sh.1

Android 一些工具

If parameter is unset or null, the
expansion of word (or a message indicating it is unset if word is omitted)
is written to standard error and the shell exits with a nonzero exit status.
Otherwise, the value of parameter is substituted.
An interactive shell need not exit.
.It ${parameter:+word}
Use Alternative Value.
If parameter is unset or null, null is
substituted; otherwise, the expansion of word is substituted.
.El
.Pp
In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omission
of the colon results in a test for a parameter that is only unset.
.Bl -tag -width aaparameterwordaaaaa
.It ${#parameter}
String Length.
The length in characters of the value of parameter.
.El
.Pp
The following four varieties of parameter expansion provide for substring
processing.
In each case, pattern matching notation (see
.Sx Shell Patterns ) ,
rather than regular expression notation, is used to evaluate the patterns.
If parameter is * or @, the result of the expansion is unspecified.
Enclosing the full parameter expansion string in double-quotes does not
cause the following four varieties of pattern characters to be quoted,
whereas quoting characters within the braces has this effect.
.Bl -tag -width aaparameterwordaaaaa
.It ${parameter%word}
Remove Smallest Suffix Pattern.
The word is expanded to produce a pattern.
The parameter expansion then results in parameter, with the
smallest portion of the suffix matched by the pattern deleted.
.It ${parameter%%word}
Remove Largest Suffix Pattern.
The word is expanded to produce a pattern.
The parameter expansion then results in parameter, with the largest
portion of the suffix matched by the pattern deleted.
.It ${parameter#word}
Remove Smallest Prefix Pattern.
The word is expanded to produce a pattern.
The parameter expansion then results in parameter, with the
smallest portion of the prefix matched by the pattern deleted.
.It ${parameter##word}
Remove Largest Prefix Pattern.
The word is expanded to produce a pattern.
The parameter expansion then results in parameter, with the largest
portion of the prefix matched by the pattern deleted.
.El
.Ss Command Substitution
Command substitution allows the output of a command to be substituted in
place of the command name itself.
Command substitution occurs when the command is enclosed as follows:
.Pp
.Dl $(command)
.Pp
or
.Po
.Dq backquoted
version
.Pc :
.Pp
.Dl `command`
.Pp
The shell expands the command substitution by executing command in a
subshell environment and replacing the command substitution with the
standard output of the command, removing sequences of one or more
.Ao newline Ac Ns s
at the end of the substitution.
(Embedded
.Ao newline Ac Ns s
before
the end of the output are not removed; however, during field splitting,
they may be translated into
.Ao space Ac Ns s ,
depending on the value of
.Ev IFS
and quoting that is in effect.)
.Ss Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value.
The format for arithmetic expansion is as follows:
.Pp
.Dl $((expression))
.Pp
The expression is treated as if it were in double-quotes, except
that a double-quote inside the expression is not treated specially.
The shell expands all tokens in the expression for parameter expansion,
command substitution, and quote removal.
.Pp
Next, the shell treats this as an arithmetic expression and
substitutes the value of the expression.
.Ss White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and
arithmetic expansion the shell scans the results of
expansions and substitutions that did not occur in double-quotes for
field splitting and multiple fields can result.
.Pp
The shell treats each character of the
.Ev IFS
as a delimiter and use the delimiters to split the results of parameter
expansion and command substitution into fields.
.Ss Pathname Expansion (File Name Generation)
Unless the
.Fl 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.
The next section describes the
patterns used for both Pathname Expansion and the
.Ic case
command.
.Ss Shell Patterns
A pattern consists of normal characters, which match themselves,
and meta-characters.
The meta-characters are
.Dq \&! ,
.Dq * ,
.Dq \&? ,
and
.Dq \&[ .
These characters lose their 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
An asterisk
.Pq Dq *
matches any string of characters.
A question mark matches any single character.
A left bracket
.Pq Dq \&[
introduces a character class.
The end of the character class is indicated by a
.Pq Dq \&] ;
if the
.Dq \&]
is missing then the
.Dq \&[
matches a
.Dq \&[
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
.Dq \&]
in a character class, make it the first character listed (after the
.Dq \&! ,
if any).
To include a minus sign, make it the first or last character listed.
.Ss Builtins
This 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 that may
be builtin for efficiency (e.g.
.Xr printf 1 ,
.Xr echo 1 ,
.Xr test 1 ,
etc).
.Bl -tag -width 5n
.It :
A null command that returns a 0 (true) exit value.
.It \&. file
The commands in the specified file are read and executed by the shell.
.It alias Op Ar name Ns Op Ar "=string ..."
If
.Ar name=string
is specified, the shell defines the alias
.Ar name
with value
.Ar string .
If just
.Ar name
is specified, the value of the alias
.Ar name
is printed.
With no arguments, the
.Ic alias
builtin prints the
names and values of all defined aliases (see
.Ic unalias ) .
.It bg [ Ar job ] ...
Continue the specified jobs (or the current job if no
jobs are given) in the background.
.It Xo command
.Op Fl p
.Op Fl v
.Op Fl V
.Ar command
.Op Ar arg ...
.Xc
Execute the specified command but ignore shell functions when searching
for it.
(This is useful when you
have a shell function with the same name as a builtin command.)
.Bl -tag -width 5n
.It Fl p
search for command using a
.Ev PATH
that guarantees to find all the standard utilities.
.It Fl V
Do not execute the command but
search for the command and print the resolution of the
command search.
This is the same as the type builtin.
.It Fl v
Do not execute the command but
search for the command and print the absolute pathname
of utilities, the name for builtins or the expansion of aliases.
.El
.It cd Op Ar directory Op Ar replace
Switch to the specified directory (default
.Ev $HOME ) .
If
.Ar replace
is specified, then the new directory name is generated by replacing
the first occurrence of
.Ar directory
in the current directory name with
.Ar replace .
Otherwise if an entry for
.Ev CDPATH
appears in the environment of the
.Ic cd
command or the shell variable
.Ev CDPATH
is set and the directory name does not begin with a slash, then the
directories listed in
.Ev CDPATH
will be searched for the specified directory.
The format of
.Ev CDPATH
is the same as that of
.Ev PATH .
In an interactive shell, the
.Ic 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
.Ev CDPATH
mechanism was used or because a symbolic link was crossed.
.It eval Ar string ...
Concatenate all the arguments with spaces.
Then re-parse and execute the command.
.It exec Op Ar command arg ...
Unless 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
.Ic exec
command are marked as permanent, so that they are not undone when the
.Ic exec
command finishes.
.It exit Op Ar exitstatus
Terminate the shell process.
If
.Ar exitstatus
is given it is used as the exit status of the shell; otherwise the
exit status of the preceding command is used.
.It export Ar name ...
.It export Fl p
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.
The shell allows the value of a variable to be set at the
same time it is exported by writing
.Pp
.Dl export name=value
.Pp
With no arguments the export command lists the names of all exported variables.
With the
.Fl p
option specified the output will be formatted suitably for non-interactive use.
.It Xo fc Op Fl e Ar editor
.Op Ar first Op Ar last
.Xc
.It Xo fc Fl l
.Op Fl nr
.Op Ar first Op Ar last
.Xc
.It Xo fc Fl s Op Ar old=new
.Op Ar first
.Xc
The
.Ic fc
builtin lists, or edits and re-executes, commands previously entered
to an interactive shell.
.Bl -tag -width 5n
.It Fl e No editor
Use the editor named by editor to edit the commands.
The editor string is a command name, subject to search via the
.Ev PATH
variable.
The value in the
.Ev FCEDIT
variable is used as a default when
.Fl e
is not specified.
If
.Ev FCEDIT
is null or unset, the value of the
.Ev EDITOR
variable is used.
If
.Ev EDITOR
is null or unset,
.Xr ed 1
is used as the editor.
.It Fl l No (ell)
List the commands rather than invoking an editor on them.
The commands are written in the sequence indicated by
the first and last operands, as affected by
.Fl r ,
with each command preceded by the command number.
.It Fl n
Suppress command numbers when listing with -l.
.It Fl r
Reverse the order of the commands listed (with
.Fl l )
or edited (with neither
.Fl l
nor
.Fl s ) .
.It Fl s
Re-execute the command without invoking an editor.
.It first
.It last
Select the commands to list or edit.
The number of previous commands that
can be accessed are determined by the value of the
.Ev HISTSIZE
variable.
The value of first or last or both are one of the following:
.Bl -tag -width 5n
.It [+]number
A positive number representing a command number; command numbers can be
displayed with the
.Fl l
option.
.It Fl number
A negative decimal number representing the command that was executed
number of commands previously.
For example, \-1 is the immediately previous command.
.El
.It string
A string indicating the most recently entered command that begins with
that string.
If the old=new operand is not also specified with
.Fl s ,
the string form of the first operand cannot contain an embedded equal sign.
.El
.Pp
The following environment variables affect the execution of fc:
.Bl -tag -width HISTSIZE
.It Ev FCEDIT
Name of the editor to use.
.It Ev HISTSIZE
The number of previous commands that are accessible.
.El
.It fg Op Ar job
Move the specified job or the current job to the foreground.
.It getopts Ar optstring var
The
.Tn POSIX
.Ic getopts
command, not to be confused with the
.Em Bell Labs
-derived
.Xr getopt 1 .
.Pp
The first argument should be a series of letters, each of which may be
optionally followed by a colon to indicate that the option requires an
argument.
The variable specified is set to the parsed option.
.Pp
The
.Ic getopts
command deprecates the older
.Xr getopt 1
utility due to its handling of arguments containing whitespace.
.Pp
The
.Ic getopts
builtin may be used to obtain options and their arguments
from a list of parameters.
When invoked,
.Ic getopts
places the value of the next option from the option string in the list in
the shell variable specified by
.Va var
and its index in the shell variable
.Ev OPTIND .
When the shell is invoked,
.Ev OPTIND
is initialized to 1.
For each option that requires an argument, the
.Ic getopts
builtin will place it in the shell variable
.Ev OPTARG .
If an option is not allowed for in the
.Va optstring ,
then
.Ev OPTARG
will be unset.
.Pp
.Va optstring
is a string of recognized option letters (see
.Xr getopt 3 ) .
If a letter is followed by a colon, the option is expected to have an
argument which may or may not be separated from it by white space.
If an option character is not found where expected,
.Ic getopts
will set the variable
.Va var
to a
.Dq \&? ;
.Ic getopts
will then unset
.Ev OPTARG
and write output to standard error.
By specifying a colon as the first character of
.Va optstring
all errors will be ignored.
.Pp
A nonzero value is returned when the last option is reached.
If there are no remaining arguments,
.Ic getopts
will set
.Va var
to the special option,
.Dq -- ,
otherwise, it will set
.Va var
to
.Dq \&? .
.Pp
The following code fragment shows how one might process the arguments
for a command that can take the options
.Op a
and
.Op b ,
and the option
.Op c ,
which requires an argument.
.Pp
.Bd -literal -offset indent
while getopts abc: f
do
	case $f in
	a | b)	flag=$f;;
	c)	carg=$OPTARG;;
	\\?)	echo $USAGE; exit 1;;
	esac
done
shift `expr $OPTIND - 1`
.Ed
.Pp
This code will accept any of the following as equivalent:
.Pp
.Bd -literal -offset indent
cmd \-acarg file file
cmd \-a \-c arg file file
cmd \-carg -a file file
cmd \-a \-carg \-\- file file
.Ed
.It hash Fl rv Ar command ...
The shell maintains a hash table which remembers the