t2

unix v7是最后一个广泛发布的研究型UNIX版本

.DS
	ps a >/tmp/ps$$
	\*(ZZ
	rm /tmp/ps$$
.DE
.IP \fB$\|!\fP 8
The process number of the last process
run in the background (in decimal).
.IP \fB$\(mi\fP 8
The current shell flags, such as
\fB\(mix\fR and \fB\(miv\|.\fR
.RE
.LP
Some variables have a special meaning to the
shell and should be avoided for general
use.
.RS
.IP \fB$\s-1MAIL\s0\fP 8
When used interactively
the shell looks at the file
specified by this variable
before it issues a prompt.
If the specified file has been modified
since it
was last looked at the shell
prints the message
\fIyou have mail\fP before prompting
for the next command.
This variable is typically set
in the file \fB.profile,\fP
in the user's login directory.
For example,
.DS
	\s-1MAIL\s0=/usr/mail/fred
.DE
.IP \fB$\s-1HOME\s0\fP 8
The default argument
for the \fIcd\fP command.
The current directory is used to resolve
file name references that do not begin with
a \fB/\|,\fR
and is changed using the \fIcd\fP command.
For example,
.DS
	cd /usr/fred/bin
.DE
makes the current directory \fB/usr/fred/bin\|.\fR
.DS
	cat wn
.DE
will print on the terminal the file \fIwn\fP
in this directory.
The command
\fIcd\fP with no argument
is equivalent to
.DS
	cd $\s-1HOME\s0
.DE
This variable is also typically set in the
the user's login profile.
.IP \fB$\s-1PATH\s0\fP 8
A list of directories that contain commands (the \fIsearch path\fR\|).
Each time a command is executed by the shell
a list of directories is searched
for an executable file.
.ne 5
If \fB$\s-1PATH\s0\fP is not set
then the current directory,
\fB/bin\fP, and \fB/usr/bin\fP are searched by default.
.ne 5
Otherwise \fB$\s-1PATH\s0\fP consists of directory
names separated by \fB:\|.\fP
For example,
.DS
	\s-1PATH\s0=\fB:\fP/usr/fred/bin\fB:\fP/bin\fB:\fP/usr/bin
.DE
specifies that the current directory
(the null string before the first \fB:\fP\|),
\fB/usr/fred/bin, /bin \fRand\fP /usr/bin\fR
are to be searched in that order.
In this way individual users
can have their own `private' commands
that are accessible independently
of the current directory.
If the command name contains a \fB/\fR then this directory search
is not used; a single attempt
is made to execute the command.
.IP \fB$\s-1PS1\s0\fP 8
The primary shell prompt string, by default, `\fB$\ \fR'.
.IP \fB$\s-1PS2\s0\fP 8
The shell prompt when further input is needed,
by default, `\fB>\ \fR'.
.IP \fB$\s-1IFS\s0\fP 8
The set of characters used by \fIblank
interpretation\fR (see section 3.4).
.RE
.SH
2.5\ The\ test\ command
.LP
The \fItest\fP command, although not part of the shell,
is intended for use by shell programs.
For example,
.DS
	test \(mif file
.DE
returns zero exit status if \fIfile\fP
exists and non-zero exit status otherwise.
In general \fItest\fP evaluates a predicate
and returns the result as its exit status.
Some of the more frequently used \fItest\fP
arguments are given here, see \fItest\fP (1)
for a complete specification.
.DS
	test s		true if the argument \fIs\fP is not the null string
	test \(mif file	true if \fIfile\fP exists
	test \(mir file	true if \fIfile\fP is readable
	test \(miw file	true if \fIfile\fP is writable
	test \(mid file	true if \fIfile\fP is a directory
.DE
.SH
2.6\ Control\ flow\ -\ while
.LP
The actions of
the \fBfor\fP loop and the \fBcase\fP
branch are determined by data available to the shell.
A \fBwhile\fP or \fBuntil\fP loop
and an \fBif then else\fP branch
are also provided whose
actions are determined by the exit status
returned by commands.
A \fBwhile\fP loop has the general form
.DS
	\fBwhile\fP \fIcommand-list\*1\fP
	\fBdo\fP \fIcommand-list\*2\fP
	\fBdone\fP
.DE
.LP
The value tested by the \fBwhile\fP command
is the exit status of the last simple command
following \fBwhile.\fP
Each time round the loop
\fIcommand-list\*1\fP is executed;
if a zero exit status is returned then
\fIcommand-list\*2\fP
is executed;
otherwise, the loop terminates.
For example,
.DS
	while test $1
	do \*(ZZ
	\*(DOshift
	done
.DE
is equivalent to
.DS
	for i
	do \*(ZZ
	done
.DE
\fIshift\fP is a shell command that
renames the positional parameters
\fB$2, $3, \*(ZZ\fR as \fB$1, $2, \*(ZZ\fR
and loses \fB$1\|.\fP
.LP
Another kind of use for the \fBwhile/until\fP
loop is to wait until some
external event occurs and then run
some commands.
In an \fBuntil\fP loop
the termination condition is reversed.
For example,
.DS
	until test \(mif file
	do sleep 300; done
	\fIcommands\fP
.DE
will loop until \fIfile\fP exists.
Each time round the loop it waits for
5 minutes before trying again.
(Presumably another process
will eventually create the file.)
.SH
2.7\ Control\ flow\ -\ if
.LP
Also available is a
general conditional branch
of the form,
.DS
	\fBif\fP \fIcommand-list
	\fBthen	\fIcommand-list
	\fBelse	\fIcommand-list
	\fBfi\fR
.DE
that tests the value returned by the last simple command
following \fBif.\fP
.LP
The \fBif\fP command may be used
in conjunction with the \fItest\fP command
to test for the existence of a file as in
.DS
	if test \(mif file
	then	\fIprocess file\fP
	else	\fIdo something else\fP
	fi
.DE
.LP
An example of the use of \fBif, case\fP
and \fBfor\fP constructions is given in
section 2.10\|.
.LP
A multiple test \fBif\fP command
of the form
.DS
	if \*(ZZ
	then	\*(ZZ
	else	if \*(ZZ
		then	\*(ZZ
		else	if \*(ZZ
			\*(ZZ
			fi
		fi
	fi
.DE
may be written using an extension of the \fBif\fP
notation as,
.DS
	if \*(ZZ
	then	\*(ZZ
	elif	\*(ZZ
	then	\*(ZZ
	elif	\*(ZZ
	\*(ZZ
	fi
.DE
.LP
The following example is the \fItouch\fP command
which changes the `last modified' time for a list
of files.
The command may be used in conjunction
with \fImake\fP (1) to force recompilation of a list
of files.
.DS
	flag=
	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
	\*(DO esac
	done
.DE
The \fB\(mic\fP flag is used in this command to
force subsequent files to be created if they do not already exist.
Otherwise, if the file does not exist, an error message is printed.
The shell variable \fIflag\fP
is set to some non-null string if the \fB\(mic\fP
argument is encountered.
The commands
.DS
	ln \*(ZZ; rm \*(ZZ
.DE
make a link to the file and then remove it
thus causing the last modified date to be updated.
.LP
The sequence
.DS
	if command1
	then	command2
	fi
.DE
may be written
.DS
	command1 && command2
.DE
Conversely,
.DS
	command1 \*(VT\*(VT command2
.DE
executes \fIcommand2\fP only if \fIcommand1\fP
fails.
In each case the value returned
is that of the last simple command executed.
.SH
2.8\ Command\ grouping
.LP
Commands may be grouped in two ways,
.DS
	\fB{\fI command-list\fB ; }\fR
.DE
and
.DS
	\fB(\fI command-list\fB )\fR
.DE
.LP
In the first \fIcommand-list\fP is simply executed.
The second form executes \fIcommand-list\fP
as a separate process.
For example,
.DS
	(cd x; rm junk )
.DE
executes \fIrm junk\fP in the directory
\fBx\fP without changing the current
directory of the invoking shell.
.LP
The commands
.DS
	cd x; rm junk
.DE
have the same effect but leave the invoking
shell in the directory \fBx.\fP
.SH
2.9\ Debugging\ shell\ procedures
.LP
The shell provides two tracing mechanisms
to help when debugging shell procedures.
The first is invoked within the procedure
as
.DS
	set \(miv
.DE
(\fBv\fP for verbose) and causes lines of the
procedure to be printed as they are read.
It is useful to help isolate syntax errors.
It may be invoked without modifying the procedure
by saying
.DS
	sh \(miv proc \*(ZZ
.DE
where \fIproc\fP is the name of the shell procedure.
This flag may be used in conjunction
with the \fB\(min\fP flag which prevents
execution of subsequent commands.
(Note that saying \fIset \(min\fP at a terminal
will render the terminal useless
until an end-of-file is typed.)
.LP
The command
.DS
	set \(mix
.DE
will produce an execution
trace.
Following parameter substitution
each command is printed as it is executed.
(Try these at the terminal to see
what effect they have.)
Both flags may be turned off by saying
.DS
	set \(mi
.DE
and the current setting of the shell flags is available as \fB$\(mi\|.\fR
.SH
2.10\ The\ man\ command
.LP
The following is the \fIman\fP command
which is used to print sections of the UNIX manual.
It is called, for example, as
.DS
		man sh
		man \(mit ed
		man 2 fork
.DE
In the first the manual section for \fIsh\fP
is printed.
Since no section is specified, section 1 is used.
The second example will typeset (\fB\(mit\fP option)
the manual section for \fIed.\fP
The last prints the \fIfork\fP manual page
from section 2.
.sp 2
.DS
	cd /usr/man

	: \'colon is the comment command\'
	: \'default is nroff ($N), section 1 ($s)\'
	N=n\ s=1

	for i
	do case $i in
.sp .5
	\*(DC[1\(mi9]\*(ST)	s=$i ;;
.sp .5
	\*(DC\(mit)	N=t ;;
.sp .5
	\*(DC\(min)	N=n ;;
.sp .5
	\*(DC\(mi\*(ST)	echo unknown flag \\\\\'$i\\\\\' ;;
.sp .5
	\*(DC\*(ST)	if test \(mif man$s/$i.$s
	\*(DC	then	${N}roff man0/${N}aa man$s/$i.$s
	\*(DC	else	: \'look through all manual sections\'
	\*(DC		found=no
	\*(DC		for j in 1 2 3 4 5 6 7 8 9
	\*(DC		do if test \(mif man$j/$i.$j
	\*(DC		\*(DOthen man $j $i
	\*(DC		\*(DO\*(THfound=yes
	\*(DC		\*(DOfi
	\*(DC		done
	\*(DC		case $found in
	\*(DC		\*(Cano) echo \\'$i: manual page not found\\'
	\*(DC		esac
	\*(DC	fi
	\*(DOesac
	done
.DE
.ce
.ft B
Figure 1. A version of the man command
.ft R