Comparisons between UNIX and other operating systems

The following sections may be useful guides if you are familiar with the VMS or MS-DOS operating systems.

Comparison between UNIX and VMS

The UNIX and VMS operating systems both have advantages and disadvantages and there is no answer to the question ``which one is better?'' The main reason that UNIX has become so popular is that it is runs on many different types of machines, not just on Digital Equipment Corporation VAXes. It is also cheaper to buy and it is typically many times faster than VMS on machines of similar cost.

Here are a few comments on the differences between UNIX and VMS from a user's perspective (albeit one biased in favour of UNIX).

• UNIX is terser. VMS tends to use easy to remember (although often verbose) English language option names, whereas UNIX uses single letters. Once you become familiar with UNIX you will appreciate the compactness of the syntax, however, it is difficult for beginners.
• UNIX distinguishes between upper and lower case letters in command and file names, i.e., the file prog.f is quite distinct from PROG.F, Prog.F, and so on. This takes a bit of getting used to. In particular, you must use lower case letters for your username when login, otherwise UNIX will assume that you only have an upper case terminal and will refuse to echo lower case characters.
• UNIX filenames are more logically organized than VMS. VMS uses a clumsy square bracket notation which makes it difficult to parse filenames. With UNIX there is no need to worry about the various fields of the file specification (e.g., disk, directory, filename, filetype).
• Some VMS commands assume default file types if you don't specify one. For example PRINT TEXT will print the file TEXT.LIS. UNIX makes no such assumptions, and if you specify the file TEXT then that is what you will get. A side effect of this is that you must specify complete filenames when using UNIX compilers. For example, when using VMS the command FORTRAN PROG will compile the program PROG.FOR, whereas the UNIX equivalent is f77 prog.f (note also that the UNIX FORTRAN compiler requires source files to end in ``.f'').
• UNIX treats all files and peripherals as streams of bytes, whereas VMS has a multitude of different file organizations (e.g., sequential, fixed-length, indexed, blocked, unformatted, relative, and so on). Using UNIX you can easily read a tape as a byte-stream, whereas with VMS you have to worry about record-lengths and buffer-sizes.
• The output from UNIX commands is designed to be easily processed by following commands. For example, a directory listing obtained with the UNIX command ls -l has a simple format, whereas the VMS equivalent DIRECTORY/SIZE/DATE/PROTECTION is not so simple. Also UNIX has the mechanisms of pipes and I/O redirection, which can not be simulated with VMS.
• UNIX has a superb wildcard facility based on a concept known as ``regular expressions'' that is considerably more flexible than VMS. For example, ls [0-9]*.dat will list all the files in the current directory that have filenames beginning with a digit and ending in ``.dat''.
• You execute UNIX programs by simply typing their name. With VMS you have to type ``RUN PROG'', and there is the added complexity that if you want the program to be able to read the command line, you have to set it up as a foreign command. UNIX is simpler and more logical in its approach. UNIX has the concept of a PATH, i.e., a list of directories in which it searches for commands to execute. VMS has no equivalent feature.
• UNIX gives you a choice of shells (a ``shell'' is simply the program that is the interface between you and the operating system) some of which are considerably more flexible than the VMS ``shell'', e.g., they have filename completion and the recall of a user-selectable number of commands. Filename completion is sufficient reason alone for abandoning VMS in favour of UNIX.
• Starting a sub-process in UNIX is very much easier and faster than the equivalent in VMS. VMS uses a clumsy system of mailboxes to transmit environment information to sub-processes, and this slows down their creation.
• UNIX displays characters on your terminal as you type them, regardless of whether the operating system is ready to accept a command. VMS will buffer, but not display, terminal input when it is busy executing the previous command.
• There is more public domain software available for UNIX. Some of this software is of very high quality, and the VMS equivalents cost many thousands of dollars.
• On the negative side, there is no doubt that UNIX documentation is far less readable than VMS documentation. The VMS HELP command is probably superior to UNIX's man. A nasty aspect of UNIX is that the various error messages that commands may produce are rarely documented, whereas every VMS message is catalogued.
• VMS has superior built-in BATCH queues. Under UNIX, batch queues are not usually required, although they can be implemented through public domain software.
• VMS files have version numbers. This provides you with a level of safety against catastrophic modification of a file. UNIX doesn't give you this protection.
• VMS compilers generally produce more highly optimized code that most UNIX compilers. VMS is reknowned for the accuracy of its floating point transcendental functions.
• VMS security and system management facilities are superior to those provided by UNIX.
• UNIX has no equivalent to VMS's &#;'136 T command. In most cases this is not a problem since you can always use &#;'136 Z to suspend the job, bg to background it, and then ps -x to see what it is up to.
• The UNIX philosophy it to do what the user asked, quietly, even if this means deleting all the user's files. Quite often UNIX commands don't do careful error checking and may not inform you that an operation succeeded or failed, for example, the command lprm for removing jobs from the print queue will not tell you that anything is wrong if you type lprm 671 when the job 671 doesn't exist. The VMS philosophy is to try and protect the user to some extent, and to give very clear indication of what a command has done and of the severity of errors.

When all is said and done, after using UNIX for a few months you will not want to use VMS again.

The following table gives rough equivalences between VMS and UNIX commands. Note: the commands mmv, hd, and less are local additions.

Comparison between UNIX and MS-DOS

  UNIX and MS-DOS have much in common, primarily because MS-DOS was partly developed using ideas from UNIX. Here are some of the major differences:

• UNIX filenames are arbitrary strings of up to 255 characters, MS-DOS filenames are limited to an 8 character file name and a 3 character file type, and have restrictions on allowable characters.
• UNIX doesn't require the notion of a disk drive modifier for a file specification, i.e., you don't need A:, B:, C:, and so on like MS-DOS. Note that MS-DOS has the JOIN command which can simulate the UNIX file system.
• UNIX uses forward slashes (/) to separate directories in a file specification, whereas MS-DOS uses backslashes ( ).
• UNIX is a full multiuser multitasking operating system, whereas MS-DOS is a single-user OS without concepts like file-ownership and process priorities.
• UNIX has a richer command set than MS-DOS.
• UNIX and MS-DOS both have the concept of a PATH.
• UNIX and MS-DOS both have pipes and I/O redirection.
• UNIX displays characters on your terminal as you type them, regardless of whether the operating system is ready to accept a command. MS-DOS will buffer, but not display, terminal input when it is busy executing the previous command.
• MS-DOS programs generally have a nicer user interface than traditional UNIX ones. This situation is changing with the emergence of X-windows applications on UNIX.

The following table gives rough equivalences between UNIX and MS-DOS commands. As you can see, there are many similarities. Note particularly the UNIX mechanism for putting a directory name into the prompt string (for more details, see man tcsh).