sh5.1

<B>Digital的Unix操作系统VAX 4.2源码</B>

Internal field separators,
normally
.BR space ,
.BR tab ,
and
.BR new-line .
.TP
.B
.SM SHELL
When the shell is invoked, it scans the environment 
for this name. For further information, see
.B Environment
below.
If it is found and there is an 'r'
in the file name part of its value, the
shell becomes a restricted shell.
.PD
.RE
.PP
The shell gives default values to
\fB\s-1PATH\s+1\fP, \fB\s-1PS1\s+1\fP, \fB\s-1PS2\s+1\fP, \fB\s-1MAILCHECK\s+1\fP and \fB\s-1IFS\s+1\fP.
.SM
.B LOGNAME,
.SM
.B HOME,
and
.SM
.B MAIL
are set by
.MS login 1 .
.SS Blank Interpretation
After parameter and command substitution,
the results of substitution are scanned for internal field separator
characters (those found in
.BR \s-1IFS\s+1 )
and split into distinct arguments where such characters are found.
Explicit null arguments (\^\fB"\^"\fP or \fB'\^'\fP\^) are retained.
Implicit null arguments,
those resulting from parameters
that have no values are removed.
.SS File Name Generation
.NXR "pattern" "matching"
Following substitution, each command
word is scanned for the characters
.BR \(** ,
.BR ? ,
and
.BR [ .
If one of these characters appears
the word is regarded as a
pattern.
The word is replaced with alphabetically sorted file
names that match the pattern.
If no file name is found that matches the pattern,
the word is left unchanged.
The character
.B .
at the start of a file name
or immediately following a
.BR / ,
as well as the character
.B /
itself,
must be matched explicitly.
.PP
.PD 0
.RS
.TP
.B \(**
Matches any string, including the null string.
.TP
.B ?
Matches any single character.
.TP
.BR [...]
Matches any one of the enclosed characters.
A pair of characters separated by
.B \-
matches any
character lexically between the pair, inclusive.
If the first character following the opening 
``[''
is a
.B "``!''"
any character not enclosed is matched.
.PD
.RE
.SS Quoting
.NXR "sh5 command interpreter" "special characters and"
The following characters have a special meaning to the shell
and cause termination of a word unless quoted:
.RS
.PP
\fB;  &  (  ) |  ^  <  >  new-line  space  tab\fP
.RE
.PP
A character may be quoted
(that is, made to stand for itself)
by preceding
it with a
.BR \e .
The pair
.B \enew-line
is ignored.
All characters enclosed between a pair of single quote marks (' '),
except a single quote,
are quoted.
Inside double quote marks
(\fB"\^"\fP),
parameter and command substitution occurs and
.B \e
quotes the characters
.BR \e ,
.BR ` ,
\fB"\fP,
and
.BR $ .
.B
"$\(**"
is equivalent to
\fB"$1 \|$2\fP \|.\|.\|.\fB"\fP,
whereas
.B
"$@"
is equivalent to
.B
"$1"\|
.B
"$2"\|
\&.\|.\|.\|.
.SS Prompting
.NXR "sh5 command interpreter" "prompts"
When used interactively,
the shell prompts with the value of
.SM
.B PS1
before reading a command.
If at any time a new-line is typed and further input is needed
to complete a command, the secondary prompt
(that is, the value of
.BR \s-1PS2\s+1 )
is issued.
.SS Input/output
.NXR "sh5 command interpreter" "directing input"
.NXR "sh5 command interpreter" "directing output"
Before a command is executed, its input and output
may be redirected using a special notation interpreted by the shell.
The following may appear anywhere in a simple command
or may precede or follow a
command and are not passed on to the invoked command.
Substitution occurs before
word or digit is used:
.PP
.PD 0
.TP 14
\fB<\fIword\fP
Use file
.I word\^
as standard input (file descriptor 0).
.TP
\fB>\fIword\fP
Use file
.I word\^
as standard output (file descriptor 1).
If the file does not exist it is created.
Otherwise, it is truncated to zero length.
.TP
\fB>\&>\&\fIword\fP
Use file
.I word\^
as standard output.
If the file exists output is appended to it,
by first seeking to the end-of-file.
Otherwise, the file is created.
.TP
\fB<<[ \- ]\fIword\fP
The shell input is read up to a line that is the same as
.IR word ,
or to an end-of-file.
The resulting document becomes
the standard input.
If any character of
.I word\^
is quoted, no interpretation
is placed upon the characters of the document.
Otherwise, parameter and command substitution occurs,
(unescaped)
.B \enew-line
is ignored,
and
.B \e
must be used to quote the characters
.BR \e ,
.BR $ ,
.BR ` ,
and the first character of
.IR word .
If
.B \-
is appended to
.BR <\h@-.3m@< ,
all leading tabs are stripped from
.I word\^
and from the document.
.TP
\fB<\h@-.1m@&\fIdigit\fP
Use the file associated with file descriptor
.I digit\^
as standard input.
Similarly for the standard output using 
.B >\h@-.1m@&\fIdigit\fP.
.TP
.B <\h@-.1m@&\h@-.1m@\-
The standard input is closed.
Similarly for the standard output using
.BR >\h@-.1m@&\h@-.1m@\- .
.PD
.PP
If any of the above is preceded by a digit,
the file descriptor which will be associated 
with the file is that specified by the digit,
instead of the default 0 or 1.
For example:
.EX
 ... 2>&1
.EE
.PP
This associates file descriptor 2
with the file currently associated with 
file descriptor 1.
.PP
The order in which redirections are specified is significant.
The shell evaluates redirections left-to-right.
For example:
.EX
 ... 1>xxx 2>&1
.EE
.PP
The first associates file descriptor 1 with file 
.IR xxx\^ .
It associates file descriptor 2 with the file associated with file
descriptor 1 (that is,
.IR xxx\^ ).
If the order of redirections were reversed,
file descriptor 2 would be associated 
with the terminal (assuming file descriptor 1 had been)
and file descriptor 
1 would be associated with file 
.IR xxx\^ .
.PP
If a command is followed by
.B &
the default standard input
for the command
is the empty file
/dev/null.
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by
input/output specifications.
.PP
Redirection of output is not allowed in the restricted shell.
.SS Environment
.NXR "sh5 command interpreter" "environment"
The environment
is a list of name-value pairs that is passed to
an executed program in the same way as a normal argument list.
For further information, see 
.MS environ 7 .
The shell interacts with the environment in several ways.
On invocation, the shell scans the environment
and creates a
parameter
for each name found,
giving it the corresponding value.
If the user modifies the value of any of these
parameters
or creates new parameters,
none of these affects the environment
unless the
.B export
command is used to bind the shell's
parameter
to the environment (see also 
.BR "set \-a" ).
A parameter may be removed from the environment
with the 
.BR unset
command.
The environment seen by any executed command is thus composed
of any unmodified name-value pairs originally inherited by the shell,
minus any pairs removed by
.BR unset ,
plus any modifications or additions,
all of which must be noted in
.B export
commands.
.PP
The environment for any simple command
may be augmented by prefixing it with one or more assignments to
parameters.
Thus:
.EX
TERM=450 cmd 				
(export TERM; TERM=450; cmd)
.EE
.PP
These are equivalent (as far as the execution of
.I cmd\^
is concerned).
.PP
If the
.B \-k
flag is set,
.I all\^
keyword arguments are placed in the environment,
even if they occur after the command name.
The following
first prints
.B "a=b c"
and
.BR c :
.EX
echo a=b c
set \-k
echo a=b c
.EE
.SS Signals
.NXR "sh5 command interpreter" "signals"
The \s-1INTERRUPT\s+1 and \s-1QUIT\s+1 signals for an invoked
command are ignored if the command is followed by
.BR & .
Otherwise signals have the values
inherited by the shell from its parent,
with the exception of signal 11.
For further information, see also
the
.B trap
command below.
.SS Execution
.NXR "sh5 command interpreter" "executing commands"
Each time a command is executed, the above substitutions are
carried out.
If the command name matches one of the 
.B Special Commands
listed below, it is executed in the shell process.
If the command name does not match a
.B Special Command,
but matches the name of a defined function, the function is executed 
in the shell process
(note how this differs from the execution of shell procedures).
The positional parameters
.BR $1 ,
.BR $2 ,
\&.\|.\|.\|.
are set to the arguments of the function.
If the command name matches neither a
.B Special Command
nor the name of a defined function,
a new process is created and an attempt is made to
execute the command via
.MS execve 2 .
.PP
The shell parameter
.B
.SM PATH
defines the search path for
the directory containing the command.
Alternative directory names are separated by
a colon
.RB ( : ).
The default path is
.B :/bin:/usr/bin
(specifying the current directory,
.BR /bin ,
and
.BR /usr/bin ,
in that order).
Note that the current directory is specified by a null path name,
which can appear immediately after the equal sign
or between the colon delimiters anywhere else in the path list.
If the command name contains a \fB/\fP the search path
is not used.
Such commands will not be executed by the restricted shell.
Otherwise, each directory in the path is
searched for an executable file.
If the file has execute permission but is not an
.B a.out
file,
it is assumed to be a file containing shell commands.
A sub-shell is spawned to read it.
A parenthesized command is also executed in
a sub-shell.
.PP
The location in the search path where a command was found is remembered by the
shell
(to help avoid unnecessary
.B exec
later).
If the command was found in a relative directory, its location must be 
re-determined whenever the current directory changes.
The shell forgets all remembered locations whenever the
.B 
.SM PATH
variable is changed or the
.B hash \-r
command is executed (see below).
.SS Special Commands
Input/output redirection is now permitted for these commands.
File descriptor 1 is the default output location.
.PP
.PD 0
.TP
.B :
No effect; the command does nothing.
A zero exit code is returned.
.br
.TP
.BI ".\| " file\^
Read and execute commands from
.I file\^
and return.
The search path
specified by
.B
.SM PATH
is used to find the directory containing
.IR file .
.TP
\fBbreak\fP [ \fIn\fP ]
.NXR "break command (System V)"
Exit from the enclosing \fBfor\fP or
.B while
loop, if any.
If
.I n\^
is specified break
.I n\^
levels.
.TP
\fBcontinue\fP [ \fIn\fP ]
.NXR "continue command (System V)"
Resume the next iteration of the enclosing
\fBfor\fP or
.B while
loop.
If
.I n\^
is specified resume at the
.IR n \-th
enclosing loop.
.TP
\fBcd\fP [ \fIarg\fP ]
.NXR "cd command (System V)"
Change the current directory to
.IR arg .
The shell
parameter
.B
.SM HOME
is the default
.IR arg .
The shell parameter
.B
.SM CDPATH
defines the search path for
the directory containing 
.IR arg .
Alternative directory names are separated by
a colon
.RB ( : ).
The default path is
.B <null>
(specifying the current directory).
Note that the current directory is specified by a null path name,
which can appear immediately after the equal sign
or between the colon delimiters anywhere else in the path list.
If 
.I arg
begins with a \fB/\fP the search path
is not used.
Otherwise, each directory in the path is