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 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 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 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 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) |
|
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} |
|
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} |
|
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 |
|
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 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 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} |
|
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} |
|
is a sequence of commands separated by &, ;, or newline. | ||
|
fn name |
{list}
|
fn 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 |
|
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 ... |
|
is set for the duration to the remainder of the argument list following file. File is searched for using $path. | ||
|
builtin |
command ...
|
as usual except that any function named command is ignored in favor of the built-in meaning. | ||
|
cd [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 ...] |
|
replaces itself with the given (non-built-in) command. |
||||
|
flag f |
[+-]
|
nor -) the flag f, where f is a single character, one of the command line flags (see Invocation, below). | |||
|
exit [ |
status]
|
is used. |
||||
|
rfork [nNeEsfFm] |
|
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] |
|
to exit. If no pid is given, all outstanding processes are waited for. | |||
|
whatis |
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 ...
|
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.)