sh.1

早期freebsd实现

directory name.
The current directory
may be indicated by an empty directory name.
.LP
Command names containing a slash are simply executed without performing any of the above searches.
.sp 2
.B Command Exit Status
.sp
.LP
Each command has an exit status that can influence the behavior
of other shell commands.  The paradigm is that a command exits
with zero for normal or success, and non-zero for failure,
error, or a false indication.  The man page for each command
should indicate the various exit codes and what they mean.
Additionally, the builtin commands return exit codes, as does
an executed function.
.sp 2
.B Complex Commands
.sp
.LP
Complex commands are combinations of simple commands
with control operators or reserved words, together creating a larger complex
command.  More generally, a command is one of the following:
.nf

  - simple command

  - pipeline

  - list or compound-list

  - compound command

  - function definition

.fi
.LP
Unless otherwise stated, the exit status of a command is
that of the last simple command executed by the command.
.sp 2
.B Pipeline
.sp
.LP
A pipeline is a sequence of one or more commands separated
by the control operator |.  The standard output of all but
the last command is connected to the standard input
of the next command.
.LP
The format for a pipeline is:
.nf

[!] command1 [ | command2 ...]

.fi
.LP
The standard output of command1 is connected to the standard
input of command2. The standard input, standard output, or
both of a command is considered to be assigned by the
pipeline before any redirection specified by redirection
operators that are part of the command.
.LP
If the pipeline is not in the background (discussed later),
the shell waits for all commands to complete.
.LP
If the reserved word ! does not precede the pipeline, the
exit status is the exit status of the last command specified
in the pipeline.  Otherwise, the exit status is the logical
NOT of the exit status of the last command.  That is, if
the last command returns zero, the exit status is 1; if
the last command returns greater than zero, the exit status
is zero.
.LP
Because pipeline assignment of standard input or standard
output or both takes place before redirection, it can be
modified by redirection.  For example:
.nf

$ command1 2>&1 | command2

.fi
sends both the standard output and standard error of command1
to the standard input of command2.
.LP
A ; or <newline> terminator causes the preceding
AND-OR-list (described next) to be executed sequentially; a & causes
asynchronous execution of the preceding AND-OR-list.
.sp 2
.B Background Commands -- &
.sp
.LP
If a command is terminated by the control operator ampersand
(&), the shell executes the command asynchronously -- that is,
the shell does not wait for
the command to finish before executing the next command.
.LP
The format for running a command in background is:
.nf

command1 & [command2 & ...]

.fi
If the shell is not interactive, the standard input of an
asynchronous command is set to /dev/null.
.sp 2
.B Lists -- Generally Speaking
.sp
.LP
A list is a sequence of zero or more commands separated by
newlines, semicolons, or ampersands,
and optionally terminated by one of these three characters.
The commands in a
list are executed in the order they are written.
If command is followed by an ampersand, the shell starts the
command and immediately proceed onto the next command;
otherwise it waits for the command to terminate before
proceeding to the next one.
.LP
``&&'' and ``||'' are AND-OR list operators.  ``&&'' executes
the first command, and then executes the second command
iff the exit status of the first command is zero.  ``||''
is similar, but executes the second command iff the exit
status of the first command is nonzero.  ``&&'' and ``||''
both have the same priority.
.LP
The syntax of the if command is
.nf

    if list
    then list
    [ elif list
    then    list ] ...
    [ else list ]
    fi

.fi
The syntax of the while command is
.nf

    while list
    do   list
    done

.fi
The two lists are executed repeatedly while the exit status of the first list is zero.  The until command is similar, but has the word until in place of while
repeat until the exit status of the first list is zero.
.LP
The syntax of the for command is
.nf

    for variable in word...
    do   list
    done

.fi
The words are expanded, and then the list is executed
repeatedly with the variable set to each word in turn.  do
and done may be replaced with ``{'' and ``}''.
.LP
The syntax of the break and continue command is
.nf

    break [ num ]
    continue [ num ]

.fi
Break terminates the num innermost for or while loops.
Continue continues with the next iteration of the innermost loop.  These are implemented as builtin commands.
.LP
The syntax of the case command is
.nf

    case word in
    pattern) list ;;
    ...
    esac

.fi
.LP
The pattern can actually be one or more patterns (see Shell
Patterns described later), separated by ``|'' characters.

.LP
Commands may be grouped by writing either
.nf

    (list)

.fi
or
.nf

    { list; }

.fi
The first of these executes the commands in a subshell.
.sp 2
.B Functions
.sp
.LP
The syntax of a function definition is
.nf

    name ( ) command

.fi
.LP
A function definition is an executable statement; when
executed it installs a function named name and returns an
exit status of zero.   The command is normally a list
enclosed between ``{'' and ``}''.
.LP
Variables may be declared to be local to a function by
using a local command.  This should appear as the first
statement of a function, and the syntax is
.nf

    local [ variable | - ] ...

.fi
Local is implemented as a builtin command.
.LP
When a variable is made local, it inherits the initial
value and exported and readonly flags from the variable
with the same name in the surrounding scope, if there is
one.  Otherwise, the variable is initially unset.   The shell
uses dynamic scoping, so that if you make the variable x
local to function f, which then calls function g, references to the variable x made inside g will refer to the
variable x declared inside f, not to the global variable
named x.
.LP
The only special parameter than can be made local is
``-''.  Making ``-'' local any shell options that are
changed via the set command inside the function to be
restored to their original values when the function
returns.
.LP
The syntax of the return command is
.nf

    return [ exitstatus ]

.fi
It terminates the currently executing function.  Return is
implemented as a builtin command.
.sp 2
.B Variables and Parameters
.sp
.LP
The shell maintains a set of parameters.  A parameter
denoted by a name is called a variable. When starting up,
the shell turns all the environment variables into shell
variables.  New variables can be set using the form
.nf

    name=value

.fi
.LP
Variables set by the user must have a name consisting solely
of alphabetics, numerics, and underscores - the first of which
must not be numeric.  A parameter can also be denoted by a number
or a special character as explained below.
.sp 2
.B Positional Parameters
.sp
.LP
A positional parameter is a parameter denoted by a number (n > 0).
The shell sets these initially to the values of its command
line arguments that follow the name of the shell script.
The set(1) builtin can also be used to set or reset them.
.sp 2
.B Special Parameters
.sp
.LP
A special parameter is a parameter denoted by one of the following
special characters.  The value of the parameter is listed
next to its character.
.TP
*
Expands to the positional parameters, starting from one.  When
the expansion occurs within a double-quoted string
it expands to a single field with the value of each parameter
separated by the first character of the IFS variable, or by a
<space> if IFS is unset.
.TP
@
Expands to the positional parameters, starting from one.  When
the expansion occurs within double-quotes, each positional
parameter expands as a separate argument.
If there are no positional parameters, the
expansion of @ generates zero arguments, even when @ is
double-quoted.  What this basically means, for example, is
if $1 is ``abc'' and $2 is ``def ghi'', then "$@" expands to
the two arguments:

"abc"   "def ghi"
.TP
#
Expands to the number of positional parameters.
.TP
?
Expands to the exit status of the most recent pipeline.
.TP
- (Hyphen)
Expands to the current option flags (the single-letter
option names concatenated into a string) as specified on
invocation, by the set builtin command, or implicitly
by the shell.
.TP
$
Expands to the process ID of the invoked shell.  A subshell
retains the same value of $ as its parent.
.TP
!
Expands to the process ID of the most recent background
command executed from the current shell.  For a
pipeline, the process ID is that of the last command in the
pipeline.
.TP
0 (Zero.)
Expands to the name of the shell or shell script.
.LP
.sp 2
.B Word Expansions
.sp
.LP
This clause describes the various expansions that are
performed on words.  Not all expansions are performed on
every word, as explained later.
.LP
Tilde expansions, parameter expansions, command substitutions,
arithmetic expansions, and quote removals that occur within
a single word expand to a single field.  It is only field
splitting or pathname expansion that can create multiple
fields from a single word. The single exception to this
rule is the expansion of the special parameter @ within
double-quotes, as was described above.
.LP
The order of word expansion is:
.LP
(1)  Tilde Expansion, Parameter Expansion, Command Substitution,
Arithmetic Expansion (these all occur at the same time).
.LP
(2)  Field Splitting is performed on fields
generated by step (1) unless the IFS variable is null.
.LP
(3)  Pathname Expansion (unless set -f is in effect).
.LP
(4)  Quote Removal.
.LP
The $ character is used to introduce parameter expansion, command
substitution, or arithmetic evaluation.
.sp 2
.B Tilde Expansion (substituting a user's home directory)
.sp	
.LP
A word beginning with an unquoted tilde character (~) is
subjected to tilde expansion.  All the characters up to
a slash (/) or the end of the word are treated as a username
and are replaced with the user's home directory.  If the
username is missing (as in ~/foobar), the tilde is replaced
with the value of the HOME variable (the current user's
home directory).

.sp 2
.B Parameter Expansion
.sp
.LP
The format for parameter expansion is as follows:
.nf

    ${expression}

.fi
where expression consists of all characters until the matching }.  Any }
escaped by a backslash or within a quoted string, and characters in
embedded arithmetic expansions, command substitutions, and variable
expansions, are not examined in determining the matching }.
.LP
The simplest form for parameter expansion is:
.nf

    ${parameter}

.fi
The value, if any, of parameter is substituted.
.LP
The parameter name or symbol can be enclosed in braces, which are
optional except for positional parameters with more than one digit or
when parameter is followed by a character that could be interpreted as
part of the name.
If a parameter expansion occurs inside
double-quotes:
.LP
1) Pathname expansion is not performed on the results of the
expansion.
.LP
2) Field splitting is not performed on the results of the
expansion, with the exception of @.
.LP
In addition, a parameter expansion can be modified by using one of the
following formats.
.sp
.TP
${parameter:-word}
Use Default Values.  If parameter is unset or
null, the expansion of word is
substituted; otherwise, the value of
parameter is substituted.
.TP
${parameter:=word}
Assign Default Values.  If parameter is unset
or null, the expansion of word is
assigned to parameter.  In all cases, the
final value of parameter is
substituted.  Only variables, not positional
parameters or special parameters, can be
assigned in this way.
.TP
${parameter:?[word]}
Indicate Error if Null or Unset.  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.
.TP
${parameter:+word}
Use Alternate Value.  If parameter is unset
or null, null is substituted;
otherwise, the expansion of word is
substituted.
.LP
In the parameter expansions shown previously, use of the colon in the