t3

unix v7是最后一个广泛发布的研究型UNIX版本

.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