ref: 866d74c0c4bb50e85e9e8bb95140c10d409e53be
dir: /man/1/sh/
.TH SH 1
.SH NAME
sh, builtin, exit, load, loaded, local, whatis, quote, run, set, unload, unquote \- command language
.SH SYNOPSIS
.B sh
[
.B -ilxvn
]
[
.B -c command
]
[
.I file
[
.I arg ...
]]
.SH DESCRIPTION
.I Sh
is a programmable user level interface (a shell) for Inferno.
It executes command lines read from a terminal or a file or, with the
.B -c
flag, from
.IR sh 's
argument list. It can also be used to give programmable functionality
to Limbo modules (see
.IR sh (2)).
.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 Sh
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 sh
variable
.BR $apid .
Whenever a command
.I not
followed by
.B &
exits or is terminated, the
.I sh
variable
.B $status
gets the process's wait message (see
.IR prog (3));
it will be the null string if the command was successful.
.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 a
.I sh
builtin or it is a braced command block (see
.IR "Compound Commands",
below), it is executed by
.IR sh .
If the first character of the name is a brace
.RB ( { ),
the shell tries to parse it and execute it as a braced command block;
if the parsing fails, an exception is raised.
Otherwise
.I sh
looks for an external program to execute.
.PP
If the name ends in
.BR .dis ,
.I sh
looks for a Dis module of that name; otherwise
it tries first to find a Dis module of
that name with
.B .dis
appended and failing that, it looks for
an executable file of the same name, which should
be a readable, executable script file.
If the name does not start with a slash
.RB ( / )
or dot-slash
.RB ( ./ ),
then the name is first looked for relative to
.BR /dis ,
and then relative to the current directory.
A Dis module will be executed only if it
implements the
.B Command
interface (see
.IR sh (1));
a script file will be executed only if it
starts with the characters
.RB `` #! ''
followed by the name of a file executable
under the rules above. In this case the
command will be executed with any following arguments mentioned
in the
.B #!
header, followed by the path of the script file,
followed by any arguments originally given to the command.
.PP
For example, to execute the simple command
.BR "ls" ,
.I sh
will look for one of the following things, in order,
stopping the search when one is found:
.RS
.IP 1)
a built-in command named
.RB `` ls ''.
.IP 2)
a Dis module named
.RB `` /dis/ls.dis '',
.IP 3)
an executable script file named
.RB `` /dis/ls '',
.IP 4)
a Dis module named
.RB `` ./ls.dis '',
.IP 5)
an executable script file named
.RB `` ./ls ''.
.RE
.SS Arguments and Variables
A number of constructions may be used where
.I sh'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. (As this character
is special to the shell, it may only be included in a pattern
if this character is quoted, as long as the leading
.B [
is not quoted).
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.
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^/*.b
.EE
matches
.BR /tmp/*.b ,
rather than matching
.B "/*.b
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
.ne 3
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)
echo (hi
there
everybody
)
.EE
Newlines within parentheses count as simple white space;
they do not terminate the command. This can be useful to give
some more freedom of layout to commands that take several
commands as arguments, for instance several of the commands
defined in
.IR sh-std (1).
.HP
.BI $ argument
.br
The
.I argument
after the
.B $
is the name of a variable whose value is substituted.
Multiple levels
of indirection are possible.
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.
Assignments to variables are described under
.I "Assignment" ,
below.
.HP
.BI $# argument
.br
The value is the number of elements in the named variable.
A variable
never assigned a value has zero elements.
.HP
\f5$"\fP\fIargument\fP
.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
.I
\f5"{\fPcommand\f5}\fP
.br
.I Sh
executes the
.I command
and reads its standard output. If backquote
.RB ( ` )
is used, it is split 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'" .
If doublequote (\f5"\fP)
is used, no tokenization takes place.
.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.
.HP
.BI ${ command }
.br
.I Command
must be a simple command with no redirections;
its first word
must be the name of a builtin substitution operator.
The operator is invoked and its value substituted.
See
.IR "Built-in Commands" ,
below, for more information on builtins.
.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
.PD
.SS Free Carets
In most circumstances,
.I sh
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 limbo -$flags $stem.b
.LP
is equivalent to
.IP
.B limbo -^$flags $stem^.b
.SS Assignment
A command of the form
.IB name = value
or
.IB name := value
assigns
.I value
to the environment variable named
.IR name .
.I Value
is either a list of arguments or an assignment statement. In
the latter case
.I value
is taken from the value assigned in the assignment statement.
If
.B :=
is used, the value is stored in the innermost local scope.
A local scope is created every time a braced block is entered,
and destroyed when the block is left. If
.B =
is used, the value is stored in the innermost scope
that contains any definition of
.IR name .
.PP
A list of names can also be used in place of
.IR name ,
which causes each element of
.I value
in turn to be assigned the respective variable name in
the list. The last variable in the list is assigned any elements
that are left over. If there are more variable names than
elements in
.IR value ,
the remaining elements are assigned the null list.
For instance, after the assignment:
.EX
(a b c) = one two three four five
.EE
.B $a
is
.BR one ,
.B $b
is
.BR two ,
and
.B $c
contains the remaining three elements
.BR "(three four five)" .
.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 by the sequence
.BI <> file \f1,
which opens the file for writing as well as reading.
Note that if
.I file
is in fact a parsed braced block, the redirection will be treated as
pipe to the given command - it is identical to the
.B "<{}"
operator mentioned above.
.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 "limbo junk.b >[2] junk" .
.PP
A file descriptor may be redirected to an already open descriptor by writing
.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 sys-dup (2))
of it.
.PP
Redirections are executed from left to right.
Therefore,
.B limbo junk.b >/dev/null >[2=1]
and
.B limbo junk.b >[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 .
.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 sequence of commands separated by
.BR & ,
.BR ; ,
or newline
may be grouped by surrounding
them with braces
.RB ( {} ),
elsewhere referred to as a
.IR "braced block" .
A braced block may be used anywhere that a simple word
is expected. If a simple command is found with
a braced block as its first word, the
variable
.B $*
is set to any following arguments,
.B $0
is set to the block itself, and the commands
are executed in sequence. If a braced block
is passed as an argument, no execution takes place:
the block is converted to a functionally equivalent
string, suitable for later re-interpretation by the shell.
The null command
.RB ( {} )
has no effect and always gives a nil status. For instance
the following commands all produce the same result:
.EX
echo hello world
{echo hello world}
'{echo hello world}'
{echo $*} hello world
sh -c {echo hello world}
{$*} {echo hello world}
{$*} {{$*} {echo hello world}}
"{echo {echo hello world}}
'{echo hello' ^ ' world}'
x := {echo hello world}; $x
.EE
It is important to note that the value of
.B $*
is lost every time a braced block is entered, so
for instance, the following command prints an empty string:
.EX
{{echo $*}} hello world
.EE
.PD
.SS Built-in Commands
The term ``built-in command'', or just ``builtin'', is used somewhat loosely
in this document to refer to any command that is executed
directly by the shell; most built-in commands are defined
by externally loaded modules; there are a few that are not,
known as ``internal'' builtins, listed below.
.PP
Given
.IR sh 's
ability to pass compound commands (braced blocks) as
arguments to other commands, most control-flow
functionality that is traditionally hard-wired into a shell
is in
.I sh
implemented by loadable modules. See
.IR sh-std (1),
.IR sh-expr (1),
and
.IR sh-tk (1)
for more details.
.PP
There are two classes of built-in commands;
the first class, known simply as ``builtins'', are used in
the same way as normal commands, the only difference
being that builtins can raise exceptions, while external
commands cannot, as they are run in a separate process.
The second class, known as
``builtin substitutions'' can only be used as the first
word of the command in the
.B ${}
operator. The two classes exist in different name-spaces:
a builtin may do something quite different from a
builtin substitution of the same name.
.PP
In general, normal builtins perform some action
or test some condition;
the return status of a normal builtin usually
indicates error status or conditional success. The
rôle of a substitution builtin is to yield a value,
(possibly a list)
which is substituted directly into place as part
of the argument list of a command.
.PP
.PD 0
.HP
.BI @ " command ..."
.br
Execute
.I command
in a subshell, allowing (for instance) the name-space to be
forked independently of main shell.
.HP
.BI run " file ..."
.br
Execute commands from
.IR file .
.B $*
is set for the duration to the remainder of the argument list following
.IR file .
.HP
.BI builtin " command ..."
.br
Execute
.I command
as usual except that any command defined by an external
module is ignored in favour of the original meaning.
This command cannot be redefined by an external module.
.HP
.B exit
.br
Terminate the current process.
.HP
.BI load " path..."
.br
.B Load
tries to load each of its arguments as a builtin module
into
.IR sh .
If a module load succeeds, each builtin
command defined by that module is
added to the list of builtin commands.
If there was a previous definition of the command,
it is replaced, with the exception of internal
.I sh
builtins, which are covered up and reappear when
the module is unloaded. If a module with
the same
.I path
has already been loaded,
.I sh
does not try to load it again.
Unless the path begins with
.B /
or
.BR ./ ,
the shell looks in the standard builtins directory
.B /dis/sh
for the module.
If a load fails, a
.B bad module
exception is raised.
The environment variable
.B $autoload
can be set to a list of Shell modules that
each instance of
.I sh
should load automatically during its initialisation.
(More precisely, the modules are loaded
when a new
.B Sh->Context
is created: see
.IR sh (2)
for details.)
.HP
.BI unload " path..."
.br
.B Unload
undoes previous load commands. To succeed,
.I path
must be the same as that given to a previous
invocation of
.BR load .
.HP
.B loaded
.br
.B Loaded
prints all the builtin commands currently
defined, along with the name of the module that defined them.
Internally defined commands are tagged with
module
.BR builtin .
.HP
.BI whatis " name ..."
.br
Print the value of each
.I name
in a form suitable for input to
.IR sh .
The forms are:
.RS 10
.TP
.I varname = "value..."
.I Varname
is a non-nil environment variable.
.TP
.BI load\ module ;\ name
.I Name
has been defined as a builtin by the externally loaded
.IR module .
.TP
.BI load\ module ;\ ${ name }
.I Name
has been defined as a builtin substitution by the externally loaded
.IR module .
.TP
.BI builtin\ name
.I Name
is defined as a builtin internally by
.IR sh .
.TP
.BI ${ name }
.I Name
is defined as a builtin substitution
internally by the shell.
.TP
.I pathname
The completed pathname of an external file.
.RE
.HP
.B ${builtin
.I command
...
.B }
.br
Does for substitution builtin commands
what
.B builtin
does for normal commands.
.HP
.B ${loaded}
.br
The
.B loaded
builtin substitution yields a list of the names of all
the modules currently loaded, as passed to
.BR load .
.HP
.BI ${quote \ list }
.br
.B Quote
yields a single element
list which if reparsed by the shell
will recreate
.IR list .
.HP
.BI ${bquote \ list }
.br
Same as
.B quote
except that items in
.I list
that are known to be
well-formed command blocks are not quoted.
.HP
.BI ${unquote \ arg}
.br
.B Unquote
reverses the operation of
.BR quote ,
yielding the original list of values. For example,
.BI "${unquote ${quote " list }}
yields
.IR list .
A list quoted with
.B bquote
can only be unquoted by parsing.
.PD
.SS Environment
The
.I environment
is a list of strings made available to externally executing commands by the
.B env
module
(see
.IR env (2)).
If the
.B env
module does not exist or cannot be loaded, no error will be
reported, but no variables can be exported to external commands.
.I Sh
creates an environment entry for each variable whose value is non-empty.
This is formatted as if it had been run through
.BR ${quote} .
Note that in order for a variable to be exported, its
name must conform to the restrictions imposed
by
.IR env (3);
names that do not will not be exported.
.PP
When
.I sh
starts executing it reads variable definitions from its
environment.
.PP
Internally, the shell holds a
.IR context ,
which holds a stack of environment variables, the
current execution flags and the list of built-in modules.
A copy is made whereever parallel access to the context might
occur. This happens for processes executing
in a pipeline,
processes run asynchronously with
.BR & ,
and in any builtin command that runs a shell command
asynchronously.
.SS Exceptions
When
.I sh
encounters an error processing its input, an exception is raised,
and if the
.B -v
flag is set, an error message is printed to
standard error.
An exception causes processing of the current command to terminate
and control to be transferred back up the invocation stack.
In an interactive shell, the central command processing loop
catches all exceptions and sets
.B $status
to the name of the exception.
Exceptions are not propagated between processes. Any
command that requires I/O redirection is run in a separate
process, namely pipes
.RB ( | ),
redirections
.RB ( > ,
.BR < ,
.BR >> ,
and
.BR <> ),
backquote substitution
(\f5`\fP, \f5"\fP)
and background processes
.RB ( & ).
Exceptions can be raised and rescued using
the
.B raise
and
.B rescue
functions in the standard builtins module,
.BR std .
(See
.IR sh-std (1)).
Names of exceptions raised by
.I sh
include:
.TP 10
.B parse error
An error has occurred trying to parse a command.
.TP
.B usage
A builtin has been passed an invalid set of arguments;
.TP
.B bad redir
An error was encountered trying to open files prior
to running a process.
.TP
.B bad $ arg
An invalid name was given to the $ or ${} operator.
.TP
.B no pipe
.I Sh
failed to make a pipe.
.TP
.B bad wait read
An error occurred while waiting for a process to exit.
.TP
.B builtin not found
A substitution builtin was named but not found.
.SS Special Variables
The following variables are set or used by
.IR sh .
.PD 0
.TP \w'\fL$promptXX'u
.B $*
Set to
.IR sh 's
argument list during initialization.
Whenever a
braced block
is executed, the current value is saved and
.B $*
receives the new argument list.
The saved value is restored on completion of the
.BR block .
.TP
.B $apid
Whenever a process is started asynchronously with
.BR & ,
.B $apid
is set to its process id.
.TP
.B $ifs
The input field separators used in backquote substitutions.
If
.B $ifs
is not set in
.IR sh 's
environment, it is initialized to blank, tab and newline.
.TP
.B $prompt
When
.I sh
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,
the return status of the last-executed builtin
(unless started with
.BR &),
or the name of the last-raised exception, whichever
is most recent.
When
.I sh
exits at end-of-file of its input,
.B $status
is its exit status.
.PD
.SS Invocation
If
.I sh
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 Sh
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 -i
If
.B -i
is present, or
.I sh
is given no arguments and its standard input is a terminal,
it runs interactively.
Commands are prompted for using
.BR $prompt .
This option implies
.BR -v .
.TP
.B -l
If
.B -l
is given
.I sh
reads commands from
.BR /lib/sh/profile ,
if it exists, and then
.BR ./lib/profile ,
if it exists, before reading its normal input.
.TP
.B -n
Normally,
.I sh
forks its namespace on startup; if
.B -n
is given, this behaviour is suppressed.
.TP
.B -v
Within a non-interactive shell, informational messages
printed to standard error are usually disabled;
giving the
.B -v
flag enables them.
.TP
.B -x
Print each simple command to stderr before executing it.
.PD
.SH SOURCE
.B /appl/cmd/sh/sh.y
.SH "SEE ALSO"
.IR sh (1),
.IR sh-std (1),
.IR sh-expr (1),
.IR sh-file2chan (1),
.IR sh-tk (1),
.IR sh-arg (1),
.IR sh-regex (1),
.IR sh-string (1),
.IR sh-csv (1),
.IR sh (2),
.IR env (2)
.SH BUGS
Due to lack of system support, appending to
a file with
.B >>
will not work correctly when there are
multiple concurrent writers (but see the
examples section of
.IR sh-file2chan (1)
for one solution to this).
.PP
While it
.I is
possible to use the shell as a general
purpose programming language, it is a very slow one!
Intensive tasks are best done in Limbo, which is a much
safer language to boot.