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KRACKEN(3F): The Ultimate Fortran Command Line Argument Cracker

John S. Urban (last change: Dec. 2013)


In Fortran 2003, GET_COMMAND(3f) provides a standard method to read command-line arguments. But this little library goes much further and lets you use Unix-like syntax very easily. You can call your command like this:

 krackenbasic -r 333.333 -f /home/urbanjs/testin -l -i 300

with very little code:

Example Usage

program krackenbasic
   use M_kracken
   character(255) filename
   logical lval

!  define command arguments, default values
!  and crack command line
   call kracken('cmd','-i 10 -r 10e3 -d 4.1123344d0 -l .false. -f input')

!  get values
   call retrev('cmd_f',filename,iflen,ier) ! get -f FILENAME
   lval = lget('cmd_l')                    ! get -l present?
   rval = rget('cmd_r')                    ! get -r RVAL
   dval = dget('cmd_d')                    ! get -d DBLEVAL
   ival = iget('cmd_i')                    ! get -i IVAL

!  all done parsing; do something with the values
   print *, "filename=",filename(:iflen)
   print *, " i=",ival, " r=",rval, " l=",lval

end program krackenbasic

KRACKEN(3f) provides:

The Routines

There are also convenience functions for getting simple values

      lval=lget(VERB_ARGNAME) !gets a "logical" value. 
      rval=rget(VERB_ARGNAME) !gets a "real" value.
      dval=rget(VERB_ARGNAME) !gets a "doubleprecision" value.
      ival=iget(VERB_ARGNAME) !gets a "integer" value
      sval=sget(VERB_ARGNAME) !gets a "character" value

Optional Routines

If you want to allow command-like input to your program you can parse your own strings using DISSECT(3f).

SUBROUTINE dissect(verb,init,pars,ipars)
!@(#) convenient call to parse() -- define defaults, then process user input
      !   verb   is the name of the command to be reset/defined  and then set
      CHARACTER(LEN=*),INTENT(IN)  ::  verb

      !   init   is a string used to add a new command or to reset an old one.
      !          This string is usually hard-set in the program.
      CHARACTER(LEN=*),INTENT(IN)  ::  init

      !   pars   is a string defining the command options to be set, usually
      !          from a user input file
      CHARACTER(LEN=*),INTENT(IN)  ::  pars

      !   ipars  is the length of the user-input string pars.
      INTEGER,INTENT(IN)           ::  ipars

   CALL DISSECT('plotsize',' -x 8.5 -y 11',my_input_string,ipars)

Extended Example

The following example is a bit long compared to typical use; but exercises all the options. The command generated will have parameters -i, -r, -l, -par1, -par2, -par3, and -- . And here is a main program that does just that ...

      program krackentest
      use M_kracken

! call with an arbitrary verb name and a prototype string that defines
! allowable switch names and default values

      call kracken('mycommand',
     &' -i 10 -r 200.3 -l -par1 "#N#" -par2 -par3 10 20 30 -- ')

! that's it. You defined your command arguments and their default
! values and parsed the user-supplied command line arguments.
! Now you can just retrieve the values as strings using
! names of the form VERB_SWITCHNAME anywhere in your program.
! Note that the special name "VERB_oo"  is for the string
! before any switch. to see how look at the SAMPLES() procedure

      call samples()

When the user types

 mycommand  one two -l -i 10 -three four -- file1 file2 file3

the program should complain about -three not being a valid option and will create names that can be used with RETREV(3f).

So lets take a look at something that calls RETREV(3F) and the convenience functions (LGET(),IGET(),RGET(),DGET(),....). For completeness, I show the use of string_to_real(3F) to convert the string returned by RETREV(3F) to a number; and the use of DELIM(3F) to break down a returned list into words. Note that as long as the routines are not called many times (because they are slower than common alternatives), you can just put the calls to RETREV() where you need the information instead of passing the values around or putting them into a COMMON or MODULE.

      SUBROUTINE samples
      USE M_kracken

      CHARACTER*255 value
      LOGICAL lget,lval

C everything before any switch is always VERB_oo
      call RETREV('mycommand_oo',value,len,ier)
      write(*,*)'before any switch=',value(:len)

C Getting an integer from -i value
      write(*,*)'value for -i =',inumber,' divided by 2 is ',inumber/2

C Getting a real number from -r value 
      write(*,*)'value for -r =',anumber,'divided by 2 is ',anumber/2

C Getting a logical value from -l value 
      write(*,*)'value for -l =',lval

C Getting a string (with a default) from -par1
      call RETREV('mycommand_par1',value,len,ier)
      write(*,*)'value for -par1 =',value(:len)

C Getting a string (with no default) from -par2
      call RETREV('mycommand_par2',value,len,ier)
      write(*,*)'value for -par2 =',value(:len)

C Getting a string with a multi-word default and splitting it from -par3
      call listem('mycommand_par3','-par3',.true.)

C Convention for last parameter because multi-word unquoted values are allowed
      call listem('mycommand_-','--'.false.)

      SUBROUTINE listem(keyword,label,toreal)
      USE M_kracken
C     Just getting fancy and showing the use of DELIM(3F)
C     SAMPLE that decomposes list of strings and optionally, numbers
C     and prints it.
C     Delimit with space, comma, or colon
      LOGICAL toreal
      CHARACTER*(*) keyword
      CHARACTER*(*) label
      CHARACTER*255 value

C     for DELIM call if you want to break down a list
      CHARACTER*255 array(132)
      INTEGER ibegin(132)
      INTEGER iterm(132)

C     get the value of the keyword and the length
      CALL RETREV(keyword,value,len,ier)
      WRITE(*,*)'value for ',label,'=',value(:len)
C     split the list into one word per array
      call DELIM(value,array,132,igot,ibegin,iterm,ilen,' ,:')
C     print and optionally convert each word into a numeric value
      DO i10=1,igot
         WRITE(*,*)i10,') ',array(i10)(:len_trim(array(i10)))
            CALL string_to_real(array(i10),anumber,ier)
               WRITE(*,*)' which is a number'
               WRITE(*,*) ' which is not a number'

This shows the expected output of the execution:

 # Remember the command was defined by
 # call kracken('mycommand',&
 # &' -i 10 -r 200.3 -l -par1 "#N#" -par2 -par3 10 20 30 -- ')
 # user types
 mycommand  one two -l -i 10 -three four -- file1 file2 file3
 # response should be
 >:E-R-R-O-R: UNKNOWN OPTION mycommand_three
 before any switch=one two
 value for -i =10
 divided by 2 is  5
 value for -r =200.3
 divided by 2 is  100.15
 value for -par1 =#N#
 value for -par2 =
 value for -par3=10 20 30
 1) 10
  which is a number
 2) 20
  which is a number
 3) 30
  which is a number
 value for --=file1 file2 file3
 1) file1
 2) file2
 3) file3

If you read this far, you probably want the source. You'll be happy to know it's annotated more thoroughly than this example is.

To get a demonstration program and the source for the kracken library download the following source files:

  # compile a demo program and the source for kracken(3F) and
  # associated routines
  g95 M_kracken.f90 krackentest.f90
  # or if you prefer ANSI-77 syntax an alternate (static) version is available
  g77   krackentest.f   kracken.f

Download files

f90+ files(preferred)

f90+ files(supplemental)

Here are some donated contributions that will be incorporated directly into M_KRACKEN(3f) in the future ...

The f77 version found here will not be changing

My purpose here is to make something very simple and portable that others can use reliably. I have made and used variants of kracken for years; for me it was just calls to mostly-f77 libraries used for a variety of other purposes. While extracting parts of the library to make a simple stand-alone version I realized anyone using this would be better served by using clean f90+ syntax. Lately being "clean" means I can compile it with an "F" compiler (if it doesn't use stuff "past" F). So I won't be maintaining the public f77 version, as I have a bigger better one I will be maintaining. And the whole idea of libraries is to not have to maintain duplicate code.

If you are having problems

The routine GET_COMMAND_ARGUMENTS() is the only one that actually reads from the command line. The f90+ version assumes your compiler has GET_COMMAND_ARGUMENTS(). The f77+ version assumes your compiler has GETARGS() and IARGC(). If neither is the case and you don't know how to make your own GET_COMMAND_ARGUMENTS() look-alike, It is VERY likely you can get a GET_COMMAND_ARGUMENT() from the F2KCLI package from WINTERACTER.

You may note that the parsing rules are not identical to Unix, although very similar. There is no way to terminate a keyword except by starting a new keyword; and the keyword -oo is implied after the verb; and you cannot combine keywords (-ir is not equivalent to -i -r, which is sometimes allowed on Unix commands). You may find the way to include a literal double-quote character (") is the most unlike Unix (see code comments for details).


If you don't need GNU/Unix syntax and you have a Fortran 2003-compliant compiler, an alternative in testing that uses NAMELIST is promising and very simple. I also have a few links to related sites in the optional "OMNIUM-GATHERIUM" section.

This is a stable version of KRACKEN(3f). If you are looking for a development version see the libCLI (Command Line Interface) home page.