sh.1

早期freebsd实现

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.
.TP
${#parameter}
String Length.  The length in characters of
the value of parameter.
.LP
The following four varieties of parameter expansion provide for substring
processing.  In each case, pattern matching notation (see 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.
(UNIMPLEMENTED IN 4.4alpha)
.TP
${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.

.TP
${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.
.TP
${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.
.TP
${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.
.LP
.sp 2
.B Command Substitution
.sp
.LP
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:
.nf

       $(command)

.fi
or (``backquoted'' version):
.nf

       `command`

.fi
.LP
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
<newline>s at the end of the substitution.  (Embedded <newline>s before
the end of the output are not removed; however, during field
splitting, they may be translated into <space>s, depending on the value
of IFS and quoting that is in effect.)

.sp 2
.B Arithmetic Expansion
.sp
.LP
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic
expansion is as follows:
.nf

       $((expression))

.fi
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.
.LP
Next, the shell treats this as an arithmetic expression and
substitutes the value of the expression.

.sp 2
.B White Space Splitting (Field Splitting)
.sp
.LP
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.
.LP
The shell treats each character of the IFS as a delimiter and use
the delimiters to split the results of parameter expansion and command
substitution into fields.

.sp 2
.B Pathname Expansion (File Name Generation)
.sp
.LP
Unless the -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 case(1) command.

.sp 2
.B Shell Patterns
.sp
.LP
A 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.
.LP
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.
.LP
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

.sp 2
.B Builtins
.sp
.LP
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. printf(1), echo(1), test(1),
etc).
.TP
alias  [ name[=string] ...  ]
If name=string is specified, the shell defines the
alias ``name'' with value ``string''.  If just ``name''
is specified, the value of the alias ``name'' is printed.
With no arguments, the alias builtin prints the
names and values of all defined aliases (see unalias).
.TP
bg [ job ] ...
Continue the specified jobs (or the current job if no
jobs are given) in the background.
.TP
command command arg...
Execute the specified builtin command.  (This is useful when you have a shell function with the same name
as a builtin command.)
.TP
cd [ directory ]
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.
.TP
\&. file
The commands in the specified file are read and executed by the shell.
.TP
eval string...
Concatenate all the arguments with spaces.  Then
re-parse and execute the command.
.TP
exec [ 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 exec command are marked as permanent, so that they are not undone when the exec command finishes.
.TP
exit [ exitstatus ]
Terminate the shell process.  If exitstatus is given
it is used as the exit status of the shell; otherwise
the exit status of the preceding command is used.
.TP
export name...
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
.nf

    export name=value

.fi
With no arguments the export command lists the names
of all exported variables.
.TP
fc  [-e editor] [first [last]]
.TP
fc  -l [-nr] [first [last]]
.TP
fc  -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands
previously entered to an interactive shell.
.RS +.5i
.TP 2
-e editor
Use the editor named by editor to edit the commands.  The
editor string is a command name, subject to search via the
PATH variable.  The value in the FCEDIT variable
is used as a default when -e is not specified.  If
FCEDIT is null or unset, the value of the EDITOR
variable is used.  If EDITOR is null or unset,
ed(1) is used as the editor.
.TP 2
-l (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 -r, with each command preceded by the command
number.
.TP 2
-n
Suppress command numbers when listing with -l.
.TP 2
-r
Reverse the order of the commands listed (with -l) or
edited (with neither -l nor -s).
.TP 2
-s
Re-execute the command without invoking an editor.
.TP 2
first
.TP 2
last
Select the commands to list or edit.  The number of
previous commands that can be accessed are determined
by the value of the HISTSIZE variable.  The value of first
or last or both are one of the following:
.TP 2
[+]number
A positive number representing a command
number; command numbers can be displayed
with the -l option.
.TP 2
-number
A negative decimal number representing the
command that was executed number of
commands previously.  For example, -1 is
the immediately previous command.
.TP 2
string
A string indicating the most recently
entered command that begins with that
string.  If the old=new operand is not also
specified with -s, the string form of the
first operand cannot contain an embedded
equal sign.
.TP
The following environment variables affect the execution of fc:
.TP 2
FCEDIT
Name of the editor to use.
.TP 2
HISTSIZE
The number of previous commands that are accessable.
.RE
.TP
fg [ job ]
Move the specified job or the current job to the
foreground.
.TP
getopts optstring var
The POSIX getopts command.
.TP
hash -rv command...
The shell maintains a hash table which remembers the
locations of commands.  With no arguments whatsoever,
the hash command  prints out the contents of this
table.  Entries which have not been looked at since
the last cd command are marked with an asterisk; it
is possible for these entries to be invalid.
.sp
With arguments, the hash command removes the specified commands from the hash table (unless they are
functions) and then locates them.   With the -v
option, hash prints the locations of the commands as
it finds them.  The -r option causes the hash command
to delete all the entries in the hash table except
for functions.
.TP
jobid [ job ]
Print the process id's of the processes in the job.
If the job argument is omitted, use the current job.
.TP
jobs
This command lists out all the background processes
which are children of the current shell process.
.TP
pwd
Print the current directory.  The builtin command may
differ from the program of the same name because the
builtin command remembers what the current directory
is rather than recomputing it each time.  This makes
it faster.  However, if the current directory is
renamed, the builtin version of pwd will continue to
print the old name for the directory.
.TP
read [ -p prompt ] [ -e ] variable...
The prompt is printed if the -p option is specified
and the standard input is a terminal.  Then a line is
read from the standard input.  The trailing newline
is deleted from the line and the line is split as
described in the section on word splitting above, and
the pieces are assigned to the variables in order.
If there are more pieces than variables, the remaining pieces (along with the characters in IFS that
separated them) are assigned to the last variable.
If there are more variables than pieces, the remaining variables are assigned the null string.
.sp
The -e option causes any backslashes in the input to
be treated specially.  If a backslash is followed by
a newline, the backslash and the newline will be
deleted.   If a backslash is followed by any other
character, the backslash will be deleted and the following character will be treated as though it were
not in IFS, even if it is.
.TP
readonly name...
The specified names are marked as read only, so that
they cannot be subsequently modified or unset.  The shell
allows the value of a variable to be set at the same
time it is marked read only by writing
.TP
readonly name=value
With no arguments the readonly command lists the
names of all read only variables.
.TP
set [ { -options | +options | -- } ] arg...
The set command performs three different functions.
.sp
With no arguments, it lists the values of all shell
variables.
.sp
If options are given, it sets the specified option
flags, or clears them as described in the section
called ``Argument List Processing''.
.sp
The third use of the set command is to set the values
of the shell's positional parameters to the specified
args.   To change the positional parameters without
changing any options, use ``--'' as the first argument to set.  If no args are present, the set command
will clear all the positional parameters (equivalent
to executing ``shift $#''.
.TP
setvar variable value
Assigns value to variable. (In general it is better
to write variable=value rather than using setvar.
Setvar is intended to be used in functions that
assign values to variables whose names are passed as
parameters.)
.TP
shift [ n ]
Shift the positional parameters n times.  A shift
sets the value of $1 to the value of $2, the value of
$2 to the value of $3, and so on, decreasing the
value of $# by one. If there are zero positional
parameters, shifting doesn't do anything.
.TP
trap [ action ] signal...
Cause the shell to parse and execute action when any
of the specified signals are received. The signals
are specified by signal number. Action may be null
or omitted; the former causes the specified signal to
be ignored and the latter causes the default action
to be taken. When the shell forks off a subshell, it
resets trapped (but not ignored) signals to the
default action. The trap command has no effect on
signals that were ignored on entry to the shell.
.TP
umask [ mask ]
Set the value of umask (see umask(2)) to the specified octal value.
If the argument is omitted, the
umask value is printed.
.TP
unalias [-a] [name]
If ``name'' is specified, the shell removes that alias.
If ``-a'' is specified, all aliases are removed.
.TP
unset name...
The specified variables and functions are unset and
unexported. If a given name corresponds to both a
variable and a function, both the variable and the
function are unset.
.TP
wait [ job ]
Wait for the specified job to complete and return the
exit status of the last process in the job. If the
argument is omitted, wait for all jobs to complete
and the return an exit status of zero.
.LP
.sp 2
.B Command Line Editing
.sp
.LP
When sh is being used interactively from a terminal, the current command
and the command history (see fc in Builtins) can be edited using vi-mode
command-line editing. This mode uses commands, described below, similar
to a subset of those described in the vi man page.
The command set -o vi enables vi-mode editing and place sh into vi
insert mode.
With vi-mode enabled, sh can be switched between insert mode and command
mode. The editor is not described in full here, but will be in a later
document. It's similar to vi: typing <ESC> will throw you into
command VI command mode. Hitting <return> while in command mode
will pass the line to the shell.