t2

unix v7是最后一个广泛发布的研究型UNIX版本

.bp
.SH
2.0\ Shell\ procedures
.LP
The shell may be used to read and execute commands
contained in a file.
For example,
.DS
	sh file [ args \*(ZZ ]
.DE
calls the shell to read commands from \fIfile.\fP
Such a file is called a \fIcommand procedure\fP
or \fIshell procedure.\fP
Arguments may be supplied with the call
and are referred to in \fIfile\fP
using the positional parameters
\fB$1, $2, \*(ZZ\|.\fR
For example, if the file \fIwg\fP contains
.DS
	who \*(VT grep $1
.DE
then
.DS
	sh wg fred
.DE
is equivalent to
.DS
	who \*(VT grep fred
.DE
.LP
UNIX files have three independent attributes,
\fIread,\fP \fIwrite\fP and \fIexecute.\fP
The UNIX command \fIchmod\fP (1) may be used
to make a file executable.
For example,
.DS
	chmod +x wg
.DE
will ensure that the file \fIwg\fP has execute status.
Following this, the command
.DS
	wg fred
.DE
is equivalent to
.DS
	sh wg fred
.DE
This allows shell procedures and programs
to be used interchangeably.
In either case a new process is created to
run the command.
.LP
As well as providing names for the positional
parameters,
the number of positional parameters in the call
is available as \fB$#\|.\fP
The name of the file being executed
is available as \fB$0\|.\fP
.LP
A special shell parameter \fB$\*(ST\fP
is used to substitute for all positional parameters
except \fB$0\|.\fP
A typical use of this is to provide
some default arguments,
as in,
.DS
	nroff \(miT450 \(mims $\*(ST
.DE
which simply prepends some arguments
to those already given.
.SH
2.1\ Control\ flow\ -\ for
.LP
A frequent use of shell procedures is to loop
through the arguments (\fB$1, $2, \*(ZZ\fR)
executing commands once for each argument.
An example of such a procedure is
\fItel\fP that searches the file
\fB/usr/lib/telnos\fR
that contains lines of the form
.DS
	\*(ZZ
	fred mh0123
	bert mh0789
	\*(ZZ
.DE
The text of \fItel\fP is
.DS
	for i
	do grep $i /usr/lib/telnos; done
.DE
The command
.DS
	tel fred
.DE
prints those lines in \fB/usr/lib/telnos\fR
that contain the string \fIfred\|.\fP
.DS
	tel fred bert
.DE
prints those lines containing \fIfred\fP
followed by those for \fIbert.\fP
.LP
The \fBfor\fP loop notation is recognized by the shell
and has the general form
.DS
	\fBfor\fR \fIname\fR \fBin\fR \fIw1 w2 \*(ZZ\fR
	\fBdo\fR \fIcommand-list\fR
	\fBdone\fR
.DE
A \fIcommand-list\fP is a sequence of one or more
simple commands separated or terminated by a newline or semicolon.
Furthermore, reserved words
like \fBdo\fP and \fBdone\fP are only
recognized following a newline or
semicolon.
\fIname\fP is a shell variable that is set
to the words \fIw1 w2 \*(ZZ\fR in turn each time the \fIcommand-list\fP
following \fBdo\fP
is executed.
If \fBin\fR \fIw1 w2 \*(ZZ\fR
is omitted then the loop
is executed once for each positional parameter;
that is, \fBin\fR \fI$\*(ST\fR is assumed.
.LP
Another example of the use of the \fBfor\fP
loop is the \fIcreate\fP command
whose text is
.DS
	for i do >$i; done
.DE
The command
.DS
	create alpha beta
.DE
ensures that two empty files
\fIalpha\fP and \fIbeta\fP exist
and are empty.
The notation \fI>file\fP may be used on its
own to create or clear the contents of a file.
Notice also that a semicolon (or newline) is required before \fBdone.\fP
.SH
2.2\ Control\ flow\ -\ case
.LP
A multiple way branch is provided for by the
\fBcase\fP notation.
For example,
.DS
	case $# in
	\*(Ca1)	cat \*(AP$1 ;;
	\*(Ca2)	cat \*(AP$2 <$1 ;;
	\*(Ca\*(ST)	echo \\'usage: append [ from ] to\\' ;;
	esac
.DE
is an \fIappend\fP command.
When called
with one argument as
.DS
	append file
.DE
\fB$#\fP is the string \fI1\fP and
the standard input is copied onto the
end of \fIfile\fP
using the \fIcat\fP command.
.DS
	append file1 file2
.DE
appends the contents of \fIfile1\fP
onto \fIfile2.\fP
If the number of arguments supplied to
\fIappend\fP is other than 1 or 2
then a message is printed indicating
proper usage.
.LP
The general form of the \fBcase\fP command
is
.DS
	\fBcase \fIword \fBin
	\*(Ca\fIpattern\|\fB)\ \fIcommand-list\fB\|;;
	\*(Ca\*(ZZ
	\fBesac\fR
.DE
The shell attempts to match
\fIword\fR with each \fIpattern,\fR
in the order in which the patterns
appear.
If a match is found the
associated \fIcommand-list\fP is
executed and execution
of the \fBcase\fP is complete.
Since \*(ST is the pattern that matches any
string it can be used for the default case.
.LP
A word of caution:
no check is made to ensure that only
one pattern matches
the case argument.
The first match found defines the set of commands
to be executed.
In the example below the commands following
the second \*(ST will never be executed.
.DS
	case $# in
	\*(Ca\*(ST) \*(ZZ ;;
	\*(Ca\*(ST) \*(ZZ ;;
	esac
.DE
.LP
Another example of the use of the \fBcase\fP
construction is to distinguish
between different forms
of an argument.
The following example is a fragment of a \fIcc\fP command.
.DS
	for i
	do case $i in
	\*(DC\(mi[ocs])	\*(ZZ ;;
	\*(DC\(mi\*(ST)	echo \\'unknown flag $i\\' ;;
	\*(DC\*(ST.c)	/lib/c0 $i \*(ZZ ;;
	\*(DC\*(ST)	echo \\'unexpected argument $i\\' ;;
	\*(DOesac
	done
.DE
.LP
To allow the same commands to be associated
with more than one pattern
the \fBcase\fP command provides
for alternative patterns
separated by a \*(VT\|.
For example,
.DS
	case $i in
	\*(Ca\(mix\*(VT\(miy)	\*(ZZ
	esac
.DE
is equivalent to
.DS
	case $i in
	\*(Ca\(mi[xy])	\*(ZZ
	esac
.DE
.LP
The usual quoting conventions apply
so that
.DS
	case $i in
	\*(Ca\\\\?)	\*(ZZ
.DE
will match the character \fB?\|.\fP
.SH
2.3\ Here\ documents
.LP
The shell procedure \fItel\fP
in section 2.1 uses the file \fB/usr/lib/telnos\fR
to supply the data
for \fIgrep.\fP
An alternative is to include this
data
within the shell procedure as a \fIhere\fP document, as in,
.DS
	for i
	do grep $i \*(HE!
	\*(DO\*(ZZ
	\*(DOfred mh0123
	\*(DObert mh0789
	\*(DO\*(ZZ
	!
	done
.DE
In this example
the shell takes the lines between \fB\*(HE!\fR and \fB!\fR
as the standard input for \fIgrep.\fP
The string \fB!\fR is arbitrary, the document
being terminated by a line that consists
of the string following \*(HE\|.
.LP
Parameters are substituted in the document
before it is made available to \fIgrep\fP
as illustrated by the following procedure
called \fIedg\|.\fP
.DS
	ed $3 \*(HE%
	g/$1/s//$2/g
	w
	%
.DE
The call
.DS
	edg string1 string2 file
.DE
is then equivalent to the command
.DS
	ed file \*(HE%
	g/string1/s//string2/g
	w
	%
.DE
and changes all occurrences of \fIstring1\fP
in \fIfile\fP to \fIstring2\|.\fP
Substitution can be prevented using \\
to quote the special character \fB$\fP
as in
.DS
	ed $3 \*(HE+
	1,\\\\$s/$1/$2/g
	w
	+
.DE
(This version of \fIedg\fP is equivalent to
the first except that \fIed\fP will print
a \fB?\fR if there are no occurrences of
the string \fB$1\|.\fP)
Substitution within a \fIhere\fP document
may be prevented entirely by quoting
the terminating string,
for example,
.DS
	grep $i \*(HE\\\\#
	\*(ZZ
	#
.DE
The document is presented
without modification to \fIgrep.\fP
If parameter substitution is not required
in a \fIhere\fP document this latter form
is more efficient.
.SH
2.4\ Shell\ variables
.LP
The shell
provides string-valued variables.
Variable names begin with a letter
and consist of letters, digits and
underscores.
Variables may be given values by writing, for example,
.DS
	user=fred\ box=m000\ acct=mh0000
.DE
which assigns values to the variables
\fBuser, box\fP and \fBacct.\fP
A variable may be set to the null string
by saying, for example,
.DS
	null=
.DE
The value of a variable is substituted
by preceding its name with \fB$\|;\fP
for example,
.DS
	echo $user
.DE
will echo \fIfred.\fP
.LP
Variables may be used interactively
to provide abbreviations for frequently
used strings.
For example,
.DS
	b=/usr/fred/bin
	mv pgm $b
.DE
will move the file \fIpgm\fP
from the current directory to the directory \fB/usr/fred/bin\|.\fR
A more general notation is available for parameter
(or variable)
substitution, as in,
.DS
	echo ${user}
.DE
which is equivalent to
.DS
	echo $user
.DE
and is used when the parameter name is
followed by a letter or digit.
For example,
.DS
	tmp=/tmp/ps
	ps a >${tmp}a
.DE
will direct the output of \fIps\fR
to the file \fB/tmp/psa,\fR
whereas,
.DS
	ps a >$tmpa
.DE
would cause the value of the variable \fBtmpa\fP
to be substituted.
.LP
Except for \fB$?\fP the following
are set initially by the shell.
\fB$?\fP is set after executing each command.
.RS
.IP \fB$?\fP 8
The exit status (return code)
of the last command executed
as a decimal string.
Most commands return a zero exit status
if they complete successfully,
otherwise a non-zero exit status is returned.
Testing the value of return codes is dealt with
later under \fBif\fP and \fBwhile\fP commands.
.IP \fB$#\fP 8
The number of positional parameters
(in decimal).
Used, for example, in the \fIappend\fP command
to check the number of parameters.
.IP \fB$$\fP 8
The process number of this shell (in decimal).
Since process numbers are unique among
all existing processes, this string is
frequently used to generate
unique
temporary file names.
For example,