ash.1

操作系统设计与实现源码

will consist of any arguments given on the first line of the program,
followed by the name of the program, followed by the arguments passed
to the program.
.h "Redirection"
Input/output redirections can be intermixed with the words in a simple
command and can be placed following any of the other commands.  When
redirection occurs, the shell saves the old values of the file descriptors
and restores them when the command completes.  The ``<'', ``>'', and ``>>''
redirections open a file for input, output, and appending, respectively.
The ``<&digit'' and ``>&digit'' makes the input or output a duplicate
of the file descriptor numbered by the digit.  If a minus sign is used
in place of a digit, the standard input or standard output are closed.
.PP
The ``<<\ word'' redirection
takes input from a
.I here
document.
As the shell encounters ``<<'' redirections, it collects them.  The
next time it encounters an unescaped newline, it reads the documents
in turn.  The word following the ``<<'' specifies the contents of the
line that terminates the document.  If none of the quoting methods
('', "", or \e) are used to enter the word, then the document is treated
like a word inside double quotes:  ``$'' and backquote are expanded
and backslash can be used to escape these and to continue long lines.
The word cannot contain any variable or command substitutions, and
its length (after quoting) must be in the range of 1 to 79 characters.
If ``<<-'' is used in place of ``<<'', then leading tabs are deleted
from the lines of the document.  (This is to allow you do indent shell
procedures containing here documents in a natural fashion.)
.PP
Any of the preceding redirection operators may be preceded by a single
digit specifying the file descriptor to be redirected.  There cannot
be any white space between the digit and the redirection operator.
.h "Path Search"
When locating a command, the shell first looks to see if it has a
shell function by that name.  Then, if PATH does not contain an
entry for "%builtin", it looks for a builtin command by that name.
Finally, it searches each entry in PATH in turn for the command.
.PP
The value of the PATH variable should be a series of entries separated
by colons.
Each entry consists of a directory name, or a directory name followed
by a flag beginning with a percent sign.
The current directory should be indicated by an empty directory name.
.PP
If no percent sign is present, then the entry causes the shell to
search for the command in the specified directory.  If the flag is
``%builtin'' then the list of shell builtin commands is searched.
If the flag is ``%func'' then the directory is searched for a file which
is read as input to the shell.  This file should define a function
whose name is the name of the command being searched for.
.PP
Command names containing a slash are simply executed without performing
any of the above searches.
.h "The Environment"
The environment of a command is a set of name/value pairs.  When the
shell is invoked, it reads these names and values, sets the shell
variables with these names to the corresponding values, and marks
the variables as exported.  The
.I export
command can be used to mark additional variables as exported.
.PP
The environment of a command is constructed by constructing name/value
pairs from all the exported shell variables, and then modifying this
set by the assignments which precede the command, if any.
.h "Expansion"
The process of evaluating words when a shell procedure is executed is
called
.IR expansion .
Expansion consists of four steps:  variable substitution, command
substitution, word splitting, and file name generation.  If a word
is the expression following the word
.B case
in a case statement, the file name
which follows a redirection symbol, or an assignment to the environment
of a command, then the word cannot be split into multiple words.  In
these cases, the last two steps of the expansion process are omitted.
.h "Variable Substitution"
To be written.
.h "Command Substitution"
.I Ash
accepts two syntaxes for command substitution:
.d
`\fIlist\fR`
.e
and
.d
$(\fIlist\fR)
.e
Either of these may be included in a word.
During the command substitution process, the command (syntactly a
.IR list )
will be executed and anything that the command writes to the standard
output will be captured by the shell.  The final newline (if any) of
the output will be deleted; the rest of the output will be substituted
for the command in the word.
.h "Word Splitting"
When the value of a variable or the output of a command is substituted,
the resulting text is subject to word splitting, unless the dollar sign
introducing the variable or backquotes containing the text were enclosed
in double quotes.  In addition, ``$@'' is subject to a special type of
splitting, even in the presence of double quotes.
.PP
Ash uses two different splitting algorithms.  The normal approach, which
is intended for splitting text separated by which space, is used if the
first character of the shell variable IFS is a space.  Otherwise an alternative
experimental algorithm, which is useful for splitting (possibly empty)
fields separated by a separator character, is used.
.PP
When performing splitting, the shell scans the replacement text looking
for a character (when IFS does not begin with a space) or a sequence of
characters (when IFS does begin with a space), deletes the character or
sequence of characters, and spits the word into two strings at that
point.  When IFS begins with a space, the shell deletes either of the
strings if they are null.  As a special case, if the word containing
the replacement text is the null string, the word is deleted.
.PP
The variable ``$@'' is special in two ways.  First, splitting takes
place between the positional parameters, even if the text is enclosed
in double quotes.  Second, if the word containing the replacement
text is the null string and there are no positional parameters, then
the word is deleted.  The result of these rules is that "$@" is
equivalent to "$1" "$2" ... "$\fIn\fR", where \fIn\fR is the number of
positional parameters.  (Note that this differs from the System V shell.
The System V documentation claims that "$@" behaves this way; in fact
on the System V shell "$@" is equivalent to "" when there are no
positional paramteters.)
.h "File Name Generation"
Unless the
.B -f
flag is set, file name generation is performed after word splitting is
complete.  Each word is viewed as a series of patterns, separated by
slashes.  The process of expansion replaces the word with the names of
all existing files whose names can be formed by replacing each pattern
with a string that matches the specified pattern.  There are two
restrictions on this:  first, a pattern cannot match a string containing
a slash, and second, a pattern cannot match a string starting with a
period unless the first character of the pattern is a period.
.PP
If a word fails to match any files and the
.B -z
flag is not set, then the word will be left unchanged (except that the
meta-characters will be converted to normal characters).  If the
.B -z
flag is set, then the word is only left unchanged if none
of the patterns contain a character that can match anything besides
itself.  Otherwise the
.B -z
flag forces the word to be replaced with the names of the files that it
matches, even if there are zero names.
.h "Patterns"
A
.I pattern
consists of normal characters, which match themselves, and meta-characters.
The meta-characters are ``!'', ``*'', ``?'', and ``[''.  These characters lose
there special meanings if they are quoted.  When command or variable
substitution is performed and the dollar sign or back quotes are not
double quoted, the value of the variable or the output of the command
is scanned for these characters and they are turned into meta-characters.
.PP
Two exclamation points at the beginning of a pattern function as a ``not''
operator, causing the pattern to match any string that the remainder of
the pattern does
.I not
match.  Other occurances of exclamation points in a pattern match
exclamation points.  Two exclamation points are required rather than one
to decrease the incompatibility with the System V shell (which does not
treat exclamation points specially).
.PP
An asterisk (``*'') matches any string of characters.
A question mark matches any single character.
A left bracket (``['') introduces a character class.  The end of the
character class is indicated by a ``]''; if the ``]'' is missing then
the ``['' matches a ``['' rather than introducing a character class.
A character class matches any of the characters between the square
brackets.  A range of characters may be specified using a minus sign.
The character class may be complemented by making an exclamation point
the first character of the character class.
.PP
To include a ``]'' in a character class, make it the first character listed
(after the ``!'', if any).
To include a minus sign, make it the first or last character listed.
.h "The /u Directory"
By convention, the name ``/u/user'' refers to the home directory of the
specified user.  There are good reasons why this feature should be supported
by the file system (using a feature such as symbolic links) rather than
by the shell, but
.I ash
is capable of performing this mapping if the file system doesn't.
If the mapping is done by
.IR ash ,
setting the
.B -f
flag will turn it off.
.h "Character Set"
.I Ash
silently discards nul characters.  Any other character will be handled
correctly by
.IR ash ,
including characters with the high order bit set.
.h "Job Names and Job Control"
The term
.I job
refers to a process created by a shell command, or in the case of a
pipeline, to the set of processes in the pipeline.  The ways to refer
to a job are:
.d
%\fInumber\fR
%\fIstring\fR
%%
\fIprocess_id\fR
.e
The first form identifies a job by job number.
When a command is run,
.I ash
assigns it a job number
(the lowest unused number is assigned).
The second form identifies a job by giving a prefix of the command used
to create the job.  The prefix must be unique.  If there is only one job,
then the null prefix will identify the job, so you can refer to the job
by writing ``%''.  The third form refers to the \fIcurrent job\fR.  The
current job is the last job to be stopped while it was in the foreground.
(See the next paragraph.)  The last form identifies a job by giving the
process id of the last process in the job.
.PP
If the operating system that
.I ash
is running on supports job control,
.I ash
will allow you to use it.
In this case, typing the suspend character (typically ^Z) while running
a command will return you to
.I ash
and will make the suspended command the current job.  You can then continue
the job in the background by typing
.IR bg ,
or you can continue it in the foreground by typing
.IR fg .
.h "Atty"
If the shell variable ATTY is set, and the shell variable TERM is not
set to ``emacs'', then \fIash\fR generates appropriate escape sequences
to talk to
.IR atty (1).
.h "Exit Statuses"
By tradition, an exit status of zero means that a command has succeeded
and a nonzero exit status indicates that the command failed.  This is
better than no convention at all, but in practice it is extremely useful
to allow commands that succeed to use the exit status to return information
to the caller.  A variety of better conventions have been proposed, but
none of them has met with universal approval.  The convention used by
\fIash\fR and all the programs included in the \fIash\fR distribution is
as follows:
.ta 1i 2i
.nf
	0	Success.
	1	Alternate success.
	2	Failure.
	129-...	Command terminated by a signal.
.fi
The \fIalternate success\fR return is used by commands to indicate various
conditions which are not errors but which can, with a little imagination,
be conceived of as less successful than plain success.  For example,
.I test
returns 1 when the tested condition is false and
.I getopts
returns 1 when there are no more options.
Because this convention is not used universally, the
.B -e
option of
.I ash
causes the shell to exit when a command returns 1 even though that
contradicts the convention described here.
.PP
When a command is terminated by a signal, the uses 128 plus the signal
number as the exit code for the command.
.h "Builtin Commands"
This concluding section lists the builtin commands which are builtin
because they need to perform some operation that can't be performed by a
separate process.  In addition to these, there are several other commands
.RI ( catf ,
.IR echo ,
.IR expr ,
.IR line ,
.IR nlecho ,
.IR test ,
.RI  `` : '',
and
.IR true )
which can optionally be compiled into the shell.  The builtin
commands described below that accept options use the System V Release 2
.IR getopt (3)
syntax.
.sp
.b bg
[
.I job
] ...
.br
Continue the specified jobs (or the current job if no jobs are given)
in the background.
This command is only available on systems with Bekeley job control.
.b command
.IR "command arg" ...
.br
Execute the specified builtin command.  (This is useful when you have a
shell function with the same name as a builtin command.)
.b cd
[
.I directory
]
.br
Switch to the specified directory (default $HOME).
If the an entry for CDPATH appears in the environment of the cd command
or the shell variable CDPATH is set and the directory name does not
begin with a slash, then the directories listed in CDPATH will be
searched for the specified directory.  The format of CDPATH is the
same as that of PATH.
In an interactive shell, the cd command will print out the name of the
directory that it actually switched to if this is different from the
name that the user gave.  These may be different either because
the CDPATH mechanism was used or because a symbolic link was crossed.
.\" .b ".\fI\h'0.1i'file"
.\" Cawf can't do \h'0.1i'
.b .
.I file
.br
The commands in the specified file are read and executed by the shell.
A path search is not done to find the file because the directories in
PATH generally contain files that are intended to be executed, not read.
.b eval
.IR string ...
.br
The strings are parsed as shell commands and executed.
(This differs from the System V shell, which concatenates the arguments
(separated by spaces) and parses the result as a single command.)
.b exec
[
.IR "command arg" ...
]
.br
Unless
.I command
is omitted,
the shell process is replaced with the specified program (which must be a real
program, not a shell builtin or function).
Any redirections on the exec command are marked as permanent, so that they
are not undone when the exec command finishes.
If the command is not found, the exec command causes the shell to exit.
.b exit
[
.I exitstatus
]
.br
Terminate the shell process.  If
.I exitstatus
is given it is used as the
exit status of the shell; otherwise the exit status of the preceding
command is used.
.b export
.IR name ...
.br
The specified names are exported so that they will appear in the environment
of subsequent commands.  The only way to un-export a variable is to unset it.
.I Ash
allows the value of a variable to be set at the same time it is exported
by writing
.d
\fBexport\fR name=value
.e
With no arguments the export command lists the names of all exported variables.
.b fg
[
.I job
]
.br