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 rcs argument 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 executing 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 otherwise a special
character.
A number-sign (#) and any following characters up to (but not
including) the next newline are ignored, except in quotation 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 rcs 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 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 rcs 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
complemented. The class may also contain pairs of characters separated
by -, standing for all characters lexically between the two. The
character / must appear explicitly in a pattern, as must the first
character of the path name components . and ... A pattern is replaced
by a list of arguments, 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 prefixing /tmp.
A quoted word is a sequence of characters surrounded by single 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 parentheses 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 indirection 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 $* 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. Subscripts 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 variable. A
variable never assigned a value has zero elements.
$"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 standard 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 construction of non-linear pipelines.
For example, the following runs two commands old and new and
uses cmp to compare their outputs cmp <{old} <{new}
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 automatically
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. 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 rcs 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 operator 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 output to fd1 of the left command and input to fd0 of the
right command.
A pair of commands separated by && or || is a command. In either case,
the left command is executed and its exit status 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 status 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 following 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}
Braces serve to alter the grouping of commands implied by
operator priorities. The body is a sequence of commands
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 command. 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 executing.
name=argument command
Any command may be preceded by a sequence of assignments
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 argument 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.
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 variable, 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 executing
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
When rc starts executing it reads variable and function definitions
from its environment.
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 completion 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 substitutions.
If $ifs is not set in rc’s environment, 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 environment, 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 program. (unless
started with &). ! and ~ also change $status. Its value is
used to control execution 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 interactively.
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 $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
/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.
By default, FreeBSD 5 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 automatically at boot
time.)
RC(1)