Appendix to the Description of Features and Implementation Status

of FreeCOM

Current Directory

  The current directory is the default directory of a drive.
  DOS stores a default directory for each drive. When a path is
  specified with a drive specification only, such as D:, it is completed
  with this default directory of that drive to construct the absolute
  path to be used.

Current Working Directory

  In opposite to the current directory the current working directory is
  the absolute path constructed out of the currently selected drive and
  current directory of that drive.

Path Specification

  In DOS an absolute path is constructed out of several components:
    1. drive,
    2. directory,
    3. filename, and
    4. file extension.
  like this: D:\DIR1\DIR2\FILENAME.EXT.
  The drive is a single letter from A through Z followed by a colon :.
  The remaining part of a path consists of similiar components delimited
  by a single backslash \. The last component is also called filename.
  Each of these components may be formed of a name, up to eight
  characters long, and an extension, up to three characters long. Both
  parts are delimited by a single dot .. Although the extension may be
  absent, the filename must have at least one character.
  Note: The term filename is not limited to files in the usual sense,
  but may apply to any name visible in a directory, such as sub-
  directories and volume labels, as well.
  To ease the way to enter a path the user may specify a relative path,
  rather than an absolute one. In such path one or more components may
  be missing:
    - If no drive is specified, what means that no colon is specified,
      the path is prefixed by the currently selected drive.
    - If the directory is not prefixed by the backslash or no directory
      is specified at all, the current directory of the drive is inserted
      right behind the colon.
    - Some programs may append an absent extension to the very last
      filename component.
  Examples, assume the current directories of

  Drive     Current Directory
  C:        \FREEDOS\HELP
  D:        \TEMP\TEXT
  The currently selected drive is C:.

    1. C:\
       The root directory of drive C:.
    2. .
       The current working directory, ergo: C:\FREEDOS\HELP.
    3. ..
       The parent directory, ergo: C:\FREEDOS.
    4. D:
       The current directory of drive D:, ergo: D:\TEMP.
    5. D:.
       The current directory of drive D:, ergo: D:\TEMP.
    6. D:..
       The parent directory of drive D:, ergo: D:\.
    7. ..\BIN
       Because there is neither a drive nor a leading backslash, both
       the currently selected drive and the current directory of that
       drive is inserted before the given path, ergo:
       The embedded component .. has the same meaning as when specified
       alone: parent directory, though, here in the context of the
       directory C:\FREEDOS\HELP\. That means that the final absolute
       path is: C:\FREEDOS\BIN.
  Path specifications that do not conform to above mentioned format
  lead to various different behaviour of the various programs, because
  there is no standard to scan, parse and interprete such patterns.
  Problems include:
    - multiple backslashes,
    - multiple dots,
    - multiple colons, or a colon at a position unequal to two,
    - The current directory . or parent directory .. special directories
      in the context of a root directory, such as C:\., C:\.., or
      C:\TEMP\..\.. .
  Note: The special directories . and .. are no phantom directories or
  virtual entries, but standard entries of every directory except the
  root directories. These entries help crash recovery tools, such as
  CHKDSK or SCANDISK, to find errors within the directory structure and
  restore it to a valid file tree. Therefore a common assumption that a
  tripple dot ... directory means parent-of-parent is incorrect, though,
  might be supported by certain programs.

Standard Rules for Options

  Options are prefixed by one forward slash "/", the following character
  identifies the option and is called option character, for instance: /A
  Some commands do accept long option names, where a complete word
  identifies the option rather than a single character, e.g. /MSG.
  Some option may be used in conjunction with an argument. The argument
  is appended to the option with one colon ":" or one equal sign "=",
  for instance: /A:hr or /P=fdexec.bat.
  Multiple options without argument maybe merged together as a single
  option with or without embedded slashes, e.g. /WS or /W/S, instead
  of /W /S.
  However, because some commands do accept long option names, the way
  with embedded slashes is more secure and is recommended therefore.
  An option with argument may be the last one of such merged options.
  Options without arguments enable or disable certain features.
  Therefore, those options are sometimes called boolean options or flags.
  Boolean options may be optionally prefixed by a plus "+" or minus "-"
  sign. So, the boolean option O can be written in three ways:
    1. /+O: The option is enabled.
    2. /-O: The option is disabled.
    3. /O:  (neither plus nor minus sign) The option is toggled or
            flipped; this means if the option is enabled currently, it
            is disabled; but if it is disabled currently, it is enabled.
  Without user invention a boolean option is disabled by default, so
  both /+O and /O behave the same most of the time. However, some
  commands allow the user to change the default settings of certain
  options, e.g. COPY and DIR.

I/O Redirection

  In DOS the standard input and output can be redirected to a file or
  another device; however, although it is common to use these I/O
  streams today, some programmers cowardly ignore them for reasons of
  speed or program size.
  If the input stream is redirected to a file or device, instead of
  polling the keyboard and request the user to interactively enter
  characters via the keyboard, those characters are read from the file
  or device. Usually these programs terminate when the file has been
  read wholely.
  If the output stream is redirected to a file or device, instead of
  issuing the information onto screen, it is dumped into the file or
  device. Per convention each program has two output streams: one
  (called standard output) to issue normal information and one (called
  standard error output) for error messages the user should not miss.
  Redirections are specified on command line and effect exactly that
  command invoked herein, regardless if the command is an external or
  internal one, an alias or BATCH script. The utter exception is the
  FOR command, which requires that the redirection is to apply to the
  command specified behind the DO keyword rather than FOR itself.
  If more than one redirection is specified on the command line and they
  effect the same stream (input, output, or error), the rightmost one
  superceed any previous one.
  Redirections are syntactically specified like this:
  operator target
    operator ::= '<' | '>' [ '>' ] [ '&' [ '>' ] ]
    target ::= file | device
  Although it is not relevant where the redirections are placed on the
  command line, it is common praxis to place them at the end of it.
  The operators have the following meaning:
    Operator    Redirection
    <     Input stream
    >     Output stream; target file is overwritten
    >>    Output stream; output is appended to target, if it
          already exists
    >&    Output and error stream; target file is overwritten
    >>&   Output and error stream; output is appended to target,
          if it already exists
    >&>   Error stream; target file is overwritten
    >>&>  Error stream; output is appended to target,
          if it already exists


Example 1:

    cmd <in1 <in2
  Input stream is redirected to file IN2, because it is the rightmost

Example 2:

    cmd <in >out
  Input stream is redirected to file IN, output and error streams are
  joined together and redireced into file OUT. If the file OUT already
  exists, the old contents is discarded and replaced by the new one;
  otherwise, the OUT is created anew.

Example 3:

    cmd <in >>out
  As example 2, but instead of replacing the contents of OUT, if the
  file already exists, the new information is appended to the end of
  the file.

Example 4:

    FOR %a IN (*.*) DO ECHO %a >out
  As mentioned earlier, FOR is an exception and passes forth the
  redirections to each invocation of the command specified right of the
  DO keyword. So this examples overwrites the output file each time the
  ECHO command is performed, thus, instead of creating a file list,
  only the last found file is recorded into OUT.

Example 5:

    IF EXIST out DEL out
    FOR %a IN (*.*) DO ECHO %a >>out
  This sequence eliminate the problem, the IF command is required to
  actually replace the file rather than appending the file list to the
  probably existent file.


  Another form of redirection is piping. Hereby, the output stream of
  one command is connected to the input stream of another command. Pipes
  can combine any number of commands this way. Because DOS is no
  multitasking system, pipes are simulated by spawning the first command
  with an output redirection capturing the issued information into a
  temporary file and then the second command with an input redirection
  from that very same temporary file, on completation of the second
  command the temporary file is deleted.


Example 1:

  cmd1 | cmd2 | cmd3

  Which is similiar to this sequence:
    cmd1 >%TEMP%\t1
    cmd2 <%TEMP%\t1 >%TEMP%\t2
    DEL %TEMP%\t1
    cmd3 <%TEMP%\t2
    DEL %TEMP%\t2

Example 2:

  The first and last command can have an input or output redirection
  respectively, like so:
    cmd1 | cmd2 | cmd3 <in >out

  Which is similiar to this sequence:
    cmd1 >%TEMP%\t1 <in
    cmd2 <%TEMP%\t1 >%TEMP%\t2
    DEL %TEMP%\t1
    cmd3 <%TEMP%\t2 >out
    DEL %TEMP%\t2

Example 3:

  The error stream can be piped as well:
    cmd1 |& cmd2 | cmd3

  Which is similiar to this sequence:
    cmd1 >&%TEMP%\t1
    cmd2 <%TEMP%\t1 >%TEMP%\t2
    DEL %TEMP%\t1
    cmd3 <%TEMP%\t2
    DEL %TEMP%\t2

  Here only the error messages of cmd1 are passed into cmd2; the error
  messages of both cmd2 and cmd3 are issued to the screen.

Nested redirections

  BATCH scripts or when external programs invoke other programs or
  another shell, redirections may be nested, e.g.:
  Consider the BATCH file BATCH.BAT:
    ECHO 1
    ECHO 2 >out_2
    ECHO 3
  which is invoked via:
    BATCH >out_1
  When the script BATCH gets executed, the actual output stream is
  redirected to the file OUT_1. Therefore the output of the first ECHO
  command is redirected into this file.
  Because the second ECHO command has its own output redirection, its
  output is redirected into the file OUT_2. On completion of ECHO the
  redirection is closed and the former one is restored.
  What causes that the output of the third ECHO command is redirected
  into OUT_1 again.


  DOS accepts certain placeholders instead of files, so you can specify
  one or more files with the same wildcard pattern. In opposite to other
  systems each program expands wildcards individually instead of let the
  shell do this; however, the shell must epxand wildcards for its
  internal commands.
  FreeCOM supports the standard DOS wildcards as supported by the DOS
  kernel itself:
  Wildcard   Meaning
    *        Any remaining body, except a dot (.)
    ?        Any character, except the dos (.); acts as *, if the last
  Wildcards are allowed in the last portion of a qualified path, that
  means behind the last backslash (\) or colon (:).
  Wildcards match either before or behind the dot delimiting the
  filename and its extension.
  Whether or not directories or volume lables or hidden and system files
  do match a pattern depends on the command expanding it.
  Letters match case-insensitively.


  Pattern        Meaning
  *.*            Any file in the current directory
  c:*.*          Any file in the current working directory of drive C:
  *              Any file without an extension. Some commands, such as
                 DIR, interprete a single * as *.*
  a*.*           Any file, which first character is an A, neither the
                 remainder of the filename nor the extension of the
                 file matters
  a?.*           Same as above
  a???.*         Same as above
  a*b.*          Same as above
  a?b.*          Any file, which first character is an A and the third
                 one is a B; the remaining characters nor the extension
  file?.txt      All files, which first four characters are FILE and
                 which extension is equal to TXT
  \scripts\*.pl  Any perl file in the directory \SCRIPTS located in the
                 root directory of the currently selected drive
  Warning: There are common mistakes about wildcards, as DOS has less
  functional wildcards than other systems, in particular:
    a*bc.*txt:   Any character right of an asterisk * up to the dot . or
                 the end of the filename is ignored; this pattern is
                 equal to: a*.*.
    a?.?:        A question mark ? as the last character of either the
                 filename or file extension has the same effect as an
                 asterisk * and does not mean exactly one any character,
                 in particular no character must be present; this pattern
                 is equal to a*.*.

BATCH Scripts

  The shell lets to automate simple tasks by writing BATCH scripts.
  These are plain text files with one command is written at one line.
  Any command, both internal and external ones, can be used in BATCH
  scripts, along with redirections, pipes ENVIRONMENT VARIABLE
  substitions a.s.o. The script is performed line-by-line.
  By default, before executing a line, it is displayed onto the screen,
  in order to prevent this, one can turn off this feature with the ECHO
  command or prefix each line with the Ad sign (@), e.g.:
    ECHO This line is displayed
    @ECHO This line is NOT displayed
  Commonly BATCH scripts start with the line:
  in order to turn off to echo each line furtherly in the script
  processing and to suppress to echo the line itself, too.
  Lines starting with a colon (:) label this particular script
  line, in order to be jumped to with the GOTO command, e.g.:
    IF "%1"=="" GOTO usage
    ECHO argument1=%1
    ECHO argument2=%2
    ECHO argument3=%3
    GOTO ende
    ECHO Usage: %0 {arguments}...
  If the script is invoked with no argument, line 2 performs the GOTO
  command and, thus, skips the lines 3 through 7 and proceeds with
  line 8.
  Otherwise the three ECHO commands are invoked and the GOTO command of
  line 6 skips the ECHO'ing of the error message in line 8 and proceeds
  in line 9.
  If the shell hits a line starting with a colon, it is silently
  skipped; not even redirections are evaluated in opposite to the REM
  When the shell reaches the end of the file, the BATCH processing
  terminates. The errorlevel of the last spawned external command
  remains unchanged. Because the shell closes the BATCH script and
  re-opens it each time a command, for both internal and external ones,
  is invoked, one can savely replace floppy disks during a BATCH
  processing, e.g.:
    ECHO Copying disk1
    COPY files\*.* C:%1
    PAUSE Insert disk #2, then hit ENTER
    IF NOT EXIST disk2 GOTO disk2
    ECHO Copying disk2
    COPY files\*.* C:%1
    PAUSE Insert disk #3, then hit ENTER
    IF NOT EXIST disk2 GOTO disk3
    ECHO Copying disk3
    COPY files\*.* C:%1
    ECHO Copying done.
  When a BATCH file is invoked within another BATCH file, the previous
  ones does not proceed once the called one terminates, regardless of
  the nesting level; this is called chaining, one chains to another
  program and does not intend to come back. To have a BATCH script
  behave like an external program in this regard, it must be invoke via
  the CALL command, e.g.:
    ECHO %0 - line 2
    CALL batch2
    ECHO %0 - line 4
    ECHO %0 - line 1

    BATCH1 - line 2
    BATCH2 - line 1  (BATCH2!)
    BATCH1 - line 4

Hidden internal commands

  There are some special internal commands, that are not directly
  visible nor accessable. They are hidden because of two main purposes:
    1. Many of them are of internal nature and should not used by
       the user.
    2. They are extensions to the normal BATCH language and may,
       therefore, clash with a particular installation. To prevent this
       clash those commands are hidden by default and can be made
       directly accessable via the ICMD command.
  Hidden internal commands can be access by prefixing the command
  with ::=. This token usually specifies a label within the BATCH
  language, given the nature of the labels, they may be comments as
  well. Due to the latter variant, most non-FreeCOM shells won't see
  the ::=, hence, ignore those commands.
    For example: C> ::=CANCEL 30
  cancels (terminates) all currently active BATCH files and assigns
  30 to the current errorlevel.

See also:

  environment variable

  Copyright © 2006 the FreeDOS team, adapted to
  FreeDOS help in 2022 by W. Spiegl.

  This file is derived from the FreeDOS Spec Command HOWTO.
  See the file H2Cpying for copying conditions.