sh.1

Android 一些工具

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.\" Kenneth Almquist.
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.\"	@(#)sh.1	8.6 (Berkeley) 5/4/95
.\"
.Dd April 17, 2004
.Os
.Dt SH 1
.Sh NAME
.Nm sh
.Nd command interpreter (shell)
.Sh SYNOPSIS
.Nm
.Bk -words
.Op Fl aCefnuvxIimqVEb
.Op Cm +aCefnuvxIimqVEb
.Ek
.Bk -words
.Op Fl o Ar option_name
.Op Cm +o Ar option_name
.Ek
.Bk -words
.Op Ar command_file Oo Ar argument ... Oc
.Ek
.Nm
.Fl c
.Bk -words
.Op Fl aCefnuvxIimqVEb
.Op Cm +aCefnuvxIimqVEb
.Ek
.Bk -words
.Op Fl o Ar option_name
.Op Cm +o Ar option_name
.Ek
.Bk -words
.Ar command_string
.Op Ar command_name Oo Ar argument ... Oc
.Ek
.Nm
.Fl s
.Bk -words
.Op Fl aCefnuvxIimqVEb
.Op Cm +aCefnuvxIimqVEb
.Ek
.Bk -words
.Op Fl o Ar option_name
.Op Cm +o Ar option_name
.Ek
.Bk -words
.Op Ar argument ...
.Ek
.Sh DESCRIPTION
.Nm
is the standard command interpreter for the system.
The current version of
.Nm
is in the process of being changed to conform with the
.Tn POSIX
1003.2 and 1003.2a specifications for the shell.
This version has many
features which make it appear similar in some respects to the Korn shell,
but it is not a Korn shell clone (see
.Xr ksh 1 ) .
Only features designated by
.Tn POSIX ,
plus a few Berkeley extensions, are being incorporated into this shell.
.\" We expect
.\" .Tn POSIX
.\" conformance by the time 4.4 BSD is released.
This man page is not intended
to be a tutorial or a complete specification of the shell.
.Ss Overview
The shell is a command that reads lines from either a file or the
terminal, interprets them, and generally executes other commands.
It is the program that is running when a user logs into the system
(although a user can select a different shell with the
.Xr chsh 1
command).
The shell implements a language that has flow control
constructs, a macro facility that provides a variety of features in
addition to data storage, along with built in history and line editing
capabilities.
It incorporates many features to aid interactive use and
has the advantage that the interpretative language is common to both
interactive and non-interactive use (shell scripts).
That is, commands
can be typed directly to the running shell or can be put into a file and
the file can be executed directly by the shell.
.Ss Invocation
If no args are present and if the standard input of the shell
is connected to a terminal (or if the
.Fl i
flag is set),
and the
.Fl c
option is not present, the shell is considered an interactive shell.
An interactive shell generally prompts before each command and handles
programming and command errors differently (as described below).
When first starting,
the shell inspects argument 0, and if it begins with a dash
.Sq - ,
the shell is also considered
a login shell.
This is normally done automatically by the system
when the user first logs in.
A login shell first reads commands
from the files
.Pa /etc/profile
and
.Pa .profile
if they exist.
If the environment variable
.Ev ENV
is set on entry to a shell, or is set in the
.Pa .profile
of a login shell, the shell next reads
commands from the file named in
.Ev ENV .
Therefore, a user should place commands that are to be executed only at
login time in the
.Pa .profile
file, and commands that are executed for every shell inside the
.Ev ENV
file.
To set the
.Ev ENV
variable to some file, place the following line in your
.Pa .profile
of your home directory
.Pp
.Dl ENV=$HOME/.shinit; export ENV
.Pp
substituting for
.Dq .shinit
any filename you wish.
Since the
.Ev ENV
file is read for every invocation of the shell, including shell scripts
and non-interactive shells, the following paradigm is useful for
restricting commands in the
.Ev ENV
file to interactive invocations.
Place commands within the
.Dq case
and
.Dq esac
below (these commands are described later):
.Pp
.Bl -item -compact -offset indent
.It
.Li case $- in *i*)
.Bl -item -compact -offset indent
.It
.Li # commands for interactive use only
.It
.Li ...
.El
.It
.Li esac
.El
.Pp
If command line arguments besides the options have been specified, then
the shell treats the first argument as the name of a file from which to
read commands (a shell script), and the remaining arguments are set as the
positional parameters of the shell ($1, $2, etc).
Otherwise, the shell
reads commands from its standard input.
.Ss Argument List Processing
All of the single letter options have a corresponding name that can be
used as an argument to the
.Fl o
option.
The set
.Fl o
name is provided next to the single letter option in
the description below.
Specifying a dash
.Dq -
turns the option on, while using a plus
.Dq +
disables the option.
The following options can be set from the command line or
with the
.Ic set
builtin (described later).
.Bl -tag -width aaaallexportfoo -offset indent
.It Fl a Em allexport
Export all variables assigned to.
.It Fl c
Read commands from the
.Ar command_string
operand instead of from the standard input.
Special parameter 0 will be set from the
.Ar command_name
operand and the positional parameters ($1, $2, etc.)
set from the remaining argument operands.
.It Fl C Em noclobber
Don't overwrite existing files with
.Dq \*[Gt] .
.It Fl e Em errexit
If not interactive, exit immediately if any untested command fails.
The exit status of a command is considered to be
explicitly tested if the command is used to control an
.Ic if ,
.Ic elif ,
.Ic while ,
or
.Ic until ;
or if the command is the left hand operand of an
.Dq \*[Am]\*[Am]
or
.Dq ||
operator.
.It Fl f Em noglob
Disable pathname expansion.
.It Fl n Em noexec
If not interactive, read commands but do not execute them.
This is useful for checking the syntax of shell scripts.
.It Fl u Em nounset
Write a message to standard error when attempting to expand a variable
that is not set, and if the shell is not interactive, exit immediately.
.It Fl v Em verbose
The shell writes its input to standard error as it is read.
Useful for debugging.
.It Fl x Em xtrace
Write each command to standard error (preceded by a
.Sq +\  )
before it is executed.
Useful for debugging.
.It Fl q Em quietprofile
If the
.Fl v
or
.Fl x
options have been set, do not apply them when reading
initialization files, these being
.Pa /etc/profile ,
.Pa .profile ,
and the file specified by the
.Ev ENV
environment variable.
.It Fl I Em ignoreeof
Ignore EOF's from input when interactive.
.It Fl i Em interactive
Force the shell to behave interactively.
.It Fl m Em monitor
Turn on job control (set automatically when interactive).
.It Fl s Em stdin
Read commands from standard input (set automatically if no file arguments
are present).
This option has no effect when set after the shell has
already started running (i.e. with
.Ic set ) .
.It Fl V Em vi
Enable the built-in
.Xr vi 1
command line editor (disables
.Fl E
if it has been set).
(See the
.Sx Command Line Editing
section below.)
.It Fl E Em emacs
Enable the built-in emacs style
command line editor (disables
.Fl V
if it has been set).
(See the
.Sx Command Line Editing
section below.)
.It Fl b Em notify
Enable asynchronous notification of background job completion.
(UNIMPLEMENTED for 4.4alpha)
.It "\ \ " Em cdprint
Make an interactive shell always print the new directory name when
changed by the
.Ic cd
command.
.El
.Ss Lexical Structure
The shell reads input in terms of lines from a file and breaks it up into
words at whitespace (blanks and tabs), and at certain sequences of
characters that are special to the shell called
.Dq operators .
There are two types of operators: control operators and redirection
operators (their meaning is discussed later).
Following is a list of operators:
.Bl -ohang -offset indent
.It "Control operators:"
.Dl \*[Am]  \*[Am]\*[Am]  \&(  \&)  \&;  ;; | || \*[Lt]newline\*[Gt]
.It "Redirection operators:"
.Dl \*[Lt]  \*[Gt]  \*[Gt]|  \*[Lt]\*[Lt]  \*[Gt]\*[Gt]  \*[Lt]\*[Am]  \*[Gt]\*[Am]  \*[Lt]\*[Lt]-  \*[Lt]\*[Gt]
.El
.Ss Quoting
Quoting is used to remove the special meaning of certain characters or
words to the shell, such as operators, whitespace, or keywords.
There are three types of quoting: matched single quotes,
matched double quotes, and backslash.
.Ss Backslash
A backslash preserves the literal meaning of the following
character, with the exception of
.Aq newline .
A backslash preceding a
.Aq newline
is treated as a line continuation.
.Ss Single Quotes
Enclosing characters in single quotes preserves the literal meaning of all
the characters (except single quotes, making it impossible to put
single-quotes in a single-quoted string).
.Ss Double Quotes
Enclosing characters within double quotes preserves the literal
meaning of all characters except dollarsign
.Pq $ ,
backquote
.Pq ` ,
and backslash
.Pq \e .
The backslash inside double quotes is historically weird, and serves to
quote only the following characters:
.Dl $  `  \*q  \e  \*[Lt]newline\*[Gt] .
Otherwise it remains literal.
.Ss Reserved Words
Reserved words are words that have special meaning to the
shell and are recognized at the beginning of a line and
after a control operator.
The following are reserved words:
.Bl -column while while while while while -offset indent
.It ! Ta elif Ta fi Ta while Ta case
.It else Ta for Ta then Ta { Ta }
.It do Ta done Ta until Ta if Ta esac
.El
.Pp
Their meaning is discussed later.
.Ss Aliases
An alias is a name and corresponding value set using the
.Ic alias
builtin command.
Whenever a reserved word may occur (see above),
and after checking for reserved words, the shell
checks the word to see if it matches an alias.
If it does, it replaces it in the input stream with its value.
For example, if there is an alias called
.Dq lf
with the value
.Dq "ls -F" ,
then the input:
.Pp
.Dl lf foobar Aq return
.Pp
would become
.Pp
.Dl ls -F foobar Aq return
.Pp
Aliases provide a convenient way for naive users to create shorthands for
commands without having to learn how to create functions with arguments.
They can also be used to create lexically obscure code.
This use is discouraged.
.Ss Commands
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to the
BNF in the
.Tn POSIX
1003.2 document).
Essentially though, a line is read and if the first
word of the line (or after a control operator) is not a reserved word,
then the shell has recognized a simple command.
Otherwise, a complex
command or some other special construct may have been recognized.
.Ss Simple Commands
If a simple command has been recognized, the shell performs
the following actions:
.Bl -enum -offset indent
.It
Leading words of the form
.Dq name=value
are stripped off and assigned to the environment of the simple command.
Redirection operators and their arguments (as described below) are
stripped off and saved for processing.
.It
The remaining words are expanded as described in
the section called
.Dq Expansions ,
and the first remaining word is considered the command name and the
command is located.
The remaining words are considered the arguments of the command.
If no command name resulted, then the
.Dq name=value
variable assignments recognized in item 1 affect the current shell.
.It
Redirections are performed as described in the next section.
.El
.Ss Redirections
Redirections are used to change where a command reads its input or sends
its output.
In general, redirections open, close, or duplicate an
existing reference to a file.
The overall format used for redirection is:
.Pp
.Dl [n] Va redir-op Ar file
.Pp
where
.Va redir-op
is one of the redirection operators mentioned previously.
Following is a list of the possible redirections.
The
.Bq n
is an optional number, as in
.Sq 3
(not
.Sq Bq 3 ) ,
that refers to a file descriptor.
.Bl -tag -width aaabsfiles -offset indent
.It [n] Ns \*[Gt] file
Redirect standard output (or n) to file.
.It [n] Ns \*[Gt]| file
Same, but override the
.Fl C
option.
.It [n] Ns \*[Gt]\*[Gt] file
Append standard output (or n) to file.
.It [n] Ns \*[Lt] file
Redirect standard input (or n) from file.
.It [n1] Ns \*[Lt]\*[Am] Ns n2
Duplicate standard input (or n1) from file descriptor n2.
.It [n] Ns \*[Lt]\*[Am]-
Close standard input (or n).
.It [n1] Ns \*[Gt]\*[Am] Ns n2
Duplicate standard output (or n1) to n2.
.It [n] Ns \*[Gt]\*[Am]-
Close standard output (or n).
.It [n] Ns \*[Lt]\*[Gt] file
Open file for reading and writing on standard input (or n).
.El
.Pp
The following redirection is often called a
.Dq here-document .
.Bl -item -offset indent
.It
.Li [n]\*[Lt]\*[Lt] delimiter
.Dl here-doc-text ...
.Li delimiter
.El
.Pp
All the text on successive lines up to the delimiter is saved away and
made available to the command on standard input, or file descriptor n if
it is specified.
If the delimiter as specified on the initial line is
quoted, then the here-doc-text is treated literally, otherwise the text is
subjected to parameter expansion, command substitution, and arithmetic
expansion (as described in the section on
.Dq Expansions ) .
If the operator is
.Dq \*[Lt]\*[Lt]-
instead of
.Dq \*[Lt]\*[Lt] ,