                                
                 Type      Count Name                                               
            character       8396 cirno                                              
              general         58 cirno-nee                                          
              general         55 cirno_(cosplay)                                    
    [Notice] Cleaning up...done  

$ sh grab.sh 'loli'                                                 
    [Notice] Downloader found: fetch                                                
                 Type      Count Name                                               
              general      16052 loli                                               
    [Notice] Cleaning up...done                                                     
$ sh grab.sh 'cirno-nee loli'                                       
    [Notice] Downloader found: fetch                                                
                 Type      Count Name                                               
                mixed          1 cirno-nee loli                                     
    [Notice] Cleaning up...done
Download Mode

Download mode activated by option '-d'. In download mode you actually downloading files. 
$ sh grab.sh 'cirno-nee loli' -d                                    
    [Notice] Downloader found: fetch                                                
    Starting grabbing for tags 'cirno-nee loli'                                     
      Switching to page 1 of 1                                                      
        Downloading file d631b6cb19abcd20ff994d6375b4e140.png (1/1)...done          
    End grabbing for tags 'cirno-nee loli'                                          
    [Notice] Cleaning up...done    
Basic usage

Grab for posts with tag 'tag1': 
$ sh grab.sh [OPTIONS] 'tag1' -d

Grab for posts with both 'tag1' and 'tag2': 
$ sh grab.sh [OPTIONS] 'tag1 tag2'

Grab for posts with 'tag1', then for posts with 'tag2': 
$ sh grab.sh [OPTIONS] 'tag1, tag2'

Search for tags, including substring 'tagname': 
$ sh grab.sh tagname

Search for tags starting with 'tag': 
$ sh grab.sh 'tag*'

Download all content, tagged with 'tag1' AND 'tag2': 
$ sh grab.sh 'tag1 tag2' -d

Download all content, tagged with 'tag1' OR 'tag2': 
$ sh grab.sh '~tag1 ~tag2' -d

Download all content, tagged with 'tag1' AND 'tag2', then download all content tagged with 'tag3' OR 'tag4': 
$ sh grab.sh 'tag1 tag2, ~tag3 ~tag4' -d

Download all content with (cirno OR reimu) AND with width > 1000 pixels: 
$ sh grab.sh 'width:>1000 ~hakurei_reimu ~cirno' -d

To grab from gelbooru, just add option -e gelbooru option: 
$ sh grab.sh 'width:>1000 ~hakurei_reimu ~cirno' -d -e gelbooru
More about tags

http://danbooru.donmai.us/help/cheatsheet 
More about options

$ sh grab.sh --help