# Redirecting unit \* to a file **URL:** \ **Category:** Help\ **Created:** [February 18, 2022, 8:45pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822 "2022-02-18T20:45:33Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![feenberg](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@feenberg](https://fortran-lang.discourse.group/u/feenberg)\ **Post date:** [February 18, 2022, 8:45pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/1 "2022-02-18T20:45:33Z")
I would like to have a program read from standard in by default or from a file if a specified. I have thought about using unit 5 for input, which I can redirect to a file in an open statement, but I understand 5 isn’t designated as standard input by the Fortran standard. Is that really a problem? Am I likely to run in to users where unit 5 is not standard in? I mean, someone with a Linux, WIndows or Mac machine. I don’t care about anything older than f2c. The program is used by random unsophisticated users, I’d like to put the redirection in the program rather than rely on a shell script I would have to explain. As it is, the best I can think of to do that fits the standard is to replace each read statement with 2 read statements, one with a \* and one with a unit number. Is that the best I can do? I notice that some compilers such as [Oracle Fortran](https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vnaf/index.html) allow a \* in the open statement: [UNIT=] u - **u** is an integer expression or an asterisk (\*) that specifies the unit number. **u** is required but does that modify read or write statements? I suppose it doesn’t matter, since so few compilers support this. Still, it seems odd if there is no way to redirect unit \*. ---
**Author:** ![Beliavsky](https://avatars.discourse-cdn.com/v4/letter/b/ba8739/32.png) [@Beliavsky](https://fortran-lang.discourse.group/u/Beliavsky)\ **Post date:** [February 18, 2022, 8:49pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/2 "2022-02-18T20:49:00Z")
> [@feenberg](#): > > I would like to have a program read from standard in by default or from a file if a specified. I have thought about using unit 5 for input, which I can redirect to a file in an open statement, but I understand 5 isn’t designated as standard input by the Fortran standard. Is that really a problem? Am I likely to run in to users where unit 5 is not standard in? Why not use `input_unit` from module [ISO\_FORTRAN\_ENV](https://gcc.gnu.org/onlinedocs/gfortran/ISO_005fFORTRAN_005fENV.html) of Fortran 2003? ---
**Author:** ![urbanjost](https://avatars.discourse-cdn.com/v4/letter/u/0ea827/32.png) [@urbanjost](https://fortran-lang.discourse.group/u/urbanjost)\ **Post date:** [February 19, 2022, 3:47am UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/3 "2022-02-19T03:47:25Z")
If you have not used INPUT\_UNIT before, here is an example that reads one line from stdin and echoes it unless a filename is given on the command line, in which case that file is read from instead. This assumes your input file is a text file; reading binary files from stdin is problematic. ```fortran program readfrom use,intrinsic :: iso_fortran_env, only : & & stdin=>input_unit, & & stdout=>output_unit,& & stderr=>error_unit implicit none integer :: count, argument_length, lun, ios character(len=:),allocatable :: filename character(len=256) :: line, msg ! get number of arguments count = command_argument_count() if(count.ne.0)then call get_command_argument(number=1,length=argument_length) ! allocate string array big enough to hold command line argument allocate(character(len=argument_length) :: filename) call get_command_argument(1, filename) open(file=filename,newunit=lun) else lun=stdin endif read(lun,'(a)',iostat=ios,iomsg=msg)line if(ios.eq.0)then write(*,*)'READ :',trim(line) else write(*,*)'',trim(msg) endif end program readfrom` ``` so if you entered “readfrom myfile” it would read the first line of file “myfile”; otherwise it would read a line from stdin. You do not have to rename the variables, but I prefer the names stdin, stdout, and stderr myself. ---
**Author:** ![feenberg](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@feenberg](https://fortran-lang.discourse.group/u/feenberg)\ **Post date:** [February 19, 2022, 12:33pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/4 "2022-02-19T12:33:18Z")
Thanks. Another demonstration that there is no problem in computer science that can not be solved with an additional level of indirection. ---
**Author:** ![sblionel](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/sblionel/32/853_2.png) [@sblionel](https://fortran-lang.discourse.group/u/sblionel)\ **Post date:** [February 19, 2022, 4:40pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/5 "2022-02-19T16:40:54Z")
If you are using Intel Fortran and the ISO\_FORTRAN\_ENV constants, be sure to compile with -standard-semantics. If you don’t, the compiler will use various negative unit numbers for unit \* (or PRINT). ---
**Author:** ![msz59](https://avatars.discourse-cdn.com/v4/letter/m/3d9bf3/32.png) [@msz59](https://fortran-lang.discourse.group/u/msz59)\ **Post date:** [February 19, 2022, 6:21pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/6 "2022-02-19T18:21:38Z")
> [@sblionel](#): > > compiler will use various negative unit numbers for unit \* (or PRINT). Would that make any difference in a program like above @urbanjost’s sample? ---
**Author:** ![sblionel](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/sblionel/32/853_2.png) [@sblionel](https://fortran-lang.discourse.group/u/sblionel)\ **Post date:** [February 19, 2022, 6:32pm UTC](https://fortran-lang.discourse.group/t/redirecting-unit-to-a-file/2822/7 "2022-02-19T18:32:28Z")
No - my comment related to @feenberg 's mention of unit \*, which is part of the standard language. Specifically, if you expected to open unit `INPUT_UNIT` and then do `READ(*)` and have that be the unit you opened, it would not work in Intel Fortran without the standard-semantics option (or `-assume noold_unit_star`)