plan9fox/sys/man/1/rc

.TH RC 1
.SH NAME
rc, cd, eval, exec, exit, flag, rfork, shift, wait, whatis, ., ~ \- command language
.SH SYNOPSIS
.B rc
[
.B -srdiIlxepvV
]
[
.B -c
.I command
]
[
.B -m
.I initial
]
[
.I file
[
.I arg ...
]]
.SH DESCRIPTION
.I Rc
is the Plan 9 shell.
It executes command lines read from a terminal or a file or, with the
.B -c
flag, from
.I rc's
argument list.
.SS Command Lines
A command line is a sequence of commands, separated by ampersands or semicolons
.RB ( &
or
.BR ; ),
terminated by a newline.
The commands are executed in sequence
from left to right.
.I Rc
does not wait for a command followed by
.B &
to finish executing before starting
the following command.
Whenever a command followed by
.B &
is executed, its process id is assigned to the
.I rc
variable
.BR $apid .
Whenever a command
.I not
followed by
.B &
exits or is terminated, the
.I rc
variable
.B $status
gets the process's wait message (see
.IR wait (2));
it will be the null string if the command was successful.
.PP
A long command line may be continued on subsequent lines by typing
a backslash
.RB ( \e )
followed by a newline.
This sequence is treated as though it were a blank.
Backslash is not otherwise a special character.
.PP
A number-sign
.RB ( # )
and any following characters up to (but not including) the next newline
are ignored, except in quotation marks.
.SS Simple Commands
A simple command is a sequence of arguments interspersed with I/O redirections.
If the first argument is the name of an
.I rc
function or of one of
.I rc's
built-in commands, it is executed by
.IR rc .
Otherwise if the name starts with a slash
.RB ( / ),
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
.BR $path .
The first executable file of the given name found
in a directory in
.B $path
is the program to be executed.
To be executable, the user must have execute permission (see
.IR 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
.BR /bin/rc ),
prefixed by
.LR #! .
.PP
The first word of a simple command cannot be a keyword unless it is
quoted or otherwise disguised.
The keywords are
.EX
	for in while if not switch fn ~ ! @
.EE
.SS Arguments and Variables
A number of constructions may be used where
.I 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.
.PP
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:
.EX
	# ; & | ^ $ = ` ' { } ( ) < >
.EE
An unquoted word that contains any of the characters
.B *
.B ?
.B [
is a pattern for matching against file names.
The character
.B *
matches any sequence of characters,
.B ?
matches any single character, and
.BI [ class ]
matches any character in the
.IR class .
If the first character of
.I class
is
.BR ~ ,
the class is complemented.
The
.I class
may also contain pairs of characters separated by
.BR - ,
standing for all characters lexically between the two.
The character
.B /
must appear explicitly in a pattern, as must the
first character of the path name components
.B .
and
.BR .. .
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,
.EX
	x=/tmp echo $x^/*.c
.EE
matches
.BR /tmp/*.c ,
rather than matching
.B "/*.c
and then prefixing
.BR /tmp .
.PP
A quoted word is a sequence of characters surrounded by single quotes
.RB ( ' ).
A single quote is represented in a quoted word by a pair of quotes
.RB ( '' ).
.PP
Each of the following is an argument.
.PD 0
.HP
.BI ( arguments )
.br
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:
.EX
	echo hi there everybody
	((echo) (hi there) everybody)
.EE
.HP
.BI $ argument
.HP
.BI $ argument ( subscript )
.br
The
.I argument
after the
.B $
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
.I argument
is a number
.IR n ,
the value is the
.IR n th
element of
.BR $* ,
unless
.B $*
doesn't have
.I n
elements, in which case the value is empty.
If
.I 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
.IB m - n
or
.IB m -
to indicate a sequence of elements.
Assignments to variables are described below.
.HP
.BI $# argument
.br
The value is the number of elements in the named variable.
A variable
never assigned a value has zero elements.
.HP
$"\c
.I argument
.br
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.
.HP
.BI `{ command }
.HP
.BI ` "split " { command }
.br
.I rc
executes the
.I command
and reads its standard output, splitting it into a list of arguments,
using characters in
.B $ifs
as separators.
If
.B $ifs
is not otherwise set, its value is
.BR "'\ \et\en'" .
In the second form of the command, split is used instead of
.BR $ifs .
.HP
.BI <{ command }
.HP
.BI >{ command }
.br
The
.I 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
.B old
and
.B new
and uses
.B cmp
to compare their outputs
.EX
	cmp <{old} <{new}
.EE
.HP
.IB argument ^ argument
.br
The
.B ^
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.
.PD
.SS Free Carets
In most circumstances,
.I rc
will insert the
.B ^
operator automatically between words that are not separated by white space.
Whenever one of
.B $
.B '
.B `
follows a quoted or unquoted word or an unquoted word follows a quoted word
with no intervening blanks or tabs,
a
.B ^
is inserted between the two.
If an unquoted word immediately follows a
.BR $ 
and contains a character other than an alphanumeric, underscore,
or
.BR * ,
a
.B ^
is inserted before the first such character.
Thus
.IP
.B cc -$flags $stem.c
.LP
is equivalent to
.IP
.B cc -^$flags $stem^.c
.SS I/O Redirections
The sequence
.BI > file
redirects the standard output file (file descriptor 1, normally the
terminal) to the named
.IR file ;
.BI >> 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
.BI < file \f1,
or from an inline `here document'
by the sequence
.BI << eof-marker\f1.
The contents of a here document are lines of text taken from the command
input stream up to a line containing nothing but the
.IR eof-marker ,
which may be either a quoted or unquoted word.
If
.I eof-marker
is unquoted, variable names of the form
.BI $ word
have their values substituted from
.I rc's
environment.
If
.BI $ word
is followed by a caret
.RB ( ^ ),
the caret is deleted.
If
.I eof-marker
is quoted, no substitution occurs.
The standard input file
may also be redirected from a file by the sequence
.BI <> file \f1,
which opens
.I file
exactly once, for reading and writing.
.PP
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
.BR "cc junk.c >[2]junk" .
.PP
A file descriptor may be redirected to an already open descriptor by writing
.BI >[ fd0 = fd1 ],
.BI <>[ fd0 = fd1 ],
or
.BI <[ fd0 = fd1 ]\f1.
.I Fd1
is a previously opened file descriptor and
.I fd0
becomes a new copy (in the sense of 
.IR dup (2))
of it.
A file descriptor may be closed by writing
.BI >[ fd0 =]
or
.BI <[ fd0 =]\f1.
.PP
Redirections are executed from left to right.
Therefore,
.B cc junk.c >/dev/null >[2=1]
and
.B cc junk.c >[2=1] >/dev/null
have different effects: the first puts standard output in
.BR /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
.BR /dev/null .
.PP
.B newconn <>/net/tcp/clone >[1=0]
opens
.B /net/tcp/clone
exactly once for reading and writing and puts it on standard input and output.
.B lpd <>[3]/net/tcp/42/data
opens
.B /net/tcp/42/data
exactly once for reading and writing and puts it on file descriptor 3.
.SS Compound Commands
A pair of commands separated by a pipe operator
.RB ( | )
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.
.BI |[ fd ]
connects the output end of the pipe to file descriptor
.I fd
rather than 1.
.BI |[ fd0 = fd1 ]
connects output to
.I fd1
of the left command and input to
.I fd0
of the right command.
.PP
A pair of commands separated by
.B &&
or
.B ||
is a command.
In either case, the left command is executed and its exit status examined.
If the operator is
.B &&
the right command is executed if the left command's status is null.
.B ||
causes the right command to be executed if the left command's status is non-null.
.PP
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
.BR ! .
.PP
The
.B |
operator has highest precedence, and is left-associative (i.e. binds tighter
to the left than the right).
.B !
has intermediate precedence, and
.B &&
and
.B ||
have the lowest precedence.
.PP
The unary
.B @
operator, with precedence equal to
.BR ! ,
causes its operand to be executed in a subshell.
.PP
Each of the following is a command.
.PD 0
.HP
.B if (
.I list
.B )
.I command
.br
A
.I list
is a sequence of commands, separated by
.BR & ,
.BR ; ,
or newline.
It is executed and
if its exit status is null, the
.I command
is executed.
.HP
.B if not
.I command
.br
The immediately preceding command must have been
.BI if( list )
.IR command .
If its condition was non-zero, the
.I command
is executed.
.HP
.BI for( name
.B in
.IB arguments )
.I command
.HP
.BI for( name )
.I command
.br
The
.I command
is executed once for each
.IR argument 
with that argument assigned to
.IR name .
If the argument list is omitted,
.B $*
is used.
.HP
.BI while( list )
.I command
.br
The
.I list
is executed repeatedly until its exit status is non-null.
Each time it returns null status, the
.I command
is executed.
An empty
.I list
is taken to give null status.
.HP
.BI "switch(" argument "){" list }
.br
The
.IR list
is searched for simple commands beginning with the word
.BR case .
(The search is only at the `top level' of the
.IR list .
That is,
.B cases
in nested constructs are not found.)
.I Argument
is matched against each word following
.B case
using the pattern-matching algorithm described above, except that
.B /
and the first characters of
.B .
and
.B ..
need not be matched explicitly.
When a match is found, commands in the list are executed up to the next
following
.B case
command (at the top level) or the closing brace.
.HP
.BI { list }
.br
Braces serve to alter the grouping of commands implied by operator
priorities.
The
.I body
is a sequence of commands separated by
.BR & ,
.BR ; ,
or newline.
.HP
.BI "fn " name { list }
.HP
.BI "fn " name
.br
The first form defines a function with the given
.IR name .
Subsequently, whenever a command whose first argument is
.I name
is encountered, the current value of
the remainder of the command's argument list will be assigned to
.BR $* ,
after saving its current value, and
.I rc
will execute the
.IR list .
The second form removes
.IR name 's
function definition.
.HP
.BI "fn " note { list }
.br
.HP
.BI "fn " note
.br
A function with a special name will be called when
.I rc
receives a corresponding note; see
.IR notify (2).
The valid note names (and corresponding notes) are
.B sighup
.RB ( hangup ),
.B sigint
.RB ( interrupt ),
.BR sigalrm
.RB ( alarm ),
and
.B sigfpe
(floating point trap).
By default
.I rc
exits on receiving any signal, except when run interactively,
in which case interrupts and quits normally cause
.I rc
to stop whatever it's doing and start reading a new command.
The second form causes
.I rc
to handle a signal in the default manner.
.I Rc
recognizes an artificial note,
.BR sigexit ,
which occurs when
.I rc
is about to finish executing.
.HP
.IB name = "argument command"
.br
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.
.PD
.SS Built-in Commands
These commands are executed internally by
.IR rc ,
usually because their execution changes or depends on
.IR rc 's
internal state.
.PD 0
.HP
.BI . " [-biq] file ..."
.br
Execute commands from
.IR file .
.B $*
is set for the duration to the remainder of the argument list following
.IR file .
.I File
is searched for using
.BR $path .
The flags
.B -b
and
.B -i
can be set for the new commands
(see description below).
The
.B -q
flag suppresses errors,
inhibiting the effect of
.B -e
and
.B -v
flags of the main interpreter.
.HP
.BI builtin " command ..."
.br
Execute
.I command
as usual except that any function named
.I command
is ignored in favor of the built-in meaning.
.HP
.BI "cd [" dir "]"
.br
Change the current directory to
.IR dir .
The default argument is
.BR $home .
.I dir
is searched for in each of the directories mentioned in
.BR $cdpath .
.HP
.BI "eval [" "arg ..." "]"
.br
The arguments are concatenated separated by spaces into a single string,
read as input to
.IR rc ,
and executed.
.HP
.BI "exec [" "command ..." "]"
.br
This instance of
.I rc
replaces itself with the given (non-built-in)
.IR command .
.HP
.BI "flag " f " [+-]"
.br
Either set
.RB ( + ),
clear
.RB ( - ),
or test (neither
.B +
nor
.BR - )
the flag
.IR f ,
where
.I f
is a single character, one of the command line flags (see Invocation, below).
.HP
.BI "exit [" status "]"
.br
Exit with the given exit status.
If none is given, the current value of
.B $status
is used.
.HP
.BR "rfork " [ nNeEsfFm ]
.br
Become a new process group using
.BI rfork( flags )
where
.I flags
is composed of the bitwise OR of the
.B rfork
flags specified by the option letters
(see
.IR fork (2)).
If no
.I flags
are given, they default to
.BR ens .
The
.I flags
and their meanings are:
.B n
is
.BR RFNAMEG ;
.B N
is
.BR RFCNAMEG ;
.B e
is
.BR RFENVG ;
.B E
is
.BR RFCENVG ;
.B s
is
.BR RFNOTEG ;
.B f
is
.BR RFFDG ;
.B F
is
.BR RFCFDG ;
and
.B m
is
.BR RFNOMNT .
.HP
.BI "shift [" n "]"
.br
Delete the first
.IR n
(default 1)
elements of
.BR $* .
.HP
.BI "wait [" pid "]"
.br
Wait for the process with the given
.I pid
to exit.
If no
.I pid
is given, all outstanding processes are waited for.
.HP
.BI whatis " name ..."
.br
Print the value of each
.I name
in a form suitable for input to
.IR rc .
The output is
an assignment to any variable,
the definition of any function,
a call to
.B builtin
for any built-in command, or
the completed pathname of any executable file.
.HP
.BI ~ " subject pattern ..."
.br
The
.I subject
is matched against each
.I pattern
in sequence.
If it matches any pattern,
.B $status
is set to zero.
Otherwise,
.B $status
is set to one.
Patterns are the same as for file name matching, except that
.B /
and the first character of
.B .
and
.B ..
need not be matched explicitly.
The
.I patterns
are not subjected to
file name matching before the
.B ~
command is executed, so they need not be enclosed in quotation marks.
.PD
.SS Environment
The
.I environment
is a list of strings made available to executing binaries by the
.B env
device
(see
.IR env (3)).
.I Rc
creates an environment entry for each variable whose value is non-empty,
and for each function.
The string for a variable entry has the variable's name followed by
.B =
and its value.
If the value has more than one component, these
are separated by nul
.RB ( '\e000' )
characters.
The string for a function is just the
.I rc
input that defines the function.
The name of a function in the environment is the function name
preceded by
.LR fn# .
.PP
When
.I rc
starts executing it reads variable and function definitions from its
environment.
.SS Special Variables
The following variables are set or used by
.IR rc .
.PD 0
.TP \w'\fL$promptXX'u
.B $*
Set to
.IR rc 's
argument list during initialization.
Whenever a
.B .
command or a function is executed, the current value is saved and
.B $*
receives the new argument list.
The saved value is restored on completion of the
.B .
or function.
.TP
.B $apid
Whenever a process is started asynchronously with
.BR & ,
.B $apid
is set to its process id.
.TP
.B $home
The default directory for
.BR cd .
.TP
.B $ifs
The input field separators used in backquote substitutions.
If
.B $ifs
is not otherwise set, its value is
.BR "'\ \et\en'" .
.TP
.B $path
The search path used to find commands and input files
for the
.B .
command.
If not set in the environment, it is initialized by
.BR "path=(.\ /bin)" .
Its use is discouraged; instead use
.IR bind (1)
to build a
.B /bin
containing what's needed.
.TP
.B $pid
Set during initialization to
.IR rc 's
process id.
.TP
.B $prompt
When
.I rc
is run interactively, the first component of
.B $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
.BR "prompt=('%\ '\ '\ ')" .
.TP
.B $status
Set to the wait message of the last-executed program.
(unless started with
.BR &).
.B !
and
.B ~
also change
.BR $status .
Its value is used to control execution in
.BR && ,
.BR || ,
.B if
and
.B while
commands.
When
.I rc
exits at end-of-file of its input or on executing an
.B exit
command with no argument,
.B $status
is its exit status.
.PD
.SS Invocation
If
.I 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
.B -c
below).
Subsequent arguments become the initial value of
.BR $* .
.I Rc
accepts the following command-line flags.
.PD 0
.TP \w'\fL-c\ \fIstring\fLXX'u
.BI -c " string"
Commands are read from
.IR string .
.TP
.B -s
Print out exit status after any command where the status is non-null.
.TP
.B -e
Exit if
.B $status
is non-null after executing a simple command.
.TP
.B -i
If
.B -i
is present, or
.I rc
is given no arguments and its standard input is a terminal,
it runs interactively.
Commands are prompted for using
.BR $prompt .
.TP
.B -I
Makes sure
.I rc
is not run interactively.
.TP
.B -l
If
.B -l
is given or the first character of argument zero is
.BR - ,
.I rc
reads commands from
.BR $home/lib/profile ,
if it exists, before reading its normal input.
.TP
.B -m
Read commands to initialize
.I rc
from
.I initial
instead of from
.BR /rc/lib/rcmain .
.TP
.B -p
A no-op.
.TP
.B -d
A no-op.
.TP
.B -v
Echo input on file descriptor 2 as it is read.
.TP
.B -x
Print each simple command before executing it.
.TP
.B -r
Print debugging information (internal form of commands
as they are executed).
.TP
.B -b
Compile the command file as a whole before executing.
This allows syntax checking of the whole file.
.PD
.SH FILES
.TF $home/lib/profile
.TP
.B $home/lib/profile
the user's local rc start script
.TF /rc/lib/rcmain
.TP
.B /rc/lib/rcmain
System rc start script
.TF /rc/lib/rcmain.local
.TP
.B /rc/lib/rcmain.local
Site specific system rc start script
.SH SOURCE
.B /sys/src/cmd/rc
.SH "SEE ALSO"
Tom Duff,
``Rc \- The Plan 9 Shell''.
.SH BUGS
There should be a way to match patterns against whole lists rather than
just single strings.
.PP
Using
.B ~
to check the value of
.B $status
changes
.BR $status .
.PP
Free carets don't get inserted next to keywords.