RC(1) RC(1)
NAME
rc, cd, eval, exec, exit, flag, rfork, shift, wait, whatis,
., ~ - command language
SYNOPSIS
rc [ -srdiIlxepvV ] [ -c command ] [ file [ arg ... ]]
DESCRIPTION
Rc is the Plan 9 shell. It executes command lines read from
a terminal or a file or, with the -c flag, from rc's argu-
ment list.
Command Lines
A command line is a sequence of commands, separated by
ampersands or semicolons (& or ;), terminated by a newline.
The commands are executed in sequence from left to right.
Rc does not wait for a command followed by & to finish exe-
cuting before starting the following command. Whenever a
command followed by & is executed, its process id is
assigned to the rc variable $apid. Whenever a command not
followed by & exits or is terminated, the rc variable
$status gets the process's wait message (see wait(3)); it
will be the null string if the command was successful.
A long command line may be continued on subsequent lines by
typing a backslash (\) followed by a newline. This sequence
is treated as though it were a blank. Backslash is not oth-
erwise a special character.
A number-sign (#) and any following characters up to (but
not including) the next newline are ignored, except in quo-
tation marks.
Simple Commands
A simple command is a sequence of arguments interspersed
with I/O redirections. If the first argument is the name of
an rc function or of one of rc's built-in commands, it is
executed by rc. Otherwise if the name starts with a slash
(/), it must be the path name of the program to be executed.
Names containing no initial slash are searched for in a list
of directory names stored in $path. The first executable
file of the given name found in a directory in $path is the
program to be executed. To be executable, the user must
have execute permission (see stat(3)) and the file must be
either an executable binary for the current machine's CPU
type, or a shell script. Shell scripts begin with a line
containing the full path name of a shell (usually /bin/rc),
prefixed by `#!'.
The first word of a simple command cannot be a keyword
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unless it is quoted or otherwise disguised. The keywords
are
for in while if not switch fn ~ ! @
Arguments and Variables
A number of constructions may be used where rc's syntax
requires an argument to appear. In many cases a
construction's value will be a list of arguments rather than
a single string.
The simplest kind of argument is the unquoted word: a
sequence of one or more characters none of which is a blank,
tab, newline, or any of the following:
# ; & | ^ $ = ` ' { } ( ) < >
An unquoted word that contains any of the characters * ? [
is a pattern for matching against file names. The character
* matches any sequence of characters, ? matches any single
character, and [class] matches any character in the class.
If the first character of class is ~, the class is comple-
mented. The class may also contain pairs of characters sep-
arated by -, standing for all characters lexically between
the two. The character / must appear explicitly in a pat-
tern, as must the first character of the path name compo-
nents . and ... A pattern is replaced by a list of argu-
ments, one for each path name matched, except that a pattern
matching no names is not replaced by the empty list, but
rather stands for itself. Pattern matching is done after
all other operations. Thus,
x=/tmp echo $x^/*.c
matches /tmp/*.c, rather than matching /*.c and then prefix-
ing /tmp.
A quoted word is a sequence of characters surrounded by sin-
gle quotes ('). A single quote is represented in a quoted
word by a pair of quotes ('').
Each of the following is an argument.
(arguments)
The value of a sequence of arguments enclosed in paren-
theses is a list comprising the members of each element
of the sequence. Argument lists have no recursive
structure, although their syntax may suggest it. The
following are entirely equivalent:
echo hi there everybody
((echo) (hi there) everybody)
$argument
$argument(subscript)
The argument after the $ is the name of a variable
whose value is substituted. Multiple levels of indi-
rection are possible, but of questionable utility.
Variable values are lists of strings. If argument is a
number n, the value is the nth element of $*, unless $*
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doesn't have n elements, in which case the value is
empty. If argument is followed by a parenthesized list
of subscripts, the value substituted is a list composed
of the requested elements (origin 1). The parenthesis
must follow the variable name with no spaces. Sub-
scripts can also take the form m-n or m- to indicate a
sequence of elements. Assignments to variables are
described below.
$#argument
The value is the number of elements in the named vari-
able. A variable never assigned a value has zero ele-
ments.
$"argument
The value is a single string containing the components
of the named variable separated by spaces. A variable
with zero elements yields the empty string.
`{command}
rc executes the command and reads its standard output,
splitting it into a list of arguments, using characters
in $ifs as separators. If $ifs is not otherwise set,
its value is ' \t\n'.
<{command}
>{command}
The command is executed asynchronously with its stan-
dard output or standard input connected to a pipe. The
value of the argument is the name of a file referring
to the other end of the pipe. This allows the con-
struction of non-linear pipelines. For example, the
following runs two commands old and new and uses cmp to
compare their outputs
cmp <{old} <{new}
<>{command}
The command is executed asynchronously with its stan-
dard input and output each connected to a pipe. The
value of the argument is a pair of file names referring
to the two other ends of the pipes, in the order corre-
sponding to the symbols < and > (first the pipe con-
nected to the command's standard output, then the pipe
connected to its standard input).
argument^argument
The ^ operator concatenates its two operands. If the
two operands have the same number of components, they
are concatenated pairwise. If not, then one operand
must have one component, and the other must be non-
empty, and concatenation is distributive.
Free Carets
In most circumstances, rc will insert the ^ operator auto-
matically between words that are not separated by white
space. Whenever one of $ ' ` follows a quoted or unquoted
word or an unquoted word follows a quoted word with no
intervening blanks or tabs, a ^ is inserted between the two.
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If an unquoted word immediately follows a $ and contains a
character other than an alphanumeric, underscore, or *, a ^
is inserted before the first such character. Thus
cc -$flags $stem.c
is equivalent to
cc -^$flags $stem^.c
I/O Redirections
The sequence >file redirects the standard output file (file
descriptor 1, normally the terminal) to the named file;
>>file appends standard output to the file. The standard
input file (file descriptor 0, also normally the terminal)
may be redirected from a file by the sequence <file, or from
an inline `here document' by the sequence <<eof-marker. The
contents of a here document are lines of text taken from the
command input stream up to a line containing nothing but the
eof-marker, which may be either a quoted or unquoted word.
If eof-marker is unquoted, variable names of the form $word
have their values substituted from rc's environment. If
$word is followed by a caret (^), the caret is deleted. If
eof-marker is quoted, no substitution occurs.
Redirections may be applied to a file-descriptor other than
standard input or output by qualifying the redirection oper-
ator with a number in square brackets. For example, the
diagnostic output (file descriptor 2) may be redirected by
writing cc junk.c >[2]junk.
A file descriptor may be redirected to an already open
descriptor by writing >[fd0=fd1] or <[fd0=fd1]. Fd1 is a
previously opened file descriptor and fd0 becomes a new copy
(in the sense of dup(3)) of it. A file descriptor may be
closed by writing >[fd0=] or <[fd0=].
Redirections are executed from left to right. Therefore, cc
junk.c >/dev/null >[2=1] and cc junk.c >[2=1] >/dev/null
have different effects: the first puts standard output in
/dev/null and then puts diagnostic output in the same place,
where the second directs diagnostic output to the terminal
and sends standard output to /dev/null.
Compound Commands
A pair of commands separated by a pipe operator (|) is a
command. The standard output of the left command is sent
through a pipe to the standard input of the right command.
The pipe operator may be decorated to use different file
descriptors. |[fd] connects the output end of the pipe to
file descriptor fd rather than 1. |[fd0=fd1] connects out-
put to fd1 of the left command and input to fd0 of the right
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command.
A pair of commands separated by && or || is a command. In
either case, the left command is executed and its exit sta-
tus examined. If the operator is && the right command is
executed if the left command's status is null. || causes
the right command to be executed if the left command's sta-
tus is non-null.
The exit status of a command may be inverted (non-null is
changed to null, null is changed to non-null) by preceding
it with a !.
The | operator has highest precedence, and is left-
associative (i.e. binds tighter to the left than the right).
! has intermediate precedence, and && and || have the lowest
precedence.
The unary @ operator, with precedence equal to !, causes its
operand to be executed in a subshell.
Each of the following is a command.
if ( list ) command
A list is a sequence of commands, separated by &, ;, or
newline. It is executed and if its exit status is
null, the command is executed.
if not command
The immediately preceding command must have been
if(list) command. If its condition was non-zero, the
command is executed.
for(name in arguments) command
for(name) command
The command is executed once for each argument with
that argument assigned to name. If the argument list is
omitted, $* is used.
while(list) command
The list is executed repeatedly until its exit status
is non-null. Each time it returns null status, the
command is executed. An empty list is taken to give
null status.
switch(argument){list}
The list is searched for simple commands beginning with
the word case. (The search is only at the `top level'
of the list. That is, cases in nested constructs are
not found.) Argument is matched against each word fol-
lowing case using the pattern-matching algorithm
described above, except that / and the first characters
of . and .. need not be matched explicitly. When a
match is found, commands in the list are executed up to
the next following case command (at the top level) or
the closing brace.
{list}
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Braces serve to alter the grouping of commands implied
by operator priorities. The body is a sequence of com-
mands separated by &, ;, or newline.
fn name{list}
fn name
The first form defines a function with the given name.
Subsequently, whenever a command whose first argument
is name is encountered, the current value of the
remainder of the command's argument list will be
assigned to $*, after saving its current value, and rc
will execute the list. The second form removes name's
function definition.
fn note{list}
fn note
A function with a special name will be called when rc
receives a corresponding note; see notify(3). The valid
note names (and corresponding notes) are sighup
(hangup), sigint (interrupt), sigalrm (alarm), and
sigfpe (floating point trap). By default rc exits on
receiving any signal, except when run interactively, in
which case interrupts and quits normally cause rc to
stop whatever it's doing and start reading a new com-
mand. The second form causes rc to handle a signal in
the default manner. Rc recognizes an artificial note,
sigexit, which occurs when rc is about to finish exe-
cuting.
name=argument command
Any command may be preceded by a sequence of assign-
ments interspersed with redirections. The assignments
remain in effect until the end of the command, unless
the command is empty (i.e. the assignments stand
alone), in which case they are effective until
rescinded by later assignments.
Built-in Commands
These commands are executed internally by rc, usually
because their execution changes or depends on rc's internal
state.
. file ...
Execute commands from file. $* is set for the duration
to the remainder of the argument list following file.
File is searched for using $path.
builtin command ...
Execute command as usual except that any function named
command is ignored in favor of the built-in meaning.
cd [dir]
Change the current directory to dir. The default argu-
ment is $home. dir is searched for in each of the
directories mentioned in $cdpath.
eval [arg ...]
The arguments are concatenated separated by spaces into
a single string, read as input to rc, and executed.
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exec [command ...]
This instance of rc replaces itself with the given
(non-built-in) command.
flag f [+-]
Either set (+), clear (-), or test (neither + nor -)
the flag f, where f is a single character, one of the
command line flags (see Invocation, below).
exit [status]
Exit with the given exit status. If none is given, the
current value of $status is used.
rfork [nNeEsfFm]
Become a new process group using rfork(flags) where
flags is composed of the bitwise OR of the rfork flags
specified by the option letters (see fork(2)). If no
flags are given, they default to ens. The flags and
their meanings are: n is RFNAMEG; N is RFCNAMEG; e is
RFENVG; E is RFCENVG; s is RFNOTEG; f is RFFDG; F is
RFCFDG; and m is RFNOMNT.
shift [n]
Delete the first n (default 1) elements of $*.
wait [pid]
Wait for the process with the given pid to exit. If no
pid is given, all outstanding processes are waited for.
whatis name ...
Print the value of each name in a form suitable for
input to rc. The output is an assignment to any vari-
able, the definition of any function, a call to builtin
for any built-in command, or the completed pathname of
any executable file.
~ subject pattern ...
The subject is matched against each pattern in
sequence. If it matches any pattern, $status is set to
zero. Otherwise, $status is set to one. Patterns are
the same as for file name matching, except that / and
the first character of . and .. need not be matched
explicitly. The patterns are not subjected to file
name matching before the ~ command is executed, so they
need not be enclosed in quotation marks.
Environment
The environment is a list of strings made available to exe-
cuting binaries by the kernel. Rc creates an environment
entry for each variable whose value is non-empty, and for
each function. The string for a variable entry has the
variable's name followed by = and its value. If the value
has more than one component, these are separated by SOH
(001) characters. The string for a function is just the rc
input that defines the function. The name of a function in
the environment is the function name preceded by `fn#'.
When rc starts executing it reads variable and function def-
initions from its environment.
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Special Variables
The following variables are set or used by rc.
$* Set to rc's argument list during initialization.
Whenever a . command or a function is executed, the
current value is saved and $* receives the new
argument list. The saved value is restored on com-
pletion of the . or function.
$apid Whenever a process is started asynchronously with
&, $apid is set to its process id.
$home The default directory for cd.
$ifs The input field separators used in backquote sub-
stitutions. If $ifs is not set in rc's environ-
ment, it is initialized to blank, tab and newline.
$path The search path used to find commands and input
files for the . command. If not set in the envi-
ronment, it is initialized by parsing the $PATH
variable (as in sh(1)) or by path=(. /bin). The
variables $path and $PATH are maintained together:
changes to one will be reflected in the other.
$pid Set during initialization to rc's process id.
$prompt When rc is run interactively, the first component
of $prompt is printed before reading each command.
The second component is printed whenever a newline
is typed and more lines are required to complete
the command. If not set in the environment, it is
initialized by prompt=('% ' ' ').
$status Set to the wait message of the last-executed pro-
gram. (unless started with &). ! and ~ also
change $status. Its value is used to control exe-
cution in &&, ||, if and while commands. When rc
exits at end-of-file of its input or on executing
an exit command with no argument, $status is its
exit status.
Invocation
If rc is started with no arguments it reads commands from
standard input. Otherwise its first non-flag argument is
the name of a file from which to read commands (but see -c
below). Subsequent arguments become the initial value of
$*. Rc accepts the following command-line flags.
-c string Commands are read from string.
-s Print out exit status after any command where the
status is non-null.
-e Exit if $status is non-null after executing a
simple command.
-i If -i is present, or rc is given no arguments and
its standard input is a terminal, it runs inter-
actively. Commands are prompted for using
$prompt.
-I Makes sure rc is not run interactively.
-l If -l is given or the first character of argument
zero is -, rc reads commands from
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$home/lib/profile, if it exists, before reading
its normal input.
-p A no-op.
-d A no-op.
-v Echo input on file descriptor 2 as it is read.
-x Print each simple command before executing it.
-r Print debugging information (internal form of
commands as they are executed).
SOURCE
/usr/local/plan9/src/cmd/rc
SEE ALSO
Tom Duff, ``Rc - The Plan 9 Shell''.
BUGS
There should be a way to match patterns against whole lists
rather than just single strings.
Using ~ to check the value of $status changes $status.
Functions that use here documents don't work.
Free carets don't get inserted next to keywords.
The <{command} syntax depends on the underlying operating
system providing a file descriptor device tree at /dev/fd.
Some FreeBSD installations does not provide file descriptors
greater than 2 in /dev/fd. To fix this, add
/fdescfs /dev/fd fdescfs rw 0 0
to /etc/fstab, and then mount /dev/fd. (Adding the line to
fstab ensures causes FreeBSD to mount the file system auto-
matically at boot time.)
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