V10/man/man1/sh.1

.ds OK [\|
.ds CK \|]
.TH SH 1
.CT 1 shell proc_man dirs files
.SH NAME
sh, cd, wait, whatis \- shell, the standard command programming language
.SH SYNOPSIS
.B sh
[
.B -acefiknpstuvx
]
[
.I args
]
.SH DESCRIPTION
.I Sh
is a command programming language
that executes commands read from a terminal
or a file.
See `Invocation' below
for the meaning of arguments to the shell.
.SS Definitions
A
.I blank
is a tab or a space.
A
.I name
is a sequence of letters, digits, or underscores beginning with a letter
or underscore.
A
.I parameter
is a name, a digit, or any of the characters
.BR \(** ,
.BR @ ,
.BR # ,
.BR ? ,
.BR - ,
.BR $ ,
and
.BR !\\^ .
A
.I word
is a sequence of characters and quoted strings set off by operators,
blanks, or newlines; see `Quoting'.
.SS Commands
A
.I simple-command
is a sequence of
.I words
separated by
.IR blanks .
The first word specifies the name of the command to
be executed.
Except as specified below,
the remaining words are passed as arguments
to the invoked command.
The command name is passed as argument 0; see
.IR exec (2).
The
.I value
of a simple-command is its exit status
if it terminates normally, or 
.BR 0200+ status
if it terminates abnormally; see
.IR signal (2)
for a list of
status values.
.PP
A
.I pipeline
is a sequence of one or more
.I commands
separated by
.BR | .
If there is more than one command, each is run
in a subshell; 
.B |
denotes a
.IR pipe (2)
connecting the standard output of one command to the standard input
of the next.
Each command is run as a separate process;
the shell waits for the last command to terminate.
The exit status of a pipeline is the exit status of the last command.
.PP
A
.I list
is a sequence of one or more
pipelines
separated by
.BR ; ,
.BR & ,
.BR && ,
or
.BR || ,
and terminated by
.B ;
or
.BR & .
Of these four symbols,
.B ;
and
.B &
have equal precedence,
which is lower than that of
.B &&
and
.BR || .
The symbols
.B &&
and
.B ||
also have equal precedence.
A semicolon
.RB ( ; )
causes sequential execution of the preceding pipeline; an ampersand
.RB ( & )
causes asynchronous execution of the pipeline; the shell does not 
wait and proceeds as if the pipeline
had returned zero exit status.
The symbol
.B &&
.RB ( || )
causes the
.I list
following it to be executed only if the preceding
pipeline
returns a zero (non-zero) exit status.
One or more newlines may follow any sequencing operator
.RB ( "; & && ||" ).
.PP
One or more newlines may always be used in place of a single semicolon,
and newlines may be freely inserted after any of
.BR "| ; & && || ;; if do then elif else fi done while until" .
.PP
A
.I command
is either a simple-command
or one of the following.
Unless otherwise stated,
the value returned by a command is that of the
last simple-command executed in the command.
.PP
.PD 0
.TP
\f5for\fP \f2name\fP \*(OK \&\f5in\fP \f2word\fP .\|.\|. \&\f5;\fP \*(CK \
\f5do\fP \f2list\fP \f5;\fP \f5done\fP
A
.L for
command executes a
.I list
of commands once for each
.I word,
with
.I name
set to each 
.I word
in turn.
If
.BI in " word"
.RB .\|.\|. " ;"
is omitted or replaced by newlines, then
the
.I list
is executed once for each positional parameter
that is set; see `Parameter Substitution'.
.TP
\f5case\fP \f2word\fP \&\f5in\fP \*(OK \f2pattern\fP \*(OK | \
\f2pattern\fP \*(CK .\|.\|. \&\f5)\fP \f2list\fP \&\f5;;\fP \*(CK .\|.\|. \f5esac\fP
A
.L case
command executes the
.I list
associated with the first
.I pattern
that matches
.IR word .
The form of the patterns is
the same as that used for
file-name generation (see `File Name Generation')
except that a slash, a leading dot, or a dot immediately
following a slash need not be matched explicitly.
Newlines may precede each 
.IR pattern 
and replace the last
.B ;;
before
.BR esac .
.TP
\f5if\fP \f2list\fP \&\f5then\fP \f2list\fP \*(OK \
\&\f5elif\fP \f2list\fP \&\f5then\fP \f2list\fP \*(CK .\|.\|. \
\*(OK \&\f5else\fP \f2list\fP \*(CK \&\f5f\&i\fP
The
.I list
following
.L if
is executed and, if it
returns a zero exit status, the
.I list
following
the first
.L then
is executed.
Otherwise, the
.I lists
in
.L elif
clauses are executed in turn until one returns zero status;
then the
.I list
following
the next
.L then
is executed.
Otherwise, the
.L else
.I list
is executed.
If no
.L else
.I list
or
.L then
.I list
is executed, then the
.B if
command returns a zero exit status.
.TP
\f5while\fP \f2list\fP \&\f5do\fP \f2list\fP \&\f5done\fP
A
.L while
command repeatedly executes the
.L while
.I list
and, if the exit status of the last command in the list is zero, executes
the
.L do
.IR list ;
otherwise the loop terminates.
If no commands in the
.L do
.I list
are executed, then the
.L while
command returns a zero exit status;
.L until
may be used in place of
.L while
to negate
the loop termination test.
.TP
\f5(\fP\f2list\^\f5)\fP
.br
Execute
.I list
in a sub-shell.
.TP
\f5{\fP\f2list\^\fP\f5}\fR
.br
.I list
is simply executed.
.TP
\f2name\fP \f5() \f2command\fP
Define a function
which is referenced by
.I name.
The body of the function
is the
.IR command .
The most useful form of
.I command
is a sequence of commands enclosed in braces
.BR "{ }" .
Execution of functions is described under
.IR Execution 
below.
.PD
.PP
These words
are only recognized as the first word of a command and when not quoted:
.B
if then else elif fi case esac for while until do done\fR.
.SS Comments
A word beginning with
.B #
causes that word and all the following characters up to a newline
to be ignored.
.SS Command Substitution
The standard output from a command enclosed in
a pair of grave accents
.B ` `
may be used as part or all
of a word;
trailing newlines are removed.
.SS Parameter Substitution
The character
.B $
is used to introduce substitutable
.IR parameters .
There are two types of parameters,
positional and keyword.
If
.I parameter
is a digit, it is a positional parameter.
Positional parameters may be assigned values by
.BR set .
Keyword parameters (also known as variables)
may be assigned values by writing:
.RS
.PP
.IB name = value
\*(OK
.IB name = value
\*(CK .\|.\|.
.RE
.PP
Pattern-matching is not performed on
.IR value .
There cannot be a function and a variable with the same
.IR name  .
.PP
.PD 0
.TP
\f5${\fP\f2parameter\^\fP\f5}\fP
The value, if any, of the parameter is substituted.
The braces are required only when
.I parameter
is followed by a letter, digit, or underscore
that is not to be interpreted as part of its name.
If
.I parameter
is
.B \(**
or
.BR @ ,
all the positional
parameters, starting with
.BR $1 ,
are substituted
(separated by spaces).
Parameter
.B $0
is set from argument zero when the shell
is invoked.
.TP
\f5${\fP\f2parameter\^\fP\f5:-\fP\f2word\^\fP\f5}\fP
If
.I parameter
is set and is non-null, substitute its value;
otherwise substitute
.IR word .
.TP
\f5${\fP\f2parameter\^\fP\f5:=\fP\f2word\^\fP\f5}\fP
If
.I parameter
is not set or is null
set it to
.IR word ;
the value of the parameter is substituted.
Positional parameters may not be assigned to
in this way.
.TP
\f5${\fP\f2parameter\^\fP\f5:?\fP\f2word\^\fP\f5}\fP
If
.I parameter
is set and is non-null, substitute its value;
otherwise, print
.I word
and exit from the shell.
If
.I word
is omitted, the message
``parameter null or not set''
is printed.
.TP
\f5${\fP\f2parameter\^\fP\f5:+\fP\f2word\^\fP\f5}\fP
If
.I parameter
is set and is non-null, substitute
.IR word ;
otherwise substitute nothing.
.PD
.PP
In the above,
.I word
is not evaluated unless it is
to be used as the substituted string,
so that, in the following example,
.B pwd
is executed only if
.B d
is not set or is null:
.IP
.B echo ${d:-\`pwd\`}
.PP
If the colon
.RB ( : )
is omitted from the above expressions, the
shell only checks whether
.I parameter
is set or not.
.PP
The following
parameters
are automatically set by the shell:
.RS
.PD 0
.TP
.B #
The number of positional parameters in decimal.
.TP
.B -
Flags supplied to the shell on invocation or by
the
.B set
command.
.TP
.B ?
The decimal value returned by the last synchronously executed command;
see
.IR exit (2).
.TP
.B $
The process number of this shell.
.TP
.B !
The process number of the last background command invoked.
.PD
.RE
.PP
The following
parameters
are used by the shell:
.RS
.PD 0
.TP
.B
.SM HOME
The default argument (home directory) for the
.I cd
command.
.TP
.B
.SM PATH
The search path for commands; see `Execution'.
.TP
.B
.SM CDPATH
The search path for the
.I cd
command.
.TP
.B
.SM MAIL
If this parameter is set to the name of a mail file
the shell informs the user of the arrival of mail
in the specified file.
The file is inspected every three minutes.
.TP
.B
.SM HISTORY
If this parameter is set to the name of a writable file,
the shell appends interactive input to the file, for use by the command
.IR = (1).
.TP
.SM
.B PS1
Primary prompt string, by default
.LR $ .
.TP
.SM
.B PS2
Secondary prompt string, by default
.LR > .
.TP
.SM
.B IFS
Internal field separators,
normally space, tab, and newline.
.PD
.RE
.PP
The shell gives default values to
\f5\s-1PATH\s+1\fP, \f5\s-1PS1\s+1\fP, \f5\s-1PS2\s+1\fP and \f5\s-1IFS\s+1\fP.
.SM
.B HOME
is set by
.IR login (8).
.SS Blank Interpretation
After parameter and command substitution,
the results of substitution are scanned for internal field separator
characters (those found in
.BR \s-1IFS\s+1 )
and split into distinct arguments where such characters are found.
Explicit null arguments (\^\f5"\^"\fP or \f5\'\^\'\fP\^) are retained.
Implicit null arguments
(those resulting from
.I parameters
that have no values) are removed.
.SS File Name Generation
Following substitution, each command
.I word
is scanned for
the characters
.BR \(** ,
.BR ? ,
and
.BR \*(OK .
If one of these characters appears
the word is regarded as a
.IR pattern .
The word is replaced with alphabetically sorted file names that match the pattern.
If no file name is found that matches the pattern,
the word is left unchanged.
The directories
.B .
and
.B ..
(initially or after a
.BR / )
are only matched by patterns beginning
with an explicit period.
The character
.B /
itself must be matched explicitly.
.PP
.PD 0
.RS
.TP
.B \(**
Matches any string, including the null string.
.TP
.B ?
Matches any single character.
.TP
.BR \*(OK .\|.\|. \*(CK
Matches any one of the enclosed characters.
A pair of characters separated by
.B -
matches any
character lexically between the pair, inclusive.
If the first character following the opening
.L \*(OK
is a
.L ^
any character not enclosed is matched.
.PD
.RE
.SS Quoting
These characters have a special meaning to the shell
and terminate a word unless quoted:
.IP
.L
;  &  (  )  |  <  >  { }
newline space tab
.PP
(The characters \f5{\fP and \f5}\fP need not be quoted inside a \f5${\^}\fP construction.)
A character may be
.I quoted
(i.e., made to stand for itself)
by preceding
it with a
.BR \e .
The pair
.BR \e newline
is ignored.
All characters enclosed between a pair of single quote marks \f5\'\^\'\fP\^
(except a single quote)
are quoted.
Inside double quote marks
\f5"\^"\fP
parameter and command substitution occurs and
.B \e
quotes the characters
.BR \e ,
.BR \` ,
\f5"\fP,
and
.BR $ .
.B
"$\(**"
is equivalent to
\f5"$1 \|$2\fP \|.\|.\|.\f5"\fP,
whereas
.B
"$@"
is equivalent to
.B
"$1"\|
.B
"$2"\|
\&.\|.\|.\|.
.SS Prompting
When used interactively,
the shell prompts with the value of
.SM
.B PS1
before reading a command.
If at any time a newline is typed and further input is needed
to complete a command, the secondary prompt
(i.e., the value of
.BR \s-1PS2\s+1 )
is issued.
.SS Input/Output
Before a command is executed, its input and output
may be redirected using a special notation interpreted by the shell.
The following may appear anywhere in a simple-command
or may precede or follow a
.I command
and are not
passed on to the invoked command;
substitution occurs before
.I word
or
.I digit
is used:
.PP
.PD 0
.TP 14
.BI < word
Use file
.I word
as standard input (file descriptor 0).
.TP
.BI > word
Use file
.I word
as standard output (file descriptor 1).
If the file does not exist it is created;
otherwise, it is truncated to zero length.
.TP
.BI >> word
Use file
.I word
as standard output.
If the file exists output is appended to it (by first seeking to the end-of-file);
otherwise, the file is created.
.TP
.BI << word
The shell input is read up to a line that is the same as
.IR word ,
or to an end-of-file.
The resulting document becomes
the standard input.
If any character of
.I word
is quoted, no interpretation
is placed upon the characters of the document;
otherwise, parameter and command substitution occurs,
(unescaped)
.BR \e newline
is ignored,
and
.B \e
must be used to quote the characters
.BR \e ,
.BR $ ,
.BR \` ,
and the first character of
.IR word .
.TP
.BI <& digit
Use the file associated with file descriptor
.I digit
as standard input.
Similarly for the standard output using
.BI >& digit .
.TP
.B <&-
The standard input is closed.
Similarly for the standard output using
.BR >&- .
.PD
.PP
If any of the above is preceded by a digit,
the
file descriptor which will be associated with the file
is that specified
by the digit
(instead of the default 0 or 1).
For example:
.IP
.RB .\|.\|. " 2>&1"
.PP
associates file descriptor 2 with the file currently associated with
file descriptor 1.
.PP
The order in which redirections are specified is significant.
The shell evaluates redirections left-to-right.
For example:
.IP
.RB .\|.\|. " 1>xxx 2>&1"
.PP
first associates file descriptor 1 with file
.LR xxx ,
then associates file descriptor 2 with the same file as
descriptor 1, namely
.LR xxx ,
while
.IP
.RB .\|.\|. " 2>&1 1>xxx"
.PP
associates file descriptor 2 
with the current value of file descriptor 1 (typically the terminal)
and file descriptor 1 with
.LR xxx .
.PP
If a command is followed by
.BR & ,
the default standard input
for the command
is the empty file
.BR /dev/null .
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by
input/output specifications.
.SS Environment
The
.I environment
is a list of strings, conventionally function definitions
and name-value pairs, that is passed to
an executed program in the same way as a normal argument list; see
.IR environ (5).
The shell interacts with the environment in several ways.
On invocation, the shell scans the environment
and creates a
parameter or function
for each name found,
giving it the corresponding value.
If the user modifies the value of any of these
parameters
or creates new parameters,
none of these affects the environment
unless the
.B export
command is used to bind the shell's
parameter
to the environment; see also
.BR "set -a" .
A parameter may be removed from the environment
with the
.B unset
command.
The environment seen by any executed command is thus composed
of any unmodified name-value pairs originally inherited by the shell,
minus any pairs removed by
.BR unset ,
plus any modifications or additions,
all of which must be noted in
.B export
commands.
.PP
The environment for any
.I simple-command
may be augmented by prefixing it with one or more assignments to
parameters (but not functions).
Thus
.L tabs
gets the same environment in both lines below,
but the shell has one less variable in the second.
.IP
.B (export TERM; TERM=450; tabs)
.br
.B TERM=450 tabs 
.PP
If the
.B -k
flag is set,
.I all
keyword arguments are placed in the environment,
even if they occur after the command name.
.SS Signals
.B SIGINT
and
.B SIGQUIT
(see
.IR signal (2))
for an invoked
command are ignored if the command is followed by
.BR & ;
otherwise signals have the values
inherited by the shell from its parent
(but see also
the
.B trap
command below).
.SS Execution
Each time a command is executed, the above substitutions are
carried out.
If the command name matches the name of a defined function, the function is executed
in the shell process.
(Note how this differs from calling a shell script.)
The positional parameters
.BR $1 ,
.BR $2 ,
\&.\|.\|.\|.
are set to the arguments of the function.
If the command name does not match a function, but matches one of the
builtin commands
listed below, it is executed in the shell process.
If the command name matches neither a
builtin command
nor the name of a defined function,
a new process is created and an attempt is made to
execute the command via
.IR exec (2).
.PP
The shell parameter
.B
.SM PATH
defines the search path for
the directory containing the command.
Alternative directory names are separated by
a colon
.RB ( : ).
The default path is
.B :/bin:/usr/bin
(specifying the current directory,
.BR /bin ,
and
.BR /usr/bin ,
in that order).
Note that the current directory is specified by a null path name,
which can appear immediately after the equal sign
or between the colon delimiters anywhere else in the path list.
If the command name contains a \f5/\fP the search path
is not used.
Otherwise, each directory in the path is
searched for an executable file.
If the file has execute permission but is not executable by
.IR exec (2),
it is assumed to be a `shell script', a file of shell commands.
A sub-shell is spawned to read it.
A parenthesized command is also executed in
a sub-shell.
.SS Builtin Commands
Input/output redirection is permitted for these commands.
File descriptor 1 is the default output location.
.PP
.PD 0
.TP
.B :
No effect; the command does nothing.
A zero exit code is returned.
.br
.TP
.BI ". " file
Read and execute commands from
.I file
and return.
The search path
specified by
.B
.SM PATH
is used to find the directory containing
.IR file .
.TP
\f5builtin\fP \*(OK \f2command\fP \*(CK
Execute the builtin
.I command
(such as
.BR break)
regardless of functions defined with the same name.
.TP
\f5break\fP \*(OK \f2n\fP \*(CK
Exit from the enclosing \f5for\fP or
.B while
loop, if any.
If
.I n
is specified break
.I n
levels.
.TP
\f5continue\fP \*(OK \f2n\fP \*(CK
Resume the next iteration of the enclosing
\f5for\fP or
.B while
loop.
If
.I n
is specified resume at the
.IR n -th
enclosing loop.
.TP
\f5cd\fP \*(OK \f2arg\fP \*(CK
Change the current directory to
.I arg.
The shell
parameter
.B
.SM HOME
is the default
.I arg.
The shell parameter
.B
.SM CDPATH
defines the search path for
the directory containing
.IR arg .
Alternative directory names are separated by
a colon
.RB ( : ).
The current directory (default) is specified by a null path name,
which can appear immediately after the equal sign
or between the colon delimiters anywhere else in the path list.
If
.I arg
begins with a
.L /
the search path
is not used.
Otherwise, each directory in the path is
searched for
.I arg.
.TP
\f5eval\fP \*(OK \f2arg\fP .\|.\|. \*(CK
The arguments are read as input
to the shell
and the resulting command(s) executed.
.TP
\f5exec\fP \*(OK \f2arg\fP .\|.\|. \*(CK
The non-builtin command specified by
the arguments is executed in place of this shell
without creating a new process.
Input/output arguments may appear and, if no other
arguments are given, cause the shell
input/output to be modified.
.TP
\f5exit\fP \*(OK \f2n\fP \*(CK
Causes a shell to exit
with the exit status specified by
.IR n .
If
.I n
is omitted the exit status is that of the last command executed
(an end-of-file will also cause the shell to exit.)
.TP
\f5export\fP \*(OK \f2name\fP .\|.\|. \*(CK
The given
.I names
are marked
for automatic export to the
.I environment
of subsequently-executed commands.
If no arguments are given, a list of all
names that are exported in this shell is printed.
.TP
\f5read\fP \*(OK \f2name\fP .\|.\|. \*(CK
One line is read from the standard input and
the first
word is assigned to the first
.I name,
the second word
to the second
.I name,
etc., with leftover words assigned to the last
.I name.
The return code is 0 unless an end-of-file is encountered.
.TP
\f5return\fP \*(OK \f2n\fP \*(CK
Causes a function to exit with the return value specified by
.I n.
If
.I n
is omitted, the return status is that of the last command executed.
.TP
\f5set\fP \*(OK \f5--aehknptuvx\fP \*(OK \f2arg\fP .\|.\|. \*(CK \*(CK
.RS
.TP
.B -a
Mark variables which are modified or created for export.
.TP
.B -e
Exit immediately if a command
exits with a non-zero exit status.
.TP
.B -f
Disable file name generation
.TP
.B -k
All keyword arguments are placed in the environment for a command,
not just those that precede the command name.
.TP
.B -n
Read commands but do not execute them.
.TP
.B -p
Remove the definitions for all functions imported from the environment,
and set
.B IFS
to blank, tab and newline.
.TP
.B -t
Exit after reading and executing one command.
.TP
.B -u
Treat unset variables as an error when substituting.
.TP
.B -v
Print shell input lines as they are read.
.TP
.B -x
Print commands and their arguments as they are executed.
.TP
.B --
Do not change any of the flags; useful in setting
.B $1
to
.BR - .
.PP
Using
.B \+
rather than
.B -
causes these flags to be turned off.
These flags can also be used upon invocation of the shell.
The current set of flags may be found in
.BR $- .
The remaining arguments are positional
parameters and are assigned, in order, to
.BR $1 ,
.BR $2 ,
\&.\|.\|.\|.
If no arguments are given the values
of all names are printed.
.RE
.TP
\f5shift\fP \*(OK \f2n\fP \*(CK
.br
The positional parameters from
.BI $ n\fR+1
\&.\|.\|.
are renamed
.B $1
\&.\|.\|.
If
.I n
is not given, it is assumed to be 1.
.TP
\f5times\fP
.br
Print the accumulated user and system times for processes
run from the shell.
.TP
\f5trap\fP \*(OK \f2arg\fP \*(CK \*(OK \f2n\fP \*(CK .\|.\|.
The command
.I arg
is to be read and executed when the shell
receives signal(s)
.IR n .
(Note that
.I arg
is scanned once when
the trap is set and once when the trap
is taken.)
Trap commands are executed in order of signal number.
Any attempt to set a trap on a signal that
was ignored on entry to the current shell
is ineffective.
If
.I arg
is absent all traps
.I n
are reset
to their original values.
If
.I arg
is the null
string this signal is ignored by the shell and by the commands
it invokes.
If
.I n
is 0 the command
.I arg
is executed
on exit from the shell.
The
.B trap
command
with no arguments prints a list
of commands associated with each signal number.
.TP
\f5umask\fP \*(OK \f2nnn\fP \*(CK
The user file-creation mask is set to
.IR nnn ;
see
.IR umask (2).
If
.I nnn
is omitted, the current value of the mask is printed.
.TP
\f5unset\fP \*(OK \f2name\fP .\|.\|. \*(CK
For each
.IR name ,
remove the corresponding variable or function.
The variables
\f5\s-1PATH\s+1\fP, \f5\s-1PS1\s+1\fP, \f5\s-1PS2\s+1\fP and \f5\s-1IFS\s+1\fP
cannot be unset.
.TP
\f5wait\fP \*(OK \f2n\fP \*(CK
Wait for the specified process and report its termination status.
If
.I n
is not given all currently active child processes are waited for
and the return code is zero.
.TP
\f5whatis\fP \*(OK \fIname\fP .\|.\|. \*(CK
For each
.IR name ,
print the associated value as a parameter, function, builtin or executable
file as appropriate.
In each case, the value is printed in a form that would yield the same
value if typed as input to the shell itself:
parameters are printed as assignments, functions as their definitions,
builtins as calls to
.BR builtin ,
and executable files as completed pathnames.
.PD
.PP
.SS Invocation
Normally the shell reads commands from the file named in its
first argument (standard input default).
The remaining arguments are interpreted as position parameters; see
`Parameter substitution' above.
If the shell is invoked through
.IR exec (2)
and the first character of argument zero
is
.BR - ,
commands are read first from
.BR $HOME/.profile ,
if it exists.
Certain options modify this behavior:
.PP
.PD 0
.TP 10
.BI -c "\| string"
Read commands from
.IR string ;
ignore remaining arguments.
.TP
.B -s
Write shell output (except for builtin commands)
on file descriptor 2.
.TP
.B -i
Interactive.
Ignore signal
.B SIGTERM
(interactive shell is immune to
.BR "kill 0" ).
Catch and ignore
.B SIGINT
.RB ( wait
is interruptible).
The shell always ignores
.BR SIGQUIT .
.PD
.PP
Other options are described under the
.B set
command above.
.SH FILES
.F $HOME/.profile
.br
.F /tmp/sh*
.br
.F /dev/null
.SH SEE ALSO
.IR = (1),
.IR echo (1),
.IR newgrp (1),
.IR test (1),
.IR dup (2),
.IR exec (2),
.IR fork (2),
.IR pipe (2),
.IR signal (2),
.IR umask (2),
.IR exit (2),
.IR environ (5)
.br
B. W. Kernighan and R. Pike,
.I The Unix Programming Environment,
Prentice-Hall, 1984
.SH DIAGNOSTICS
Errors detected by the shell, such as syntax errors,
cause the shell
to return a non-zero exit status.
If the shell is being used non-interactively
execution of the shell file is abandoned.
Otherwise, the shell returns the exit status of
the last command executed; see also the
.B exit
command.
.SH BUGS
Errors arising from builtins terminate shell scripts.