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-.\"	$NetBSD: t3,v 1.3 2010/08/22 02:19:07 perry Exp $
-.\"
-.\" Copyright (C) Caldera International Inc. 2001-2002.  All rights reserved.
-.\"
-.\" Redistribution and use in source and binary forms, with or without
-.\" modification, are permitted provided that the following conditions are
-.\" met:
-.\"
-.\" Redistributions of source code and documentation must retain the above
-.\" copyright notice, this list of conditions and the following
-.\" disclaimer.
-.\"
-.\" Redistributions in binary form must reproduce the above copyright
-.\" notice, this list of conditions and the following disclaimer in the
-.\" documentation and/or other materials provided with the distribution.
-.\"
-.\" All advertising materials mentioning features or use of this software
-.\" must display the following acknowledgement:
-.\"
-.\" This product includes software developed or owned by Caldera
-.\" International, Inc.  Neither the name of Caldera International, Inc.
-.\" nor the names of other contributors may be used to endorse or promote
-.\" products derived from this software without specific prior written
-.\" permission.
-.\"
-.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
-.\" INTERNATIONAL, INC.  AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
-.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
-.\" DISCLAIMED.  IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
-.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
-.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
-.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
-.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
-.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
-.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-.\"
-.\"	@(#)t3	8.1 (Berkeley) 6/8/93
-.\"
-.SH
-3.0\ Keyword\ parameters
-.LP
-Shell variables may be given values
-by assignment
-or when a shell script is invoked.
-An argument to a command of the form
-\fIname=value\fP
-that precedes the command name
-causes \fIvalue\fP
-to be assigned to \fIname\fP
-before execution of the command 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.
-.\" Removed by Perry Metzger because -k is not in POSIX
-.\"
-.\" 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 script.
-For example,
-.DS
-	set\ \(mi\(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\(mi, ensures correct treatment
-when the first file name begins with a \(mi\|.
-.LP
-.SH
-3.1\ Parameter\ transmission
-.LP
-When a command is executed both positional parameters
-and shell variables may be set on invocation.
-Variables are also made available implicitly
-to a command
-by specifying in advance that such parameters
-are to be exported from the invoking shell.
-For example,
-.DS
-	export\ user\ box=red
-.DE
-marks the variables \fBuser\fP and \fBbox\fP
-for export (setting \fBbox\fP to ``red'' in the process).
-When a command is invoked
-copies are made of all exportable variables
-(also known as \fIenvironment variables\fP)
-for use within the invoked program.
-Modification of such variables
-within an invoked command does not
-affect the values in the invoking shell.
-It is generally true of
-a shell script or other program
-that it
-cannot modify the state
-of its caller without explicit
-actions on the part of the caller.
-.\" Removed by Perry Metzger because this is confusing to beginners.
-.\"
-.\" (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[=value] \*(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 not null 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 or null.
-Similarly
-.DS
-	echo ${d:\(mi$1}
-.DE
-will echo the value of \fBd\fP if it is set and not null
-and the value (if any) of \fB$1\fP otherwise.
-.LP
-The notation ${d:+\fB.\fR} performs the inverse operation. It
-substitutes `\fB.\fP' if \fBd\fP is set or not null, and otherwise
-substitutes null.
-.LP
-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 or null
-then it will be set to the string `\fB.\fP'\|.
-.LP
-If there is no sensible default then
-the notation
-.DS
-	echo ${d:?\fImessage\fP}
-.DE
-will echo the value of the variable \fBd\fP if it is set and not null,
-otherwise \fImessage\fP is printed by the shell and
-execution of the shell script is abandoned.
-If \fImessage\fP is absent then a standard message
-is printed.
-A shell script that requires some variables
-to be set might start as follows:
-.DS
-	:\ ${user:?}\ ${acct:?}\ ${bin:?}
-	\*(ZZ
-.DE
-Colon (\fB:\fP) is a command
-that is
-built in to the shell and does nothing
-once its arguments have been evaluated.
-If any of the variables \fBuser, acct\fP
-or \fBbin\fP are not set then the shell
-will abandon execution of the script.
-.SH
-3.3\ Command\ substitution
-.LP
-The standard output from a command can be
-substituted in a similar way to parameters.
-The command \fIpwd\fP prints on its standard
-output the name of the current directory.
-For example, if the current directory is
-\fB/usr/fred/bin\fR
-then the commands
-.DS
-	d=$(pwd)
-.DE
-(or the older notation d=\`pwd\`)
-is equivalent to
-.DS
-	d=/usr/fred/bin
-.DE
-.LP
-The entire string inside $(\*(ZZ)\| (or between grave accents \`\*(ZZ\`)
-is taken as the command
-to be executed
-and is replaced with the output from
-the command.
-(The difference between the $(\*(ZZ) and \`\*(ZZ\` notations is that
-the former may be nested, while the latter cannot be.)
-.LP
-The command is written using the usual quoting conventions,
-except that inside \`\*(ZZ\`
-a \fB\`\fR must be escaped using
-a \fB\\\|\fR.
-For example,
-.DS
-	ls $(echo "$HOME")
-.DE
-is equivalent to
-.DS
-	ls $HOME
-.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 scripts.
-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 \(mi\(mi\| \`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\ Arithmetic\ Expansion
-.LP
-Within a $((\*(ZZ)) construct, integer arithmetic operations are
-evaluated.
-(The $ in front of variable names is optional within $((\*(ZZ)).
-For example:
-.DS
-	x=5; y=1
-	echo $(($x+3*2))
-	echo $((y+=x))
-	echo $y
-.DE
-will print `11', then `6', then `6' again.
-Most of the constructs permitted in C arithmetic operations are
-permitted though some (like `++') are not universally supported \(em
-see the shell manual page for details.
-.SH
-3.5\ 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 or \fB\`pwd\`\fP
-.IP \(bu
-arithmetic expansion, e.g. \fB$(($count+1))\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$()\fP	command substitution
-	\fB\`\fP	command substitution
-	\fB"\fP	ends the quoted string
-	\fB\e\fP	quotes the special characters \fB$ \` " \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\|\fR, following
-substitution.
-.SH
-3.6\ 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.
-.\" Removed by Perry Metzger, obsolete and excess detail
-.\"
-.\" (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 script.
-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\(dg
-.ft
-.FS
-\(dg Additional signals have been added in modern Unix.
-See \fIsigvec\fP(2) or \fIsignal\fP(3) for an up-to-date list.
-.FE
-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
-3.7\ Fault\ handling
-.LP
-shell scripts 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 script.
-The \fIexit\fP is required; otherwise,
-after the trap has been taken,
-the shell will resume executing
-the script
-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 script, 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
-	#!/bin/sh
-
-	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	cp\ $i\ junk$$;\ mv\ junk$$ $i
-	\*(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 script.
-.LP
-A script 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
-script 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 script \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
-3.8\ 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.)
-.\" Removed by Perry Metzger, most of this is now obsolete
-.\"
-.\" .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
-3.9\ 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
-3.10\ Job\ Control
-.LP
-When a command or pipeline (also known as a \fIjob\fP) is running in
-the foreground, entering the stop character (typically
-\s-1CONTROL-Z\s0 but user settable with the \fIstty\fP(1) command)
-will usually cause the job to stop.
-.LP
-The jobs associated with the current shell may be listed by entering
-the \fIjobs\fP command.
-Each job has an associated \fIjob number\fP.
-Jobs that are stopped may be continued by entering
-.DS
-	bg %\fIjobnumber\fP
-.DE
-and jobs may be moved to the foreground by entering
-.DS
-	fg %\fIjobnumber\fP
-.DE
-If there is a sole job with a particular name (say only one instance
-of \fIcc\fP running), \fIfg\fP and \fIbg\fP may also use name of the
-command in place of the number, as in:
-.DS
-	bg %cc
-.DE
-If no `\fB%\fP' clause is entered, most recently stopped job
-(indicated with a `+' by the \fIjobs\fP command) will be assumed.
-See the manual page for the shell for more details.
-.SH
-3.11\ Aliases
-.LP
-The \fIalias\fP command creates a so-called shell alias, which is an
-abbreviation that macro-expands at run time into some other command.
-For example:
-.DS
-	alias ls="ls -F"
-.DE
-would cause the command sequence \fBls -F\fP to be executed whenever
-the user types \fBls\fP into the shell.
-Note that if the user types \fBls -a\fP, the shell will in fact
-execute \fBls -F -a\fP.
-The command \fBalias\fP on its own prints out all current aliases.
-The \fIunalias\fP command, as in:
-.DS
-	unalias ls
-.DE
-will remove an existing alias.
-Aliases can shadow pre-existing commands, as in the example above.
-They are typically used to override the interactive behavior of
-commands in small ways, for example to always invoke some program with
-a favorite option, and are almost never found in scripts.
-.SH
-3.12\ Command\ Line\ Editing\ and\ Recall
-.LP
-When working interactively with the shell, it is often tedious to
-retype previously entered commands, especially if they are complicated.
-The shell therefore maintains a so-called \fIhistory\fP, which is
-stored in the file specified by the \fB\s-1HISTFILE\s0\fP environment
-variable if it is set.
-Users may view, edit, and re-enter previous lines of input using
-a small subset of the commands of the \fIvi\fP(1) or
-\fIemacs\fP(1)\(dg editors.
-.FS
-Technically, vi command editing is standardized by POSIX while emacs
-is not.
-However, all modern shells support both styles.
-.FE
-Emacs style editing may be selected by entering
-.DS
-	set -o emacs
-.DE
-and vi style editing may be selected with
-.DS
-	set -o vi
-.DE
-The details of how command line editing works are beyond the scope of
-this document.
-See the shell manual page for details.
-.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
-.[
-$LIST$
-.]