.bp
.SH
3.0\ Keyword\ parameters
.LP
Shell variables may be given values
by assignment
or when a shell procedure is invoked.
An argument to a shell procedure of the form
\fIname=value\fP
that precedes the command name
causes \fIvalue\fP
to be assigned to \fIname\fP
before execution of the procedure begins.
The value of \fIname\fP in the invoking
shell is not affected.
For example,
.DS
	user=fred\ command
.DE
will execute \fIcommand\fP with
\fBuser\fP set to \fIfred\fP.
The \fB\(mik\fR flag causes arguments of the form
\fIname=value\fP to be interpreted in this way
anywhere in the argument list.
Such \fInames\fP are sometimes
called keyword parameters.
If any arguments remain they
are available as positional
parameters \fB$1, $2, \*(ZZ\|.\fP
.LP
The \fIset\fP command
may also be used to set positional parameters
from within a procedure.
For example,
.DS
	set\ \(mi\ \*(ST
.DE
will set \fB$1\fP to the first file name
in the current directory, \fB$2\fP to the next,
and so on.
Note that the first argument, \(mi, ensures correct treatment
when the first file name begins with a \(mi\|.
.LP
.SH
3.1\ Parameter\ transmission
.LP
When a shell procedure is invoked both positional
and keyword parameters may be supplied with the call.
Keyword parameters are also made available implicitly
to a shell procedure
by specifying in advance that such parameters
are to be exported.
For example,
.DS
	export\ user\ box
.DE
marks the variables \fBuser\fP and \fBbox\fP
for export.
When a shell procedure is invoked
copies are made of all exportable variables
for use within the invoked procedure.
Modification of such variables
within the procedure does not
affect the values in the invoking shell.
It is generally true of
a shell procedure
that it
may not modify the state
of its caller without explicit
request on the part of the caller.
(Shared file descriptors are an
exception to this rule.)
.LP
Names whose value is intended to remain
constant may be declared \fIreadonly\|.\fP
The form of this command is the same as that of the \fIexport\fP
command,
.DS
	readonly name \*(ZZ
.DE
Subsequent attempts to set readonly variables
are illegal.
.SH
3.2\ Parameter\ substitution
.LP
If a shell parameter is not set
then the null string is substituted for it.
For example, if the variable \fBd\fP
is not set
.DS
	echo $d
.DE
or
.DS
	echo ${d}
.DE
will echo nothing.
A default string may be given
as in
.DS
	echo ${d\(mi\fB.\fR}
.DE
which will echo
the value of the variable \fBd\fP
if it is set and `\fB.\fP' otherwise.
The default string is evaluated using the usual
quoting conventions so that
.DS
	echo ${d\(mi\'\*(ST\'}
.DE
will echo \fB\*(ST\fP if the variable \fBd\fP
is not set.
Similarly
.DS
	echo ${d\(mi$1}
.DE
will echo the value of \fBd\fP if it is set
and the value (if any) of \fB$1\fP otherwise.
A variable may be assigned a default value
using
the notation
.DS
	echo ${d=\fB.\fR}
.DE
which substitutes the same string as
.DS
	echo ${d\(mi\fB.\fR}
.DE
and if \fBd\fP were not previously set
then it will be set to the string `\fB.\fP'\|.
(The notation ${\*(ZZ=\*(ZZ}
is not available for positional parameters.)
.LP
If there is no sensible default then
the notation
.DS
	echo ${d?message}
.DE
will echo the value of the variable \fBd\fP if it has
one, otherwise \fImessage\fP is printed by the shell and
execution of the shell procedure is abandoned.
If \fImessage\fP is absent then a standard message
is printed.
A shell procedure that requires some parameters
to be set might start as follows.
.DS
	:\ ${user?}\ ${acct?}\ ${bin?}
	\*(ZZ
.DE
Colon (\fB:\fP) is a command
that is
built in to the shell and does nothing
once its arguments have been evaluated.
If any of the variables \fBuser, acct\fP
or \fBbin\fP are not set then the shell
will abandon execution of the procedure.
.SH
3.3\ Command\ substitution
.LP
The standard output from a command can be
substituted in a similar way to parameters.
The command \fIpwd\fP prints on its standard
output the name of the current directory.
For example, if the current directory is
\fB/usr/fred/bin\fR
then the command
.DS
	d=\`pwd\`
.DE
is equivalent to
.DS
	d=/usr/fred/bin
.DE
.LP
The entire string between grave accents (\`\*(ZZ\`)
is taken as the command
to be executed
and is replaced with the output from
the command.
The command is written using the usual
quoting conventions
except that a \fB\`\fR must be escaped using
a \fB\\\|.\fR
For example,
.DS
	ls \`echo "$1"\`
.DE
is equivalent to
.DS
	ls $1
.DE
Command substitution occurs in all contexts
where parameter substitution occurs (including \fIhere\fP documents) and the
treatment of the resulting text is the same
in both cases.
This mechanism allows string
processing commands to be used within
shell procedures.
An example of such a command is \fIbasename\fP
which removes a specified suffix from a string.
For example,
.DS
	basename main\fB.\fPc \fB.\fPc
.DE
will print the string \fImain\|.\fP
Its use is illustrated by the following
fragment from a \fIcc\fP command.
.DS
	case $A in
	\*(Ca\*(ZZ
	\*(Ca\*(ST\fB.\fPc)	B=\`basename $A \fB.\fPc\`
	\*(Ca\*(ZZ
	esac
.DE
that sets \fBB\fP to the part of \fB$A\fP
with the suffix \fB.c\fP stripped.
.LP
Here are some composite examples.
.RS
.IP \(bu
.ft B
for i in \`ls \(mit\`; do \*(ZZ
.ft R
.br
The variable \fBi\fP is set
to the names of files in time order,
most recent first.
.IP \(bu
.ft B
set \`date\`; echo $6 $2 $3, $4
.ft R
.br
will print, e.g.,
.ft I
1977 Nov 1, 23:59:59
.ft R
.RE
.SH
3.4\ Evaluation\ and\ quoting
.LP
The shell is a macro processor that
provides parameter substitution, command substitution and file
name generation for the arguments to commands.
This section discusses the order in which
these evaluations occur and the
effects of the various quoting mechanisms.
.LP
Commands are parsed initially according to the grammar
given in appendix A.
Before a command is executed
the following
substitutions occur.
.RS
.IP \(bu
parameter substitution, e.g. \fB$user\fP
.IP \(bu
command substitution, e.g. \fB\`pwd\`\fP
.RS
.LP
Only one evaluation occurs so that if, for example, the value of the variable
\fBX\fP
is the string \fI$y\fP
then
.DS
	echo $X
.DE
will echo \fI$y\|.\fP
.RE
.IP \(bu
blank interpretation
.RS
.LP
Following the above substitutions
the resulting characters
are broken into non-blank words (\fIblank interpretation\fP).
For this purpose `blanks' are the characters of the string
\fB$\s-1IFS\s0\fP.
By default, this string consists of blank, tab and newline.
The null string
is not regarded as a word unless it is quoted.
For example,
.DS
	echo \'\'
.DE
will pass on the null string as the first argument to \fIecho\fP,
whereas
.DS
	echo $null
.DE
will call \fIecho\fR with no arguments
if the variable \fBnull\fP is not set
or set to the null string.
.RE
.IP \(bu
file name generation
.RS
.LP
Each word
is then scanned for the file pattern characters
\fB\*(ST, ?\fR and \fB[\*(ZZ]\fR
and an alphabetical list of file names
is generated to replace the word.
Each such file name is a separate argument.
.RE
.RE
.LP
The evaluations just described also occur
in the list of words associated with a \fBfor\fP
loop.
Only substitution occurs
in the \fIword\fP used
for a \fBcase\fP branch.
.LP
As well as the quoting mechanisms described
earlier using \fB\\\fR and \fB\'\*(ZZ\'\fR
a third quoting mechanism is provided using double quotes.
Within double quotes parameter and command substitution
occurs but file name generation and the interpretation
of blanks does not.
The following characters
have a special meaning within double quotes
and may be quoted using \fB\\\|.\fP
.DS
	\fB$	\fPparameter substitution
	\fB\`\fP	command substitution
	\fB"\fP	ends the quoted string
	\fB\e\fP	quotes the special characters \fB$ \` " \e\fP
.DE
For example,
.DS
	echo "$x"
.DE
will pass the value of the variable \fBx\fP as a
single argument to \fIecho.\fP
Similarly,
.DS
	echo "$\*(ST"
.DE
will pass the positional parameters as a single
argument and is equivalent to
.DS
	echo "$1 $2 \*(ZZ"
.DE
The notation \fB$@\fP
is the same as \fB$\*(ST\fR
except when it is quoted.
.DS
	echo "$@"
.DE
will pass the positional parameters, unevaluated, to \fIecho\fR
and is equivalent to
.DS
	echo "$1" "$2" \*(ZZ
.DE
.LP
The following table gives, for each quoting mechanism,
the shell metacharacters that are evaluated.
.DS
.ce
.ft I
metacharacter
.ft
.in 1.5i
	\e	$	*	\`	"	\'
\'	n	n	n	n	n	t
\`	y	n	n	t	n	n
"	y	y	n	y	t	n

	t	terminator
	y	interpreted
	n	not interpreted

.in
.ft B
.ce
Figure 2. Quoting mechanisms
.ft
.DE
.LP
In cases where more than one evaluation of a string
is required the built-in command \fIeval\fP
may be used.
For example,
if the variable \fBX\fP has the value
\fI$y\fP, and if \fBy\fP has the value \fIpqr\fP
then
.DS
	eval echo $X
.DE
will echo the string \fIpqr\|.\fP
.LP
In general the \fIeval\fP command
evaluates its arguments (as do all commands)
and treats the result as input to the shell.
The input is read and the resulting command(s)
executed.
For example,
.DS
	wg=\\'eval who\*(VTgrep\\'
	$wg fred
.DE
is equivalent to
.DS
	who\*(VTgrep fred
.DE
In this example,
\fIeval\fP is required
since there is no interpretation
of metacharacters, such as \fB\*(VT\|,\fP following
substitution.
.SH
3.5\ Error\ handling
.LP
The treatment of errors detected by
the shell depends on the type of error
and on whether the shell is being
used interactively.
An interactive shell is one whose
input and output are connected
to a terminal (as determined by
\fIgtty\fP (2)).
A shell invoked with the \fB\(mii\fP
flag is also interactive.
.LP
Execution of a command (see also 3.7) may fail
for any of the following reasons.
.IP \(bu
Input output redirection may fail.
For example, if a file does not exist
or cannot be created.
.IP \(bu
The command itself does not exist
or cannot be executed.
.IP \(bu
The command terminates abnormally,
for example, with a "bus error"
or "memory fault".
See Figure 2 below for a complete list
of UNIX signals.
.IP \(bu
The command terminates normally
but returns a non-zero exit status.
.LP
In all of these cases the shell
will go on to execute the next command.
Except for the last case an error
message will be printed by the shell.
All remaining errors cause the shell
to exit from a command procedure.
An interactive shell will return
to read another command from the terminal.
Such errors include the following.
.IP \(bu
Syntax errors.
e.g., if \*(ZZ then \*(ZZ done
.IP \(bu
A signal such as interrupt.
The shell waits for the current
command, if any, to finish execution and
then either exits or returns to the terminal.
.IP \(bu
Failure of any of the built-in commands
such as \fIcd.\fP
.LP
The shell flag \fB\(mie\fP
causes the shell to terminate
if any error is detected.
.DS
1	hangup
2	interrupt
3*	quit
4*	illegal instruction
5*	trace trap
6*	IOT instruction
7*	EMT instruction
8*	floating point exception
9	kill (cannot be caught or ignored)
10*	bus error
11*	segmentation violation
12*	bad argument to system call
13	write on a pipe with no one to read it
14	alarm clock
15	software termination (from \fIkill\fP (1))

.DE
.ft B
.ce
Figure 3. UNIX signals
.ft

Those signals marked with an asterisk
produce a core dump
if not caught.
However,
the shell itself ignores quit which is the only
external signal that can cause a dump.
The signals in this list of potential interest
to shell programs are 1, 2, 3, 14 and 15.
.SH
3.6\ Fault\ handling
.LP
Shell procedures normally terminate
when an interrupt is received from the
terminal.
The \fItrap\fP command is used
if some cleaning up is required, such
as removing temporary files.
For example,
.DS
	trap\ \'rm\ /tmp/ps$$; exit\'\ 2
.DE
sets a trap for signal 2 (terminal
interrupt), and if this signal is received
will execute the commands
.DS
	rm /tmp/ps$$; exit
.DE
\fIexit\fP is
another built-in command
that terminates execution of a shell procedure.
The \fIexit\fP is required; otherwise,
after the trap has been taken,
the shell will resume executing
the procedure
at the place where it was interrupted.
.LP
UNIX signals can be handled in one of three ways.
They can be ignored, in which case
the signal is never sent to the process.
They can be caught, in which case the process
must decide what action to take when the
signal is received.
Lastly, they can be left to cause
termination of the process without
it having to take any further action.
If a signal is being ignored
on entry to the shell procedure, for example,
by invoking it in the background (see 3.7) then \fItrap\fP
commands (and the signal) are ignored.
.LP
The use of \fItrap\fP is illustrated
by this modified version of the \fItouch\fP
command (Figure 4).
The cleanup action is to remove the file \fBjunk$$\fR\|.
.DS
	flag=
	trap\ \'rm\ \(mif\ junk$$;\ exit\'\ 1 2 3 15
	for i
	do\ case\ $i\ in
	\*(DC\(mic)	flag=N ;;
	\*(DC\*(ST)	if\ test\ \(mif\ $i
	\*(DC	then	ln\ $i\ junk$$;\ rm\ junk$$
	\*(DC	elif\ test\ $flag
	\*(DC	then	echo\ file\ \\\\\'$i\\\\\'\ does\ not\ exist
	\*(DC	else	>$i
	\*(DC	fi
	\*(DOesac
	done
.DE
.sp
.ft B
.ce
Figure 4. The touch command
.ft
.sp
The \fItrap\fP command
appears before the creation
of the temporary file;
otherwise it would be
possible for the process
to die without removing
the file.
.LP
Since there is no signal 0 in UNIX
it is used by the shell to indicate the
commands to be executed on exit from the
shell procedure.
.LP
A procedure may, itself, elect to
ignore signals by specifying the null
string as the argument to trap.
The following fragment is taken from the
\fInohup\fP command.
.DS
	trap \'\' 1 2 3 15
.DE
which causes \fIhangup, interrupt, quit \fRand\fI kill\fR
to be ignored both by the
procedure and by invoked commands.
.LP
Traps may be reset by saying
.DS
	trap 2 3
.DE
which resets the traps for signals 2 and 3 to their default values.
A list of the current values of traps may be obtained
by writing
.DS
	trap
.DE
.LP
The procedure \fIscan\fP (Figure 5) is an example
of the use of \fItrap\fP where there is no exit
in the trap command.
\fIscan\fP takes each directory
in the current directory, prompts
with its name, and then executes
commands typed at the terminal
until an end of file or an interrupt is received.
Interrupts are ignored while executing
the requested commands but cause
termination when \fIscan\fP is
waiting for input.
.DS
	d=\`pwd\`
	for\ i\ in\ \*(ST
	do\ if\ test\ \(mid\ $d/$i
	\*(DOthen\ cd\ $d/$i
	\*(DO\*(THwhile\ echo\ "$i:"
	\*(DO\*(TH\*(WHtrap\ exit\ 2
	\*(DO\*(TH\*(WHread\ x
	\*(DO\*(THdo\ trap\ :\ 2;\ eval\ $x;\ done
	\*(DOfi
	done
.DE
.sp
.ft B
.ce
Figure 5. The scan command
.ft
.sp
\fIread x\fR is a built-in command that reads one line from the
standard input
and places the result in the variable \fBx\|.\fP
It returns a non-zero exit status if either
an end-of-file is read or an interrupt
is received.
.SH
3.7\ Command\ execution
.LP
To run a command (other than a built-in) the shell first creates
a new process using the system call \fIfork.\fP
The execution environment for the command
includes input, output and the states of signals, and
is established in the child process
before the command is executed.
The built-in command \fIexec\fP
is used in the rare cases when no fork
is required
and simply replaces the shell with a new command.
For example, a simple version of the \fInohup\fP
command looks like
.DS
	trap \\'\\' 1 2 3 15
	exec $\*(ST
.DE
The \fItrap\fP turns off the signals specified
so that they are ignored by subsequently created commands
and \fIexec\fP replaces the shell by the command
specified.
.LP
Most forms of input output redirection have already been
described.
In the following \fIword\fP is only subject
to parameter and command substitution.
No file name generation or blank interpretation
takes place so that, for example,
.DS
		echo \*(ZZ >\*(ST.c
.DE
will write its output into a file whose name is \fB\*(ST.c\|.\fP
Input output specifications are evaluated left to right
as they appear in the command.
.IP >\ \fIword\fP 12
The standard output (file descriptor 1)
is sent to the file \fIword\fP which is
created if it does not already exist.
.IP \*(AP\ \fIword\fP 12
The standard output is sent to file \fIword.\fP
If the file exists then output is appended
(by seeking to the end);
otherwise the file is created.
.IP <\ \fIword\fP 12
The standard input (file descriptor 0)
is taken from the file \fIword.\fP
.IP \*(HE\ \fIword\fP 12
The standard input is taken from the lines
of shell input that follow up to but not
including a line consisting only of \fIword.\fP
If \fIword\fP is quoted then no interpretation
of the document occurs.
If \fIword\fP is not quoted
then parameter and command substitution
occur and \fB\\\fP is used to quote
the characters \fB\\\fP \fB$\fP \fB\`\fP and the first character
of \fIword.\fP
In the latter case \fB\\newline\fP is ignored (c.f. quoted strings).
.IP >&\ \fIdigit\fP 12
The file descriptor \fIdigit\fP is duplicated using the system
call \fIdup\fP (2)
and the result is used as the standard output.
.IP <&\ \fIdigit\fP 12
The standard input is duplicated from file descriptor \fIdigit.\fP
.IP <&\(mi 12
The standard input is closed.
.IP >&\(mi 12
The standard output is closed.
.LP
Any of the above may be preceded by a digit in which
case the file descriptor created is that specified by the digit
instead of the default 0 or 1.
For example,
.DS
	\*(ZZ 2>file
.DE
runs a command with message output (file descriptor 2)
directed to \fIfile.\fP
.DS
	\*(ZZ 2>&1
.DE
runs a command with its standard output and message output
merged.
(Strictly speaking file descriptor 2 is created
by duplicating file descriptor 1 but the effect is usually to
merge the two streams.)
.LP
The environment for a command run in the background such as
.DS
	list \*(ST.c \*(VT lpr &
.DE
is modified in two ways.
Firstly, the default standard input
for such a command is the empty file \fB/dev/null\|.\fR
This prevents two processes (the shell and the command),
which are running in parallel, from trying to
read the same input.
Chaos would ensue
if this were not the case.
For example,
.DS
	ed file &
.DE
would allow both the editor and the shell
to read from the same input at the same time.
.LP
The other modification to the environment of a background
command is to turn off the QUIT and INTERRUPT signals
so that they are ignored by the command.
This allows these signals to be used
at the terminal without causing background
commands to terminate.
For this reason the UNIX convention
for a signal is that if it is set to 1
(ignored) then it is never changed
even for a short time.
Note that the shell command \fItrap\fP
has no effect for an ignored signal.
.SH
3.8\ Invoking\ the\ shell
.LP
The following flags are interpreted by the shell
when it is invoked.
If the first character of argument zero is a minus,
then commands are read from the file \fB.profile\|.\fP
.IP \fB\(mic\fP\ \fIstring\fP
.br
If the \fB\(mic\fP flag is present then
commands are read from \fIstring\|.\fP
.IP \fB\(mis\fP
If the \fB\(mis\fP flag is present or if no
arguments remain
then commands are read from the standard input.
Shell output is written to
file descriptor 2.
.IP \fB\(mii\fP
If the \fB\(mii\fP flag is present or
if the shell input and output are attached to a terminal (as told by \fIgtty\fP)
then this shell is \fIinteractive.\fP
In this case TERMINATE is ignored (so that \fBkill 0\fP
does not kill an interactive shell) and INTERRUPT is caught and ignored
(so that \fBwait\fP is interruptable).
In all cases QUIT is ignored by the shell.
.SH
Acknowledgements
.LP
The design of the shell is
based in part on the original UNIX shell
.[
unix command language thompson
.]
and the PWB/UNIX shell,
.[
pwb shell mashey unix
.]
some
features having been taken from both.
Similarities also exist with the
command interpreters
of the Cambridge Multiple Access System
.[
cambridge multiple access system hartley
.]
and of CTSS.
.[
ctss
.]
.LP
I would like to thank Dennis Ritchie
and John Mashey for many
discussions during the design of the shell.
I am also grateful to the members of the Computing Science Research Center
and to Joe Maranzano for their
comments on drafts of this document.
.SH
.[
$LIST$
.]
