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.Ss Search and Execution
There are three types of commands: shell functions, builtin commands, and
normal programs -- and the command is searched for (by name) in that order.
They each are executed in a different way.
.Pp
When a shell function is executed, all of the shell positional parameters
(except $0, which remains unchanged) are set to the arguments of the shell
function.
The variables which are explicitly placed in the environment of
the command (by placing assignments to them before the function name) are
made local to the function and are set to the values given.
Then the command given in the function definition is executed.
The positional parameters are restored to their original values
when the command completes.
This all occurs within the current shell.
.Pp
Shell builtins are executed internally to the shell, without spawning a
new process.
.Pp
Otherwise, if the command name doesn't match a function or builtin, the
command is searched for as a normal program in the file system (as
described in the next section).
When a normal program is executed, the shell runs the program,
passing the arguments and the environment to the program.
If the program is not a normal executable file (i.e., if it does
not begin with the "magic number" whose
.Tn ASCII
representation is "#!", so
.Xr execve 2
returns
.Er ENOEXEC
then) the shell will interpret the program in a subshell.
The child shell will reinitialize itself in this case,
so that the effect will be as if a
new shell had been invoked to handle the ad-hoc shell script, except that
the location of hashed commands located in the parent shell will be
remembered by the child.
.Pp
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic
number as a "shell procedure".
.Ss Path Search
When locating a command, the shell first looks to see if it has a shell
function by that name.
Then it looks for a builtin command by that name.
If a builtin command is not found, one of two things happen:
.Bl -enum
.It
Command names containing a slash are simply executed without performing
any searches.
.It
The shell searches each entry in
.Ev PATH
in turn for the command.
The value of the
.Ev PATH
variable should be a series of entries separated by colons.
Each entry consists of a directory name.
The current directory may be indicated
implicitly by an empty directory name, or explicitly by a single period.
.El
.Ss Command Exit Status
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 shell function.
.Pp
If a command consists entirely of variable assignments then the
exit status of the command is that of the last command substitution
if any, otherwise 0.
.Ss Complex Commands
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:
.Bl -bullet
.It
simple command
.It
pipeline
.It
list or compound-list
.It
compound command
.It
function definition
.El
.Pp
Unless otherwise stated, the exit status of a command is that of the last
simple command executed by the command.
.Ss Pipelines
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.
The standard output of the last
command is inherited from the shell, as usual.
.Pp
The format for a pipeline is:
.Pp
.Dl [!] command1 [ | command2 ...]
.Pp
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.
.Pp
If the pipeline is not in the background (discussed later), the shell
waits for all commands to complete.
.Pp
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.
.Pp
Because pipeline assignment of standard input or standard output or both
takes place before redirection, it can be modified by redirection.
For example:
.Pp
.Dl $ command1 2\*[Gt]\*[Am]1 | command2
.Pp
sends both the standard output and standard error of command1
to the standard input of command2.
.Pp
A ; or
.Aq newline
terminator causes the preceding AND-OR-list (described
next) to be executed sequentially; a \*[Am] causes asynchronous execution of
the preceding AND-OR-list.
.Pp
Note that unlike some other shells, each process in the pipeline is a
child of the invoking shell (unless it is a shell builtin, in which case
it executes in the current shell -- but any effect it has on the
environment is wiped).
.Ss Background Commands -- \*[Am]
If a command is terminated by the control operator ampersand (\*[Am]), the
shell executes the command asynchronously -- that is, the shell does not
wait for the command to finish before executing the next command.
.Pp
The format for running a command in background is:
.Pp
.Dl command1 \*[Am] [command2 \*[Am] ...]
.Pp
If the shell is not interactive, the standard input of an asynchronous
command is set to
.Pa /dev/null .
.Ss Lists -- Generally Speaking
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.
.Ss Short-Circuit List Operators
.Dq \*[Am]\*[Am]
and
.Dq ||
are AND-OR list operators.
.Dq \*[Am]\*[Am]
executes the first command, and then executes the second command if and only
if the exit status of the first command is zero.
.Dq ||
is similar, but executes the second command if and only if the exit status
of the first command is nonzero.
.Dq \*[Am]\*[Am]
and
.Dq ||
both have the same priority.
Note that these operators are left-associative, so
.Dq true || echo bar && echo baz
writes
.Dq baz
and nothing else.
This is not the way it works in C.
.Ss Flow-Control Constructs -- if, while, for, case
The syntax of the if command is
.Bd -literal -offset indent
if list
then list
[ elif list
then    list ] ...
[ else list ]
fi
.Ed
.Pp
The syntax of the while command is
.Bd -literal -offset indent
while list
do   list
done
.Ed
.Pp
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, which causes it to
repeat until the exit status of the first list is zero.
.Pp
The syntax of the for command is
.Bd -literal -offset indent
for variable in word ...
do   list
done
.Ed
.Pp
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
.Dq {
and
.Dq } .
.Pp
The syntax of the break and continue command is
.Bd -literal -offset indent
break [ num ]
continue [ num ]
.Ed
.Pp
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.
.Pp
The syntax of the case command is
.Bd -literal -offset indent
case word in
pattern) list ;;
\&...
esac
.Ed
.Pp
The pattern can actually be one or more patterns (see
.Sx Shell Patterns
described later), separated by
.Dq \*(Ba
characters.
.Ss Grouping Commands Together
Commands may be grouped by writing either
.Pp
.Dl (list)
.Pp
or
.Pp
.Dl { list; }
.Pp
The first of these executes the commands in a subshell.
Builtin commands grouped into a (list) will not affect the current shell.
The second form does not fork another shell so is slightly more efficient.
Grouping commands together this way allows you to redirect
their output as though they were one program:
.Pp
.Bd -literal -offset indent
{ echo -n \*q hello \*q ; echo \*q world" ; } \*[Gt] greeting
.Ed
.Pp
Note that
.Dq }
must follow a control operator (here,
.Dq \&; )
so that it is recognized as a reserved word and not as another command argument.
.Ss Functions
The syntax of a function definition is
.Pp
.Dl name ( ) command
.Pp
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
.Dq {
and
.Dq } .
.Pp
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
.Pp
.Dl local [ variable | - ] ...
.Pp
Local is implemented as a builtin command.
.Pp
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.
.Pp
The only special parameter that can be made local is
.Dq - .
Making
.Dq -
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.
.Pp
The syntax of the return command is
.Pp
.Dl return [ exitstatus ]
.Pp
It terminates the currently executing function.
Return is implemented as a builtin command.
.Ss Variables and Parameters
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
.Pp
.Dl name=value
.Pp
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.
.Ss Positional Parameters
A positional parameter is a parameter denoted by a number (n \*[Gt] 0).
The shell sets these initially to the values of its command line arguments
that follow the name of the shell script.
The
.Ic set
builtin can also be used to set or reset them.
.Ss Special Parameters
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.
.Bl -tag -width thinhyphena
.It *
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
.Ev IFS
variable, or by a
.Aq space
if
.Ev IFS
is unset.
.It @
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
.Dq abc
and $2 is
.Dq def ghi ,
then
.Qq $@
expands to
the two arguments:
.Pp
.Sm off
.Dl \*q abc \*q \  \*q def\ ghi \*q
.Sm on
.It #
Expands to the number of positional parameters.
.It \&?
Expands to the exit status of the most recent pipeline.
.It - (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.
.It $
Expands to the process ID of the invoked shell.
A subshell retains the same value of $ as its parent.
.It \&!
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.
.It 0 (Zero.)
Expands to the name of the shell or shell script.
.El
.Ss Word Expansions
This clause describes the various expansions that are performed on words.
Not all expansions are performed on every word, as explained later.
.Pp
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.
.Pp
The order of word expansion is:
.Bl -enum
.It
Tilde Expansion, Parameter Expansion, Command Substitution,
Arithmetic Expansion (these all occur at the same time).
.It
Field Splitting is performed on fields
generated by step (1) unless the
.Ev IFS
variable is null.
.It
Pathname Expansion (unless set
.Fl f
is in effect).
.It
Quote Removal.
.El
.Pp
The $ character is used to introduce parameter expansion, command
substitution, or arithmetic evaluation.
.Ss Tilde Expansion (substituting a user's home directory)
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
.Pa ~/foobar ) ,
the tilde is replaced with the value of the
.Va HOME
variable (the current user's home directory).
.Ss Parameter Expansion
The format for parameter expansion is as follows:
.Pp
.Dl ${expression}
.Pp
where expression consists of all characters until the matching
.Dq } .
Any
.Dq }
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
.Dq } .
.Pp
The simplest form for parameter expansion is:
.Pp
.Dl ${parameter}
.Pp
The value, if any, of parameter is substituted.
.Pp
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:
.Bl -enum
.It
Pathname expansion is not performed on the results of the expansion.
.It
Field splitting is not performed on the results of the
expansion, with the exception of @.
.El
.Pp
In addition, a parameter expansion can be modified by using one of the
following formats.
.Bl -tag -width aaparameterwordaaaaa
.It ${parameter:-word}
Use Default Values.
If parameter is unset or null, the expansion of word
is substituted; otherwise, the value of parameter is substituted.
.It ${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.
.It ${parameter:?[word]}
Indicate Error if Null or Unset.