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NAME
rc - shell
SYNOPSIS
rc [-deiIlnopsvx] [-c command] [arguments]
DESCRIPTION
rc is a command interpreter and programming language similar to sh(1).
It is based on the AT&T Plan 9 shell of the same name. The shell
offers a C-like syntax (much more so than the C shell), and a powerful
mechanism for manipulating variables. It is reasonably small and
reasonably fast, especially when compared to contemporary shells. Its
use is intended to be interactive, but the language lends itself well
to scripts.
OPTIONS
-c If -c is present, commands are executed from the immediately
following argument. Any further arguments to rc are placed in
$*. Thus:
rc -c 'echo $*' 1 2 3
prints out
1 2 3
-d This flag causes rc not to ignore SIGQUIT or SIGTERM. Thus rc
can be made to dump core if sent SIGQUIT. This flag is only
useful for debugging rc.
-e If the -e flag is present, then rc will exit if the exit status
of a command is false (nonzero). rc will not exit, however, if a
conditional fails, e.g., an if() command.
-i If the -i flag is present or if the input to rc is from a
terminal (as determined by isatty(3)) then rc will be in
interactive mode. That is, a prompt (from $prompt(1)) will be
printed before an input line is taken, and rc will ignore SIGINT.
-I If the -I flag is present, or if the input to rc is not from a
terminal, then rc will not be in interactive mode. No prompts
will be printed, and SIGINT will cause rc to exit.
-l If the -l flag is present, or if rc's argv[0][0] is a dash (-),
then rc will behave as a login shell. That is, it will run
commands from $home/.rcrc, if this file exists, before reading
any other input.
-n This flag causes rc to read its input and parse it, but not to
execute any commands. This is useful for syntax checking on
scripts. If used in combination with the -x flag, rc will print
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each command as it is parsed in a form similar to the one used
for exporting functions into the environment.
-o This flag prevents the usual practice of trying to open /dev/null
on file descriptors 0, 1, and 2, if any of those descriptors are
inherited closed.
-p This flag prevents rc from initializing shell functions from the
environment. This allows rc to run in a protected mode, whereby
it becomes more difficult for an rc script to be subverted by
placing false commands in the environment. (Note that the
presence of this flag does not mean that it is safe to run setuid
rc scripts; the usual caveats about the setuid bit still apply.)
-s This flag causes rc to read from standard input. Any arguments
are placed in $*.
-v This flag causes rc to echo its input to standard error as it is
read.
-x This flag causes rc to print every command on standard error
before it is executed. It can be useful for debugging rc
scripts.
COMMANDS
A simple command is a sequence of words, separated by white space
(space and tab) characters that ends with a newline, semicolon (;), or
ampersand (&). The first word of a command is the name of that
command. If the name begins with /, ./, or ../, then the name is used
as an absolute path name referring to an executable file. Otherwise,
the name of the command is looked up in a table of shell functions,
builtin commands, or as a file in the directories named by $path.
Background Tasks
A command ending with & is run in the background; that is, the shell
returns immediately rather than waiting for the command to complete.
Background commands have /dev/null connected to their standard input
unless an explicit redirection for standard input is used.
Subshells
A command prefixed with an at-sign (@) is executed in a subshell.
This insulates the parent shell from the effects of state changing
operations such as a cd or a variable assignment. For example:
@ {cd ..; make}
will run make(1) in the parent directory (..), but leaves the shell
running in the current directory.
Line continuation
A long logical line may be continued over several physical lines by
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terminating each line (except the last) with a backslash (\). The
backslash-newline sequence is treated as a space. A backslash is not
otherwise special to rc. (In addition, inside quotes a backslash
loses its special meaning even when it is followed by a newline.)
Quoting
rc interprets several characters specially; special characters
automatically terminate words. The following characters are special:
# ; & | ^ $ = ` ' { } ( ) < >
The single quote (') prevents special treatment of any character other
than itself. All characters, including control characters, newlines,
and backslashes between two quote characters are treated as an
uninterpreted string. A quote character itself may be quoted by
placing two quotes in a row. The minimal sequence needed to enter the
quote character is ''''. The empty string is represented by ''.
Thus:
echo 'What''s the plan, Stan?'
prints out
What's the plan, Stan?
The number sign (#) begins a comment in rc. All characters up to but
not including the next newline are ignored. Note that backslash
continuation does not work inside a comment, i.e., the backslash is
ignored along with everything else.
Grouping
Zero or more commands may be grouped within braces (``{'' and ``}''),
and are then treated as one command. Braces do not otherwise define
scope; they are used only for command grouping. In particular, be
wary of the command:
for (i) {
command
} | command
Since pipe binds tighter than for, this command does not perform what
the user expects it to. Instead, enclose the whole for statement in
braces:
{for (i) command} | command
Fortunately, rc's grammar is simple enough that a (confident) user can
understand it by examining the skeletal yacc(1) grammar at the end of
this man page (see the section entitled GRAMMAR).
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Input and output
The standard output may be redirected to a file with
command > file
and the standard input may be taken from a file with
command < file
Redirections can appear anywhere in the line: the word following the
redirection symbol is the filename and must be quoted if it contains
spaces or other special characters. These are all equivalent.
echo 1 2 3 > foo
> foo echo 1 2 3
echo 1 2 > foo 3
File descriptors other than 0 and 1 may be specified also. For
example, to redirect standard error to a file, use:
command >[2] file
In order to duplicate a file descriptor, use >[n=m]. Thus to redirect
both standard output and standard error to the same file, use
command > file >[2=1]
As in sh, redirections are processed from left to right. Thus this
sequence
command >[2=1] > file
is usually a mistake. It first duplicates standard error to standard
output; then redirects standard output to a file, leaving standard
error wherever standard output originally was.
To close a file descriptor that may be open, use >[n=]. For example,
to close file descriptor 7:
command >[7=]
Note that no spaces may appear in these constructs:
command > [2] file
would send the output of the command to a file named [2], with the
intended filename appearing in the command's argument list.
In order to place the output of a command at the end of an already
existing file, use:
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command >> file
If the file does not exist, then it is created.
``Here documents'' are supported as in sh with the use of
command << 'eof-marker'
Subsequent lines form the standard input of the command, till a line
containing just the marker, in this case eof-marker, is encountered.
If the end-of-file marker is enclosed in quotes, then no variable
substitution occurs inside the here document. Otherwise, every
variable is substituted by its space-separated-list value (see Flat
Lists, below), and if a ^ character follows a variable name, it is
deleted. This allows the unambiguous use of variables adjacent to
text, as in
$variable^follow
To include a literal $ in a here document when an unquoted end-of-file
marker is being used, enter it as $$.
Additionally, rc supports ``here strings'', which are like here
documents, except that input is taken directly from a string on the
command line. Their use is illustrated here:
cat <<< 'this is a here string' | wc
(This feature enables rc to export functions using here documents into
the environment; the author does not expect users to find this feature
useful.)
Pipes
Two or more commands may be combined in a pipeline by placing the
vertical bar (|) between them. The standard output (file descriptor
1) of the command on the left is tied to the standard input (file
descriptor 0) of the command on the right. The notation |[n=m]
indicates that file descriptor n of the left process is connected to
file descriptor m of the right process. |[n] is a shorthand for
|[n=0]. As an example, to pipe the standard error of a command to
wc(1), use:
command |[2] wc
As with file redirections, no spaces may occur in the construct
specifying numbered file descriptors.
The exit status of a pipeline is considered true if and only if every
command in the pipeline exits true.
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Commands as Arguments
Some commands, like cmp(1) or diff(1), take their arguments on the
command line, and do not read input from standard input. It is
convenient sometimes to build nonlinear pipelines so that a command
like cmp can read the output of two other commands at once. rc does
it like this:
cmp <{command} <{command}
compares the output of the two commands in braces. Note: since this
form of redirection is implemented with some kind of pipe, and since
one cannot lseek(2) on a pipe, commands that use lseek(2) will hang.
For example, some versions of diff(1) use lseek(2) on their inputs.
Data can be sent down a pipe to several commands using tee(1) and the
output version of this notation:
echo hi there | tee >{sed 's/^/p1 /'} >{sed 's/^/p2 /'}
CONTROL STRUCTURES
The following may be used for control flow in rc:
If-Else Statements
if (test) {
cmd
} else cmd
The test is executed, and if its return status is zero, the first
command is executed, otherwise the second is. Braces are not
mandatory around the commands. However, an else statement is
valid only if it follows a close-brace on the same line.
Otherwise, the if is taken to be a simple-if:
if (test)
command
While and For Loops
while (test) cmd
rc executes the test and performs the command as long as the test
is true.
for (var in list) cmd
rc sets var to each element of list (which may contain variables
and backquote substitutions) and runs cmd. If ``in list'' is
omitted, then rc will set var to each element of $*. For
example:
for (i in `{ls -F | grep '\*$' | sed 's/\*$//'}) { commands }
will set $i to the name of each file in the current directory
that is executable.
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Switch
switch (list) { case ... }
rc looks inside the braces after a switch for statements
beginning with the word case. If any of the patterns following
case match the list supplied to switch, then the commands up
until the next case statement are executed. The metacharacters
*, [ or ? should not be quoted; matching is performed only
against the strings in list, not against file names. (Matching
for case statements is the same as for the ~ command.)
Logical Operators
There are a number of operators in rc which depend on the exit status
of a command.
command && command
executes the first command and then executes the second command if and
only if the first command exits with a zero exit status (``true'' in
Unix).
command || command
executes the first command and then executes the second command if and
only if the first command exits with a nonzero exit status (``false''
in Unix).
! command
negates the exit status of a command.
PATTERN MATCHING
There are two forms of pattern matching in rc. One is traditional
shell globbing. This occurs in matching for file names in argument
lists:
command argument argument ...
When the characters *, [ or ? occur in an argument or command, rc
looks at the argument as a pattern for matching against files.
(Contrary to the behavior other shells exhibit, rc will only perform
pattern matching if a metacharacter occurs unquoted and literally in
the input. Thus,
foo='*'
echo $foo
will always echo just a star. In order for non-literal metacharacters
to be expanded, an eval statement must be used in order to rescan the
input.) Pattern matching occurs according to the following rules: a *
matches any number (including zero) of characters. A ? matches any
single character, and a [ followed by a number of characters followed
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by a ] matches a single character in that class. The rules for
character class matching are the same as those for ed(1), with the
exception that character class negation is achieved with the tilde
(~), not the caret (^), since the caret already means something else
in rc.
rc also matches patterns against strings with the ~ command:
~ subject pattern pattern ...
~ sets $status to zero if and only if a supplied pattern matches any
single element of the subject list. Thus
~ foo f*
sets status to zero, while
~ (bar baz) f*
sets status to one. The null list is matched by the null list, so
~ $foo ()
checks to see whether $foo is empty or not. This may also be achieved
by the test
~ $#foo 0
Note that inside a ~ command rc does not match patterns against file
names, so it is not necessary to quote the characters *, [ and ?.
However, rc does expand the subject against filenames if it contains
metacharacters. Thus, the command
~ * ?
returns true if any of the files in the current directory have a
single-character name. If the ~ command is given a list as its first
argument, then a successful match against any of the elements of that
list will cause ~ to return true. For example:
~ (foo goo zoo) z*
is true.
LISTS AND VARIABLES
The primary data structure in rc is the list, which is a sequence of
words. Parentheses are used to group lists. The empty list is
represented by (). Lists have no hierarchical structure; a list
inside another list is expanded so the outer list contains all the
elements of the inner list. Thus, the following are all equivalent
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one two three
(one two three)
((one) () ((two three)))
Note that the null string, '', and the null list, (), are two very
different things. Assigning the null string to a variable is a valid
operation, but it does not remove its definition.
null = '' empty = () echo $#null $#empty
produces the output
1 0
List Concatenation
Two lists may be joined by the concatenation operator (^).
Concatenation works according to the following rules: if the two lists
have the same number of elements, then concatenation is pairwise:
echo (a- b- c-)^(1 2 3)
produces the output
a-1 b-2 c-3
Otherwise, at least one of the lists must have a single element, and
then the concatenation is distributive:
cc -^(O g c) (malloc alloca)^.c
has the effect of performing the command
cc -O -g -c malloc.c alloca.c
A single word is a list of length one, so
echo foo^bar
produces the output
foobar
Free Carets
rc inserts carets (concatenation operators) for free in certain
situations, in order to save some typing on the user's behalf. For
example, the above example could also be typed in as:
opts=(O g c) files=(malloc alloca) cc -$opts $files.c
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rc takes care to insert a free-caret between the ``-'' and $opts, as
well as between $files and .c. The rule for free carets is as
follows: if a word or keyword is immediately followed by another
word, keyword, dollar-sign or backquote, then rc inserts a caret
between them.
Variables
A list may be assigned to a variable, using the notation:
var = list
The special variable * may also be assigned to using this notation; rc
has no set builtin.
Any non-empty sequence of characters, except a sequence including only
digits, may be used as a variable name. Any character except = may be
used, but special characters must be quoted. All user-defined
variables are exported into the environment.
The value of a variable is referenced with the dollar ($) operator:
$var
Any variable which has not been assigned a value returns the null
list, (), when referenced. Multiple references are allowed:
a = foo
b = a
echo $ $ b
prints
foo
A variable's definition may also be removed by assigning the null list
to a variable:
var=()
For ``free careting'' to work correctly, rc must make certain
assumptions about what characters may appear in a variable name. rc
assumes that a variable name consists only of alphanumeric characters,
underscore (_) and star (*). To reference a variable with other
characters in its name, quote the variable name. Thus:
echo $'we$Ird:Variab!le'
Local Variables
Any number of variable assignments may be made local to a single
command by typing:
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a=foo b=bar ... command
The command may be a compound command, so for example:
path=. ifs=() {
...
}
sets path to . and removes ifs for the duration of one long compound
command.
Variable Subscripts
Variables may be subscripted with the notation
$var(n)
where n is a list of integers (origin 1). The opening parenthesis
must immediately follow the variable name. The list of subscripts
need not be in order or even unique. Thus,
a=(one two three)
echo $a(3 3 3)
prints
three three three
If n references a nonexistent element, then $var(n) returns the null
list. The notation $n, where n is an integer, is a shorthand for
$*(n). Thus, rc's arguments may be referred to as $1, $2, and so on.
Note also that the list of subscripts may be given by any of rc's list
operations:
$var(`{awk 'BEGIN{for(i=1;i<=10;i++)print i;exit; }'})
returns the first 10 elements of $var.
To count the number of elements in a variable, use
$#var
This returns a single-element list, with the number of elements in
$var.
Flat Lists
In order to create a single-element list from a multi-element list,
with the components space-separated, use the dollar-caret ($^)
operator:
$^var
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This is useful when the normal list concatenation rules need to be
bypassed. For example, to append a single period at the end of $path,
use:
echo $^path.
Backquote Substitution
A list may be formed from the output of a command by using backquote
substitution:
`{ command }
returns a list formed from the standard output of the command in
braces. $ifs is used to split the output into list elements. By
default, $ifs has the value space-tab-newline. The braces may be
omitted if the command is a single word. Thus `ls may be used instead
of `{ls}. This last feature is useful when defining functions that
expand to useful argument lists. A frequent use is:
fn src { echo *.[chy] }
followed by
wc `src
(This will print out a word-count of all C source files in the current
directory.)
In order to override the value of $ifs for a single backquote
substitution, use:
`` (ifs-list) { command }
$ifs will be temporarily ignored and the command's output will be
split as specified by the list following the double backquote. For
example:
`` ($nl :) {cat /etc/passwd}
splits up /etc/passwd into fields, assuming that $nl contains a
newline as its value.
SPECIAL VARIABLES
Several variables are known to rc and are treated specially. In the
following list, ``default'' indicates that rc gives the variable a
default value on startup; ``no-export'' indicates that the variable is
never exported; and ``read-only'' indicates that an attempt to set the
variable will silently have no effect.
Also, ``alias'' means that the variable is aliased to the same name in
capitals. For example, an assignment to $cdpath causes an automatic
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assignment to $CDPATH, and vice-versa. If $CDPATH is set when rc is
started, its value is imported into $cdpath. $cdpath and $path are rc
lists; $CDPATH and $PATH are colon-separated lists. Only the names
spelt in capitals are exported into the environment.
* (no-export)
The argument list of rc. $1, $2, etc. are the same as $*(1),
$*(2), etc.
0 (default no-export)
The variable $0 holds the value of argv[0] with which rc was
invoked. Additionally, $0 is set to the name of a function for
the duration of the execution of that function, and $0 is also
set to the name of the file being interpreted for the duration of
a . command. $0 is not an element of $*, and is never treated
as one.
apid (no-export)
The process ID of the last process started in the background.
apids (no-export read-only)
A list whose elements are the process IDs of all background
processes which are still alive, or which have died and have not
been waited for yet.
bqstatus (no-export)
The exit status of the rc forked to execute the most recent
backquote substitution. Note that, unlike $status, $bqstatus is
always a single element list (see EXIT STATUS below). For
example:
echo foo |grep bar; whatis status
prints
status=(0 1)
whereas
x=`{echo foo |grep bar}; whatis bqstatus
prints
bqstatus=1
cdpath (alias)
A list of directories to search for the target of a cd command.
The empty string stands for the current directory. Note that if
the $cdpath variable does not contain the current directory, then
the current directory will not be searched; this allows directory
searching to begin in a directory other than the current
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directory.
history
$history contains the name of a file to which commands are
appended as rc reads them. This facilitates the use of a stand-
alone history program (such as history(1)) which parses the
contents of the history file and presents them to rc for
reinterpretation. If $history is not set, then rc does not
append commands to any file.
home (alias)
The default directory for the builtin cd command, and the
directory in which rc looks to find its initialization file,
.rcrc, if rc has been started up as a login shell.
ifs (default)
The internal field separator, used for splitting up the output of
backquote commands for digestion as a list. On startup, rc
assigns the list containing the characters space, tab, and
newline to $ifs.
path (alias)
This is a list of directories to search in for commands. The
empty string stands for the current directory. If neither $PATH
nor $path is set at startup time, $path assumes a default value
suitable for your system. This is typically (/usr/local/bin
/usr/bin /usr/ucb /bin .)
pid (default no-export)
On startup, $pid is initialized to the numeric process ID of the
currently running rc.
prompt (default)
This variable holds the two prompts (in list form, of course)
that rc prints. $prompt(1) is printed before each command is
read, and $prompt(2) is printed when input is expected to
continue on the next line. rc sets $prompt to ('; ' '') by
default. The reason for this is that it enables an rc user to
grab commands from previous lines using a mouse, and to present
them to rc for re-interpretation; the semicolon prompt is simply
ignored by rc. The null $prompt(2) also has its justification:
an rc script, when typed interactively, will not leave
$prompt(2)'s on the screen, and can therefore be grabbed by a
mouse and placed directly into a file for use as a shell script,
without further editing being necessary.
prompt (function)
If this function is defined, then it gets executed every time rc
is about to print $prompt(1).
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status (no-export read-only)
The exit status of the last command. If the command exited with
a numeric value, that number is the status. If the command died
with a signal, the status is the name of that signal; if a core
file was created, the string ``+core'' is appended. The value of
$status for a pipeline is a list, with one entry, as above, for
each process in the pipeline. For example, the command
ls | wc
usually sets $status to (0 0).
version (default)
On startup, the first element of this list variable is
initialized to a string which identifies this version of rc. The
second element is initialized to a string which can be found by
ident(1) and the what command of sccs(1).
FUNCTIONS
rc functions are identical to rc scripts, except that they are stored
in memory and are automatically exported into the environment. A
shell function is declared as:
fn name { commands }
rc scans the definition until the close-brace, so the function can
span more than one line. The function definition may be removed by
typing
fn name
(One or more names may be specified. With an accompanying definition,
all names receive the same definition. This is sometimes useful for
assigning the same signal handler to many signals. Without a
definition, all named functions are deleted.) When a function is
executed, $* is set to the arguments to that function for the duration
of the command. Thus a reasonable definition for l, a shorthand for
ls(1), could be:
fn l { ls -FC $* }
but not
fn l { ls -FC } # WRONG
INTERRUPTS AND SIGNALS
rc recognizes a number of signals, and allows the user to define shell
functions which act as signal handlers. rc by default traps SIGINT
when it is in interactive mode. SIGQUIT and SIGTERM are ignored,
unless rc has been invoked with the -d flag. However, user-defined
signal handlers may be written for these and all other signals. The
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way to define a signal handler is to write a function by the name of
the signal in lower case. Thus:
fn sighup { echo hangup; rm /tmp/rc$pid.*; exit }
In addition to Unix signals, rc recognizes the artificial signal
SIGEXIT which occurs as rc is about to exit.
In order to remove a signal handler's definition, remove it as though
it were a regular function. For example:
fn sigint
returns the handler of SIGINT to the default value. In order to
ignore a signal, set the signal handler's value to {}. Thus:
fn sigint {}
causes SIGINT to be ignored by the shell. Only signals that are being
ignored are passed on to programs run by rc; signal functions are not
exported.
On System V-based Unix systems, rc will not allow you to trap SIGCLD.
BUILTIN COMMANDS
Builtin commands execute in the context of the shell, but otherwise
behave exactly like other commands. Although !, ~ and @ are not
strictly speaking builtin commands, they can usually be used as such.
. [-i] file [arg ...]
Reads file as input to rc and executes its contents. With a -i
flag, input is interactive. Thus from within a shell script,
. -i /dev/tty
does the ``right thing''.
break
Breaks from the innermost for or while, as in C. It is an error
to invoke break outside of a loop. (Note that there is no break
keyword between commands in switch statements, unlike C.)
builtin command [arg ...]
Executes the command ignoring any function definition of the same
name. This command is present to allow functions with the same
names as builtins to use the underlying builtin or binary. For
example:
fn ls { builtin ls -FC $* }
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is a reasonable way to pass a default set of arguments to ls(1),
whereas
fn ls { ls -FC $* } # WRONG
is a non-terminating recursion, which will cause rc to exhaust
its stack space and (eventually) terminate if it is executed.
cd [directory]
Changes the current directory to directory. The variable $cdpath
is searched for possible locations of directory, analogous to the
searching of $path for executable files. With no argument, cd
changes the current directory to $home.
echo [-n] [--] [arg ...]
Prints its arguments to standard output, terminated by a newline.
Arguments are separated by spaces. If the first argument is -n
no final newline is printed. If the first argument is --, then
all other arguments are echoed literally. This is used for
echoing a literal -n.
eval [list]
Concatenates the elements of list with spaces and feeds the
resulting string to rc for re-scanning. This is the only time
input is rescanned in rc.
exec [arg ...]
Replaces rc with the given command. If the exec contains only
redirections, then these redirections apply to the current shell
and the shell does not exit. For example,
exec >[2] err.out
places further output to standard error in the file err.out.
exit [status]
Cause the current shell to exit with the given exit status. If
no argument is given, the current value of $status is used.
limit [-h] [resource [value]]
Similar to the csh(1) limit builtin, this command operates upon
the BSD-style resource limits of a process. The -h flag
displays/alters the hard limits. The resources which can be
shown or altered are cputime, filesize, datasize, stacksize,
coredumpsize, memoryuse, and, where supported, descriptors,
memoryuse, memoryrss, maxproc, memorylocked, and filelocks. For
example:
limit coredumpsize 0
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disables core dumps. To set a soft limit equal to the hard
limit:
limit `{limit -h datasize}
newpgrp
Puts rc into a new process group. This builtin is useful for
making rc behave like a job-control shell in a hostile
environment. One example is the NeXT Terminal program, which
implicitly assumes that each shell it forks will put itself into
a new process group.
return [n]
Returns from the current function, with status n, where n is a
valid exit status, or a list of them. Thus it is legal to have
return (sigpipe 1 2 3)
(This is commonly used to allow a function to return with the
exit status of a previously executed pipeline of commands.) If n
is omitted, then $status is left unchanged. It is an error to
invoke return when not inside a function.
shift [n]
Deletes n elements from the beginning of $* and shifts the other
elements down by n. n defaults to 1.
umask [mask]
Sets the current umask (see umask(2)) to the octal mask. If no
argument is present, the current mask value is printed.
wait [pid]
Waits for process with the specified pid, which must have been
started by rc, to exit. If no pid is specified, rc waits for all
its child processes to exit.
whatis [-b] [-f] [-p] [-s] [-v] [--] [name ...]
Prints a definition of the named objects. For builtins, builtin
foo is printed; for functions, including signal handlers, their
definitions are printed; for executable files, path names are
printed; and for variables, their values are printed. The flags
restrict output to builtins, functions, executable programs,
signal handlers, and variables, respectively. If no names are
specified, rc lists all objects of that type. (This is not
permitted for -p.) Without arguments, whatis is equivalent to
whatis -fv, and prints the values of all shell variables and
functions.
Note that whatis output is suitable for input to rc; by saving
the output of whatis in a file, it should be possible to recreate
the state of rc by sourcing this file with a . command. Another
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note: whatis -s > file cannot be used to store the state of rc's
signal handlers in a file, because builtins with redirections are
run in a subshell, and rc always restores signal handlers to
their default value after a fork().
Since whatis uses getopt(3) to parse its arguments, you can use
the special argument -- to terminate its flags. This allows you
to use names beginning with a dash, such as the history(1)
commands. For example,
whatis -- -p
EXAMPLES
The shift builtin only shifts $*. This function can shift any
variable (except $lshift).
fn lshift { lshift=$*; *=$$1; shift $lshift(2); $lshift(1)=$* }
With this definition in place,
walrus = (shoes ships sealing-wax cabbages kings)
lshift walrus 3
whatis walrus
prints
walrus=(cabbages kings)
The $^var operator flattens a list by separating each element with a
space. This function allows the separator to be an arbitrary string.
fn lflat {
lflat=$*; *=$$1
while () {
echo -n $1; shift
~ $#* 0 && break
echo -n $lflat(2)
}
With this definition in place,
hops=(uunet mcvax ukc tlg)
lflat hops !
prints (with no final newline)
uunet!mcvax!ukc!tlg
EXIT STATUS
The exit status of rc is normally the same as that of the last command
executed. If the last command was a pipeline, rc exits 0 if every
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command in the pipeline did; otherwise it exits 1.
rc can be made to exit with a particular status using the exit
builtin.
GRAMMAR
Here is rc's grammar, edited to remove semantic actions.
%term ANDAND BACKBACK BANG CASE COUNT DUP ELSE END FLAT FN FOR IF IN
%term OROR PIPE REDIR SUB SUBSHELL SWITCH TWIDDLE WHILE WORD HUH
%left WHILE ')' ELSE
%left ANDAND OROR '\n'
%left BANG SUBSHELL
%left PIPE
%right '$'
%left SUB
%start rc
%%
rc: line end
| error end
end: END /* EOF */ | '\n'
cmdsa: cmd ';' | cmd '&'
line: cmd | cmdsa line
body: cmd | cmdsan body
cmdsan: cmdsa | cmd '\n'
brace: '{' body '}'
paren: '(' body ')'
assign: first '=' word
epilog: /* empty */ | redir epilog
redir: DUP | REDIR word
case: CASE words ';' | CASE words '\n'
cbody: cmd | case cbody | cmdsan cbody
iftail: cmd %prec ELSE
| brace ELSE optnl cmd
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cmd : /* empty */ %prec WHILE
| simple
| brace epilog
| IF paren optnl iftail
| FOR '(' word IN words ')' optnl cmd
| FOR '(' word ')' optnl cmd
| WHILE paren optnl cmd
| SWITCH '(' word ')' optnl '{' cbody '}'
| TWIDDLE optcaret word words
| cmd ANDAND optnl cmd
| cmd OROR optnl cmd
| cmd PIPE optnl cmd
| redir cmd %prec BANG
| assign cmd %prec BANG
| BANG optcaret cmd
| SUBSHELL optcaret cmd
| FN words brace
| FN words
optcaret: /* empty */ | '^'
simple: first | simple word | simple redir
first: comword | first '^' sword
sword: comword | keyword
word: sword | word '^' sword
comword: '$' sword
| '$' sword SUB words ')'
| COUNT sword
| FLAT sword
| '`' sword
| '`' brace
| BACKBACK word brace | BACKBACK word sword
| '(' words ')'
| REDIR brace
| WORD
keyword: FOR | IN | WHILE | IF | SWITCH
| FN | ELSE | CASE | TWIDDLE | BANG | SUBSHELL
words: /* empty */ | words word
optnl: /* empty */ | optnl '\n'
FILES
$HOME/.rcrc, /tmp/rc*, /dev/null
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CREDITS
rc was written by Byron Rakitzis, with valuable help from Paul Haahr,
Hugh Redelmeier and David Sanderson. The design of this shell was
copied from the rc that Tom Duff wrote at Bell Labs.
BUGS
There is a compile-time limit on the number of ; separated commands in
a line: usually 500. This is sometimes a problem for automatically
generated scripts: substituting the newline character for ; avoids the
limit.
On modern systems that support /dev/fd or /proc/self/fd, <{foo} style
redirection is implemented that way. However, on older systems it is
implemented with named pipes. Allegedly, it is sometimes possible to
foil rc into removing the FIFO it places in /tmp prematurely, or it is
even possible to cause rc to hang. (The current maintainer has never
seen this, but then he doesn't use systems which lack /dev/fd any
more. If anybody can reproduce this problem, please let the
maintainer know.)
The echo command does not need to be a builtin. It is one for reasons
of performance and portability (of rc scripts).
There should be a way to avoid exporting a variable.
Extra parentheses around a ~ expression or a ! expression are a
syntax error. Thus, this code is illegal.
while ((~ $1 -*) && (! ~ $1 --)) { ...
The redundant inner parentheses must be omitted.
Variable subscripting cannot be used in here documents.
The limit builtin silently ignores extra arguments.
Bug reports should be mailed to <tjg@star.le.ac.uk>.
INCOMPATIBILITIES
Here is a list of features which distinguish this incarnation of rc
from the one described in the Bell Labs manual pages:
The Tenth Edition rc does not have the else keyword. Instead, if is
optionally followed by an if not clause which is executed if the
preceding if test does not succeed.
Backquotes are slightly different in Tenth Edition rc: a backquote
must always be followed by a left-brace. This restriction is not
present for single-word commands in this rc.
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For . file, the Tenth Edition rc searches $path for file. This rc
does not, since it is not considered useful.
The list flattening operator, $^foo, is spelt " " """ .Xf R R "$foo" "
" " "" "" "" "" "" "" in those versions of the Bell Labs rc which have
it.
The following are all new with this version of rc: The -n flag, here
strings (they facilitate exporting of functions with here documents
into the environment), the return and break keywords, the echo
builtin, the bqstatus and version variables, the support for the GNU
readline(3) library, and the support for the prompt function. This rc
also sets $0 to the name of a function being executed/file being
sourced.
SEE ALSO
``rc - A Shell for Plan 9 and UNIX Systems'', Unix Research System,
Tenth Edition, Volume 2. (Saunders College Publishing)
http://www.star.le.ac.uk/~tjg/rc/misc/td, an updated version of the
above paper.
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