|
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(2)); 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(2)) 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,
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
|
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
is equivalent to
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. The standard
input file may also be redirected from a file by the sequence
<>file, which opens file exactly once, for reading and writing.
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], <>[fd0=fd1], or <[fd0=fd1]. Fd1 is a previously
opened file descriptor and fd0 becomes a new copy (in the sense
of dup(2)) 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.
newconn <>/net/tcp/clone >[1=0] opens /net/tcp/clone exactly once
for reading and writing and puts it on standard input and output.
lpd <>[3]/net/tcp/42/data opens /net/tcp/42/data exactly once for
reading and writing and puts it on file descriptor 3.
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(2). 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 env device (see env(3)). 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 ctrl–a ('\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 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 path=(. /bin). Its use is discouraged; instead use bind(1)
to build a /bin containing what's needed.
$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
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.
–m Read commands to initialize rc from initial instead of from /rc/lib/rcmain.
–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).
|