rc shell Frequently Asked Questions

rc shell Frequently Asked Questions

Post by Alan Wats » Sat, 07 May 1994 10:53:34



Archive-name: unix-faq/shell/rc
Comp-unix-shell-archive-name: rc-FAQ
Version: 2.16
Last-modified: 08 Apr 1994

This document attempts to answer some Frequently Asked Questions about
rc, a clean, portable, and freely-available shell for UNIX systems (see
1.1 for a very brief overview of rc).  It in no way replaces either the
formal documentation for rc (see 1.2) or the mailing list archives (see
2.2).

This document draws heavily on the collective wisdom of the rc mailing
list; the quality of the correspondence is such that often all that was
required was a summary of one or two messages.  The rc mailing list
archive is recommend reading for newcomers to rc (see 2.2).
Contributors are acknowledged at the end of this document.

Corrections, comments, and suggestions concerning this document are
welcome (see 3.2).

NOTE: Neither the editor nor any of the contributors to this document
are responsible for any consequences resulting from the advice in this
document.

Table of Contents

1.      Implementations of rc
1.1     What is rc?
1.2     Where is rc documented?
1.3     How do I obtain rc?
1.4     Are there any patches to rc?
1.5     Can I compile rc on a ... machine with ... operating system and
        ... compiler?
1.6     Should rc be dynamically or statically linked?
1.7     To whom do I report bugs?
1.8     Does anyone have an implementation of /dev/fd for my UNIX?

2.      The rc mailing list
2.1     How do I subscribe to the rc mailing list?
2.2     How do I obtain the rc mailing list archive?

3.      The rc FAQ list
3.1     How do I obtain the current version of the rc FAQ list?
3.2     To whom do I make suggestions about the rc FAQ list?

4.      Does rc have ... ?
4.1     Does rc have job control?
4.2     Does rc have command-line editing?
4.3     Does rc have a history mechanism?
4.4     Does rc have file name completion?
4.5     Does rc have home directory expansion?
4.6     Does rc check for new mail?
4.7     Does rc have hierarchical lists?
4.8     Does rc have cartesian products?
4.9     Does rc cache the location of executables in the path?
4.10    Does rc have C-style \ escapes?
4.11    Does rc have ... built in?
4.12    Does rc have echo built in?
4.13    Does rc have test built in?
4.14    Does rc have true, false, and `:' built in?

5.      Using rc as a login shell
5.1     How do I make rc my login shell?
5.2     How do I use rc in a heterogeneous cluster?
5.3     What problems occur with rc on Apollos?
5.4     What problems occur with rc on SYSV-derived systems?
5.5     What problems occur with rc on NeXTs?
5.6     How do I keep down the size of my .rcrc file?
5.7     How do I speed up the execution of my .rcrc file?

6.      Interactions between rc and the rest of UNIX
6.1     How do I get each new interactive rc to do ...?
6.2     How do I get rc to do ... in each new xterm?
6.3     How do I get rc to read .rcrc when I use rsh?
6.4     How do I get at or batch to run an rc script?

7.      Programming in rc
7.1     What makes rc such a good scripting language?
7.2     What are the trade-offs between functions and scripts?
7.3     What can I do if I have deficient sed, awk, test, ... ?
7.4     Where do I obtain the GNU tools and other software?
7.5     How do I perform arithmetic in rc?
7.6     How do I find the first occurrence of an element in a list?
7.7     How do I use ranges in array subscripts in rc?
7.8     How do I perform the equivalent of read in rc?

1.      Implementations of rc

1.1     What is rc?

        rc is a shell designed by Tom Duff of Bell Labs to replace the
        venerable Bourne sh in Plan 9.  (Plan 9 is an experimental
        operating and networking system developed at Bell Labs.)  In
        addition to Plan 9, Duff's rc runs on SunOS and v10 UNIX.  The
        design of rc is extremely clean, in that precisely those
        features of Bourne's sh that have caused problems over the years
        (the slightly awkward flow control syntax, the Byzantine quoting
        rules, the failure to export all variables into the environment)
        were reworked, and the temptation to add superfluous features
        was strongly resisted.

        Byron Rakitzis has written a version of rc that is portable
        between many version on UNIX and is largely compatible with
        Duff's design.  Rakitzis' implementation has been successfully
        used on UNIX platforms ranging from PCs running Linux to Cray
        Y-MPs running Unicos.

        Most of this document deals with issues that have arisen from
        use of Rakitzis' rc, and most of the example code will follow
        his minor modifications to rc.  Henceforth, a plain `rc' refers
        to Rakitzis' rather than Duff's rc.

1.2     Where is rc documented?

        rc is documented in its man page, distributed with the source
        (see 1.3).

        The Plan 9 manual pages, including those on Duff's rc, are
        available as research.att.com:/dist/plan9man/manual.ps.Z by
        anonymous FTP.  The full manual is almost 600 pages long, but
        the manual pages for Duff's rc are physical pages 142 to 147
        (logical pages 116 to 121).

        An early version of Duff's rc is described in the paper `rc -- a
        Shell for Plan 9 and UNIX Systems', written for a UKUUG meeting.
        Duff has kindly allowed the paper to be distributed by anonymous
        FTP as viz.tamu.edu:/pub/rc/plan9.ps.  It provides an excellent
        introduction and tutorial for Duff's rc, and describes the
        motivations for the differences between Duff's rc and Bourne's
        sh.

        When reading Duff's paper and the Plan 9 manual pages, bear in
        mind that Rakitzis' implementation of rc differs in detail from
        Duff's (these differences are documented in the rc man page).

        Another valuable source of information on rc is the rc mailing
        list archive (see 2.2).

1.3     How do I obtain rc?

        The sources are available by anonymous FTP in the directory
        viz.tamu.edu:/pub/rc.  Be sure to get the most recent version.
        At the time this document was last updated, the most recent
        version of rc was v1.4 announced on 21 May 1992, available as
        viz.tamu.edu:/pub/rc/rc-1.4.tar.Z.  (A beta release of v1.5 is
        on the horizon.)

        rc is distributed on very liberal terms, although it has not
        been placed in the public domain (see the file COPYRIGHT
        distributed with the sources).

        Duff's rc can be obtained by acquiring either a Plan 9 or a v10
        UNIX system from Bell Labs.  Plan 9 can be licensed by academic
        institutions, which should contact:

            Neera Kuckreja
            RM 2C557
            ATT Bell Laboratories
            Murray Hill
            NJ 07974
            USA
            +1 908 582-3855
            ne...@research.att.com

1.4     Are there any patches to rc?

        Although the current version of rc is remarkably mature, a small
        number of bugs have been found since its release and patches
        have been posted to the rc mailing list (see 2.2).  Other
        patches posted to the mailing list include work-arounds for
        broken compilers or libraries and assorted features.

        An up-to-date set of patches extracted from the mailing list
        archive, together with a script to apply those patches, is
        available as oldp.astro.wisc.edu:/pub/rc/patches.tar.Z by
        anonymous FTP.

1.5     Can I compile rc on a ... machine with ... operating system and
        ... compiler?

        rc is written in portable ANSI C, but it can be compiled by many
        older C compilers after passing the sources through unproto (get
        ftp.win.tue.nl:/pub/unix/unproto4.shar.Z by anonymous FTP).

        If you use gcc, use v1.39 or later.

        rc has been successfully compiled on systems with the following
        combinations of architectures, operating systems, and compilers,
        although this list is probably not exhaustive:

            DEC VAX        Ultrix v3.x            gcc v1
            DEC MIPS       Ultrix v4.[23]         gcc v2
            DEC MIPS       Ultrix v4.[23]         cc          [a]
            DEC AXP        OSF/1 v1.2             cc          [a][b]
            DEC AXP        OSF/1 v1.2             cc -std1    [b]

            Sun SPARC      Solaris v2             gcc v2      [b][c]
            Sun SPARC                             acc         [b][c]
            Sun SPARC      SunOS v4.1.[123]       cc          [a][b][c]
            Sun SPARC      SunOS v4.1.[123]       gcc v2      [b][c]
            Sun SPARC      BSD                                [b][c]
            Sun 68K        SunOS v4.1.1           gcc v2      [b][c]
            Sun 68K        SunOS v3.5             gcc v1      [b][c]

            SGI Iris       Irix v[45].x           gcc v2

            IBM RS/6000    AIX v3.1.5             cc
            IBM RS/6000    AIX v3.2.[013]         cc

            HP PA-RISC     HP-UX v8.0             gcc v2      [d][e]
            HP PA-RISC     HP-UX v9.0             gcc v2      [d]
            HP PA-RISC     OSF/1 v1.0.2           cc

            i386/i486      SCO-ODT SVR3           gcc v2      [d][e]
            i386/i486      SCO-ODT SVR3           cc          [d][e]
            i386           Minix v1.5                         [d]
            i386/i486      NetBSD                 gcc v[12]
            i386/i486      Linux                  gcc v2      [d]
            i386/i486      386BSD r0.0            gcc         [j]
            i386/i486      SCO Xenix SVR2.3x                  [f]

            NeXT           NeXT OS v2.x           gcc v[12]   [g]
            NeXT           NeXT OS v3.0           gcc v2      [g]

            Apollo 68K     DomainOS v10.4 (BSD)   cc          [h]
            Apollo 10000   DomainOS v10.4 (BSD)   cc          [h]

            Sony MIPS      NEWS/OS                cc          [a]
            Sony 68K       NEWS/OS v4.0.1         cc          [a]

            Convex C1      ConvexOS v10.1         cc

            Pyramid 9810   OSx 5.1a (BSD)         cc -Xa      [i]

            Cray Y-MP      Unicos 6.1.7           scc         [e][j]

            Sequent        Dynix v3.2.0 (BSD)     gcc v2

            Apple 68K      A/UX v3.0              cc

            Encore MIPS    Umax v4.3              gcc v2

            Acorn ARM      iX v1.21a              cc

            Motorola 88K   System V R32V3         gcc v2
            Motorola 88K   System V R32V3         cc          [a][k]

        Notes:

            [a] Sources pre-processed with unproto.
            [b] See 1.6.
            [c] Sun's yacc has a memory leak -- use another
                implementation such as GNU bison (see 7.4).
            [d] Requires intervention by hand to get mksignal to work,
                because of of the non-standard signal.h file.
            [e] See 4.13.
            [f] See 5.4.
            [g] See 5.5.
            [h] See 5.3
            [i] This compiler has no stdarg.h, but vararg.h sufficed.
            [j] Minor modifications required to prototypes.
            [k] Greenhills cc, used without any optimization.

1.6     Should rc be dynamically or statically linked?

        As far as one can generalize, statically linked executables fork
        faster than dynamically linked executables.  This suggests that
        shells, which fork a great deal, should probably be statically
        linked (i.e., you should not link them with shared libraries).
        rc makes so little use of libc functions that static linking
        results in only a small increase in the size of the executable
        on disc.

        Versions of UNIX which support shared libraries include SVR4,
        SunOS, Solaris, OSF/1, and Irix.

1.7     To whom do I report bugs?

        Byron Rakitzis (by...@netapp.com) or the rc mailing list
        (r...@hawkwind.utcs.toronto.edu).  Please check the documentation
        before reporting bugs, and check the mailing list archive in
        case the bug is already known.

1.8     Does anyone have an implementation of /dev/fd for my UNIX?

        /dev/fd pseudo-devices allow a more reliable implementation of
        non-linear pipelines (i.e., <{...}  and >{...}) than named
        pipes, but they are not present in all versions of UNIX.

        An implementation of Ultrix v4.2 and later was posted to the
        mailing list in March 1993 (see 2.2).

        An implementation for SunOS v4.1.1 is also available (although
        it may well work with v4.1.2 and v4.1.3) by anonymous FTP as
        ftp.cs.arizona.edu:/devfd.shar.

        An implementation for SunOS v4.1.2 is available by anonymous FTP
        as emx.cc.utexas.edu:/pub/mnt/source/sun/devfd.shar.  (This is
        packaged as a loadable module, so you do not have to rebuild
        your kernel.)

        An implementation for NeXT OS 2.1 is available by anonymous FTP
        as ftp.sys.toronto.edu:/pub/rc/devfd-next.shar.gz.

        You will, of course, need to rebuild your kernel to install
        these pseudo-devices.

2.      The rc mailing list

2.1     How do I subscribe to the rc mailing list?

        Send mail to

            rc-requ...@hawkwind.utcs.toronto.edu.

        Chris Siebenmann maintains the mailing list, and can be reached
        at the above address in the event of problems with the mailing
        list or to unsubscribe.

        The rc mailing list represents hundreds of user-years of
        experience with rc.  Clearly, it is a courtesy to the list and
        to your own advantage to check the mailing list archive before
        posting to the list, to benefit from the experience and mistakes
        of those before you, and to prevent topics being continually
        rehashed.

2.2     How do I obtain the rc mailing list archive?

        The archive can be obtained by anonymous FTP.  At the moment, it
        is split chronologically between the original and current
        maintainers of the mailing list.

        Period              File
        Jun 91 to Dec 91    viz.tamu.edu:/pub/rc/rc-list.1991.Z
        Jan 92 to May 92    viz.tamu.edu:/pub/rc/rc-list
        May 92 to present   ftp.sys.toronto.edu:/pub/rc/rc-list

3.      The rc FAQ list

3.1     How do I obtain the current version of the rc FAQ list?

        The current version of the rc FAQ list is available by anonymous
        FTP as oldp.astro.wisc.edu:/pub/rc/rc-FAQ.  This FAQ is also
        archived automatically and is available by anonymous FTP as
        rtfm.mit.edu:/pub/usenet/comp.unix.shell/rc-FAQ.

3.2     To whom do I make suggestions about the rc FAQ list?

        To its editor, Alan Watson (a...@oldp.astro.wisc.edu), or the rc
        mailing list if you think your suggestion warrants a wider
        audience.

4.      Does rc have ... ?

4.1     Does rc have job control?

        No.  This has been discussed at length in the rc mailing list.

        Now that personal workstations with window systems are common
        place, the case for job control is less compelling.  However, if
        you have to use rc on a traditional terminal, you may wish to
        obtain the screens program from a GNU archive (see 7.4), the sm
        program that was posted to alt.sources in mid-1992 (see 7.4), or
        investigate shell escape mechanisms in vi and shell mode in
        emacs.

4.2     Does rc have command-line editing?

        If you wish.  Hooks are provided for the readline or editline
        libraries which provide command-line editing.  The default is no
        command-line editing.

        The readline library can be obtained from a GNU archive (see
        7.4).

        The editline library is similar to the readline library, but is
        a fraction of the size and offers fewer features.  The sources
        are available as ftp.sys.toronto.edu:/pub/rc/editline.shar by
        anonymous FTP).

        Alternatively, you can use fep, ile, atty, or Rk to implement
        editing on a per-terminal basis.  These have been discussed in
        on the mailing list at one time or another, and modifications to
        ile have been posted there.

4.3     Does rc have a history mechanism?

        Yes.  While, rc does not have a built-in csh-like history
        mechanism, rather it is able to write interactive commands to a
        file (see the history special variable in the man page),
        allowing a history mechanism similar to the v8 `=' command to be
        implemented as an external program (one such implementation is
        supplied with the source of rc).

        In Plan 9, `:' was the history program (the name was probably
        changed from the `=' used in v8 because `=' is a special
        character in rc, and needs quoting if it refers to a command).
        However, this form of history was apparently used so little that
        the ability to write commands to a file was removed from Duff's
        rc.  The history program distributed with the rc source is
        invoked with `-'.

        Both editline and readline have command recall.

4.4     Does rc have file name completion?

        If you wish.  Plain rc does not have file name completion (as it
        obtains its command input with read), but the readline library
        does.

4.5     Does rc have home directory expansion?

        No.  This has been hotly debated on the rc mailing list, and a
        patch to implement this was posted to the rc mailing list in
        November 1992 (see 2.2 and 1.4).

4.6     Does rc check for new mail?

        No.  However, you can make it do so with the following prompt
        function:

            fn prompt { mail -e && echo You have mail. }

        Alternatively, consider using biff or xbiff.

4.7     Does rc have hierarchical lists?

        No.  Each element of an rc list must be a string, and cannot be
        another list.  Whenever a list appears inside another list, it
        is expanded so that the outer list contains its elements.
        Similarly, whenever two lists are adjacent, they are
        concatenated to form a single list (with one exception).

        In rc terms, the arguments to a command form a list, even though
        they may not have explicit `(' and `)' delimiters around them.
        Hierarchical lists do not correspond well to the `char **argv'
        model for arguments to UNIX commands, although simple lists do.

        The single exception to these rules are the arguments to the `~'
        built-in command, which rc interprets as two lists -- a subject
        list and a pattern list.  Normal expansion and concatenation is
        performed on the two lists separately.  A consequence of this is
        that the following command:

                if ( ~ $1 () '' ) ...

        does not check if $1 is either the null list or the null string,
        as normal list concatenation occurs on the pattern list leading
        to a pattern list with one element -- the null string.

4.8     Does rc have cartesian products?

        No.  If you really need them, they can be obtained in the
        following manner:

            ; x = (a b c)
            ; y = (1 2 3)
            ; product = ()
            ; for ( elem in $x ) product = ( $product $elem^$y )
            ; echo $product
            a1 a2 a3 b1 b2 b3 c1 c2 c3

        rc's concatenation operator works in a distributive manner if
        one of the lists has only a single element and in a pairwise
        manner if both lists have the same number of elements:

            ; echo a^( x y z )
            ax ay az
            ; echo ( a b c )^( x y z )
            ax by cz

4.9     Does rc cache the location of executables in the path?

        No.  This has been discussed on the mailing list.  As far as one
        can generalize, searching the path is not a performance
        bottleneck on most modern UNIX systems.

4.10    Does rc have C-style \ escapes?

        No.  One of the most attractive features of rc is its single
        quoting rule.

        If you need to embed control characters in your strings,
        investigate tr and printf (see 7.3 if your system lacks the
        latter).

4.11    Does rc have ... built in?

        It is not by accident that rc has fewer built-in commands than
        most shells other than the original Bourne shell.  The
        underlying philosophy, eloquently summarized by Boyd Roberts, is
        that a shell is there to run other programs, not to have other
        programs built into it.  By having a well-defined role, rc has
        remained small and simple, properties that lead to better
        performance, fewer bugs, and a more thorough understanding of
        the shell by its users.

        Most of the rc built-in commands perform functions that are
        essential for a shell and cannot be performed by external
        programs (e.g., cd, eval, limit, umask, wait).

        rc does include hooks for users to add new built-in commands,
        and these have been used to add test, access, kill, read, and
        even sync and haltsys commands (see the mailing list and the
        examples file for more details).  Such additions are unsupported
        and potentially unportable.

4.12    Does rc have echo built in?

        If you wish.  While most of rc's built-in commands could not
        easily be replaced by external commands, echo is the one
        exception.  The justification is that echo is used so much that
        providing it as a built-in command results in a significant gain
        in performance, although this opinion is by no means universally
        held.  By default, echo is built in.

        It has also been noted that rc's echo is more portable that
        /bin/echo, as the v7 and SYSV implementations of echo differ in
        the method that must be used to suppress the final newline.
        (However, rc's echo differs from both, as it uses `--' to
        indicate that all other arguments are to be echoed literally.)

        People who dislike the notion of a built-in echo command have
        the option of excluding it from the compilation.  Alternatively,
        the built-in echo can be subverted with the following function,
        although this will not defeat an explicit call to `builtin
        echo'.

            fn echo { /bin/echo $* }

        See also 7.3.

4.13    Does rc have test built in?

        No.  You can use the built-in `~' command to match strings, and
        fall back on /bin/expr or /bin/test for numeric comparisons and
        /bin/test for querying the file system.

        If you are using rc on a system that lacks a stand-alone
        /bin/test and /bin/[ and has test built into sh, the following
        serves as an interim replacement:

            #! /bin/sh
            `basename $0` ${1+"$@"}
            # end-of-file

        This applies to Unicos and HP-UX v8.  Note that trip.rc, the
        script provided with the rc sources to check basic functionality
        after compilation, will fail unless a suitable test command
        exists in the path.

        SCO UNIX implements /bin/test in precisely this way.  You can
        almost certainly get better performance by using the GNU
        implementation of /bin/test (see 7.4), although this is
        obviously more important on an i386 machine than on a Cray.

        It is possible to get at the file system obliquely through the
        globbing mechanism and the builtin cd command, and in some
        circumstances these can be of use (see, for example, 5.7).  Such
        speed hacks need to be carefully justified.

        See also 7.3.

4.14    Does rc have true, false, and `:' built in?

        No.  The following functions are just as fast:

            fn true  { return 0 }
            fn false { return 1 }
            fn : { return 0 }

        Note that an indefinite loop in rc does not require the use of
        an explicit `true' command:

            while () {
                ...
            }

5.      Using rc as a login shell

5.1     How do I make rc my login shell?

        On most systems, you or the system administrator will have to
        add a line to /etc/shells containing the full path to rc.  This
        will need to be done on each machine on which you wish to use
        rc.

        Next, use the chsh command to specify your new preference.  If
        your machine is running Yellow Pages (YP), you will have to do
        this on the YP server, otherwise you will have to do this on
        each machine individually.  (To determine if your machine is
        using YP, type `ypwhich -m passwd' that will either print the
        name of your YP server or give an error if your machine is not
        running YP.)  Often, you will also need to change your password
        (using the yppasswd command) on the YP server to propagate the
        new /etc/passwd entry contain your selected shell; changing your
        password to itself will normally suffice.

        An alternative method is to invoke rc from your .profile file or
        its equivalent, but this has so many drawbacks that I cannot
        recommend it.

5.2     How do I use rc in a heterogeneous cluster?

        There are two issues: finding the correct executable when you
        login, and getting the correct path in scripts run using the #!
        kernel hack.

        If you have root privileges on every machine on the cluster, by
        far the simplest solution is the place a copy of rc in /bin and
        add /bin/rc to /etc/shells on each machine.  Use `#! /bin/rc' in
        shell scripts.

        Otherwise, use a sh wrapper around rc.  Create a shell script
        called, say, rc-login, and make this your login shell.  The
        purpose of this script is to set SHELL and PATH to point
        appropriately according to the machine, and then to exec rc.

        The following script is suitable for adaptation:

            #! /bin/sh

            # determine OS
            TMP=/tmp/$$
            ( exec >$TMP ; echo OS=`uname -m` )
            . $TMP
            rm -f $TMP

            # determine location of rc executable
            case $OS in
            RISC)
               PATH=$HOME/bin/mips:$PATH
               SHELL=$HOME/bin/mips/rc
               ;;
            sun4c)
               PATH=$HOME/bin/sparc:$PATH
               SHELL=$HOME/bin/sparc/rc
               ;;
            alpha)
               PATH=$HOME/bin/alpha:$PATH
               SHELL=$HOME/bin/alpha/rc
               ;;
            *)
               echo 1>&2 "WARNING: unknown architecture"
               SHELL=/bin/sh
               ;;
            esac

            # determine if this is a login shell
            case $SHELL in
            */rc)
              case $0 in
              -*)
                 FLAG=-l
                 ;;
              esac
              ;;
            esac

            export PATH
            export SHELL

            exec $SHELL $FLAG ${1+"$@"}

            # end-of-file

        The dance to capture the output of uname is necessary because of
        a bug whereby redirection does not work in the original v7 sh if
        stdout is closed.  (This bug lives on in the Ultrix /bin/sh.)

        The easiest way to manage shell scripts is to keep a master copy
        in a central directory, say $home/bin/share, and have slave
        scripts in machine-specific directories which source the master
        script.  For example, for a command called `foo' create the
        following in each machine-specific directory:

            #! /full/path/to/machine/specific/rc
            . $home/bin/share/foo $*
            # end-of-file

        See 5.3 if your cluster contains Apollos.

5.3     What problems occur with rc on Apollos?

        Apollos require the login shell to be a binary, so you cannot
        use a sh script wrapper around rc.

5.4     What problems occur with rc on SYSV-derived systems?

        Many SYSV-derived sh and ksh are `restricted' if the final
        component of SHELL contains the letter `r'.  A restricted shell
        will not allow you to cd, perform output redirection, set PATH,
        or specify absolute paths to executables, meaning that many
        shell scripts written in sh or ksh will fail.  This is also
        applies to BSD-derived systems that also ship with a SYSV-
        derived shell (for example, /bin/sh5 on Ultrix).

        One solution is to link rc to a file that does not contain the
        letter `r', and use that file as your SHELL.  You can achieve
        this automatically by placing the following commands in your
        .rcrc:

            rcdir = `{ dirname $SHELL } {
                test -f $rcdir/RC || ln $rcdir/rc $rcdir/RC
                SHELL = $rcdir/RC
            }

        Make sure when you install a new version of rc that you remove
        the previous RC.

        This behaviour is also a potential problem if you use a sh
        wrapper around rc as your login shell (see 5.2 and 6.3).  Make
        sure that the name of the wrapper does not contain the letter
        `r'.

5.5     What problems occur with rc on NeXTs?

        The NeXT window system assumes that all shells are `job control'
        shells and will place themselves in a new process group.
        Contrary to this assumption, rc is not a job control shell and
        so does not do this.  The result is that a signal to one (such
        as an interrupt) will be sent to all.

        The `newpgrp' builtin-in command was provided to deal with this
        problem.  You can use it to ensure that each new interactive rc
        belongs to a new process group by using a prompt function to run
        code in each new interactive instance of rc (see 6.1 and 6.2).
        Other more elegant solutions (such as a wrapper around rc) are
        precluded by the uncooperative and inflexible nature of the NeXT
        window system.

        See also 6.1 and 6.2.

5.6     How do I keep down the size of my .rcrc file?

        Given the nature of rc, the natural place to define functions is
        in the .rcrc file.  However, as the number of functions grows,
        it becomes more awkward to maintain them all in a single file.
        One solution is to break out the functions into separate files
        and automatically load them within .rcrc:

            fnlib = $home/fnlib
            fn load { ~ $#* 1 && builtin . $fnlib/$1 }
            builtin cd $fnlib
            name = () for ( name in * ) load $name
            builtin cd

        See also 5.7.

5.7     How do I speed up the execution of my .rcrc file?

        If you have lots of functions, you may wish to use lazy function
        loading:

            fnlib = $home/fnlib
            fn load { ~ $#* 1 && builtin . $fnlib/$1 }
            builtin cd $fnlib
            name = () for ( name in * ) fn $name { load $0 && $0 $* }
            builtin cd

        Often, though, a more substantial amount of time is spent
        testing directories before they are added to a path: rc does not
        have a built-in test command, so each test implies a fork and an
        exec.  In some circumstances, you can replace the test with code
        similar to the following:

            path = ()
            dir = () for ( dir in ... )
                builtin cd $dir >[1=] >[2=] && path = ( $path $dir )
            builtin cd

        See also 5.6 and 4.13.

6.      Interactions between rc and the rest of UNIX

6.1     How do I get each new interactive rc to do ...?

        Rather than set the `-l' flag and place the code in .rcrc, you
        can use a prompt function like the following:

            fn prompt {
                if ( ! ~ $ipid $pid ) {
                    ipid=$pid
                    ...
                }
            }

        This will run ... at the start of each new interactive instance
        of rc, but not in non-interactive instances of rc.

        See also 6.2 and 5.5.

6.2     How do I get rc to do ... in each new xterm?

        Since xterm puts WINDOWID into the environment, you can use a
        prompt function like the following:

            fn prompt {
                if ( ! ~ $wid $WINDOWID && ~ $TERM xterm* ) {
                    wid=$WINDOWID
                    ...
                }
            }

        See also 6.1.

6.3     How do I get rc to read .rcrc when I use rsh?

        rshd does not place a `-' in the first character of argv[0] when
        it execs a shell, so .rcrc is not run and this can cause
        problems.

        One solution is to have a false login shell called login-shell
        that explicitly supplies a -l flag to rc, i.e., login-shell
        contains:

            #! /path/to/rc
            exec /path/to/rc -l $*
            # end-of-file

        See also 5.2.

6.4     How do I get at or batch to run an rc script?

        at and batch only accept sh or csh scripts, so you need to
        package your rc script in a sh wrapper.  The following two
        functions accomplish this.  Note that they do not completely
        emulate the standard commands, as they will not accept a file
        name on the command line, but rather only read commands from
        stdin.  A simple further enhancement might be to mail the stdout
        and stderr of the job back to the user.

            fn at {
                if ( ~ $#* 0 ) {
                    echo >[2=1] \
                        'usage: at [ time [day] | -r job ... | -l [job ...] ]'
                    return 1
                }
                switch ( $1 ) {
                case -r
                    builtin at $*
                case -l
                    builtin at $* >[2=1]
                case *
                    {
                        echo 'SHELL='^$SHELL
                        echo 'export SHELL'
                        echo 'sed ''s/^-//'' <<''+'' | '^$SHELL
                        sed 's/^/-/'
                        echo '+'
                    } | SHELL = /bin/sh builtin at $*
                }
            }

            fn batch {
                if ( ! ~ $#* 0 ) {
                    echo >[1=2] 'usage: batch'
                    return 1
                }
                {
                    echo 'SHELL='^$SHELL
                    echo 'export SHELL'
                    echo 'sed ''s/^-//'' <<''+'' | '^$SHELL
                    sed 's/^/-/'
                    echo '+'
                } | SHELL = /bin/sh builtin batch
            }

7.      Programming in rc

7.1     What makes rc such a good scripting language?

        Brian Kernighan and Rob Pike wrote of the original v7 Bourne
        shell:

            The UNIX shell isn't typical of command interpreters:
            although it lets you run commands in the usual way, because
            it is a programming language it can accomplish much more.

        Their comments apply equally to rc, perhaps more so.

        The two aspects of rc that contribute most to its utility as a
        scripting language are its simple quoting rule coupled with the
        absence of rescanning and the fact that the language is small,
        simple, and well defined.

        The former allows one to write scripts with complete control
        over the lexical aspects of the process: when and if globbing,
        pattern matching, and list formation take place.  Tom Duff wrote
        that rc is `not a macro processor'.  The lack or rescanning,
        together with improvements to backquote substitutions, allowed
        him to collapse Bourne's four types of quoting down to one,
        eliminating all of their complex interactions.

        The latter means that the whole language can be readily
        understood by the user.  Its grammar is well defined, in sharp
        contrast to sh.  (Indeed, a skeleton yacc grammar is included as
        an appendix to rc's manual page.)  The language is small and
        well defined; there are, to quote John Mackin, no `dark
        corners'.  Although the language is smaller and simpler, it
        retains all the power and flexibility of sh, and improves and
        extends on it in several areas.

7.2     What are the trade-offs between functions and scripts?

        Functions are run in the process that invokes them, so they can
        make persistent changes to the environment of the interpreter.
        They are essential for some actions (reading a variable, for
        example).

        Functions make excellent wrappers around standard commands, as
        one can use the `builtin' command to invoke the standard command
        from within the function.  Invoking a standard command from
        within a script of the same name involves either hard-wiring the
        name into the script or using a more elaborate method, such as
        the `pathos' script posted to the mailing list in June 1992.

        There is less overhead in invoking a function, as the kernel
        need not exec rc once again.

        However, functions take up space in the environment.  There is
        often a hard limit on the size of the environment.  Although on
        some systems this limit is very large, on others it is not.  In
        addition, very large environments can degrade the performance of
        fork.

        Functions cannot be exec-ed directly by a C program, but must be
        invoked through rc, although it is probably a good idea to do
        this anyway, as responsibility for globbing and variable
        substitution properly resides with the shell.

7.3     What can I do if I have deficient sed, awk, test, ... ?

        Shell programming on UNIX rests on two foundations: the shell
        itself and the toolkit of programs provided in /bin and
        /usr/bin.  Unfortunately, many vendors provide out-of-date tools
        and there are many incompatibilities between BSD- and SYSV-
        derived systems, so writing portable scripts is a problem.  This
        problem can be especially acute in heterogeneous clusters.

        There are two approaches to this problem: one is to use only
        those tools and options that were present in v7, and the other
        is to use only those tools and options that are present in
        POSIX.2, and to replace the vendor-supplied tools with those
        from GNU if the former do not conform (see 7.4).

7.4     Where do I obtain the GNU tools and other software?

        Get prep.ai.mit.edu:/pub/gnu/GNUinfo/FTP by anonymous FTP for a
        list of sites that archive GNU software.

        The article `How to find sources' lists sites that archive
        software posted to Usenet source newgroups.  This article is
        regularly cross-posted to comp.sources.wanted, comp.answers, and
        news.answers, and is also available via anonymous FTP as
        rtfm.mit.edu:/pub/usenet/comp.answers/finding-sources.

7.5     How do I perform arithmetic in rc?

        You can use /bin/expr, /bin/awk, or unary arithmetic with lists
        (see 7.6 for an example).

7.6     How do I find the first occurrence of an element in a list?

        Use an `index' function, as in:

            ; x = ( a b a b )
            ; index b $x
            2

        This implementation of index uses a list to count up to the
        index of the element:

            fn index {
                switch ( $#* ) {
                case 0 1
                    echo >[1=2] 'usage: index pattern list ...'
                    return 1
                case *
                    elem = $1 prefix = () {
                        shift
                        if ( ! ~ $elem $* ) {
                            echo 0
                            return 1
                        }
                        while ( prefix = ( $prefix $1 ) && ! ~ $elem $1 )
                            shift
                        echo $#prefix
                        return 0
                    }
                }
            }

7.7     How do I use ranges in array subscripts in rc?

        Use a `seq' function, as in:

            ; x = ( a b c d e f g h )
            ; echo $x( `{seq 2 5} )
            b c d e

        There are many implementations of seq, most of which use awk,
        like this one:

            fn seq {
                switch ( $#* ) {
                case 1
                    * = ( 1 $1 )
                case 2
                case *
                    echo >[1=2] usage: seq [start] stop
                    return 1
                }
                echo $1 $2 | awk '{
                    if ( $1 <= $2 )
                        for ( i = $1; i <= $2; i++ ) print i;
                    else
                        for ( i = $1; i >= $2; i-- ) print i;
                }'
            }

7.8     How do I perform the equivalent of read in rc?

        Use a `read' function, to read a single line from stdin, remove
        the trailing newline, and assign it to a variable:

            ; read foo
            Do the funky gibbon!
            ; echo $foo
            Do the funky gibbon!
            ; echo $#foo
            1

        The following implementation of read returns 1 if EOF is seen,
        like the read in the v7 sh:

            fn read {
                if ( ! ~ $#* 1 ) {
                    echo >[1=2] 'usage: read variable'
                    return 1
                }
                if ( ~ $1 '*' () *'='* ) {
                    echo >[1=2] 'read: bad variable name'
                    return 1
                }
                lines = () nl = () {
                    ifs = () { nl = `{ echo } }
                    ifs = $nl { lines = `{ line ; echo $status } }
                    if ( ~ $#lines 2 ) {
                        $1 = $lines(1)
                        return 0
                    } else {
                        $1 = ''
                        return $lines(1)
                    }
                }
            }

        If your UNIX does not have a line command, the following is an
        straightforward substitute.

            #include <unistd.h>
            int main ()
            {
                unsigned char c;

                while (read (0, &c, 1) == 1 && c != '\n')
                    write (1, &c, 1);
                write (1, "\n", 1);
                return c != '\n';
            }

        Attempts to use standard UNIX utilities (e.g., "sed 1q", "head
        -1", and "awk '{ print; exit; }'") to replace the line command
        fail because line does not buffer its input.  This was discussed
        extensively in the mailing list in September 1991 (see 2.2).

        Alternatively, a patch to add a builtin read was posted to the
        mailing list in September 1993 (see 1.4).

Acknowledgements

Many people have contributed ideas, code, and information to this
document, either directly or indirectly, through the rc mailing list and
its influence on my understanding of rc.

People who have directly contributed to this document include Vincent
Broman, Raymond Chen, Tom Culliton, Stefan Dalibor, Jim Davis, Matt Day,
Matthew Farwell, David Fiander, Tim Goodwin, Paul Haahr, Charles Hannum,
David Hogan, Noel Hunt, John LoVerso, Hamish Macdonald, John Mackin,
Byron Rakitzis, Boyd Roberts, Rich Salz, Chris Siebenmann, Emin Gun
Sirer, Icarus Sparry, Gerry Tomlinson, Malte Uhl, Christopher Vance, and
Victor Zandy.  My apologies to anyone I have missed.

As a user of rc, I am indebted to Tom Duff for its design, to Steve
Bourne for its foundation, and to Byron Rakitzis for its implementation.

Alan Watson (a...@oldp.astro.wisc.edu)