POSH(1) User commands POSH(1)NAMEposh - Policy-compliant Ordinary SHell
SYNOPSISposh [+-aCefilnuvx] [+-o option]
[[-c command-string [command-name] | -s | file ] [argument...]]
DESCRIPTIONposh is a reimplementation of the Bourne shell, a command interpreter
for both interactive and script use.
Shell Startup
The following options can be specified only on the command line:
-c command-string
the shell executes the command(s) contained in command-string
-i
interactive mode — see below
-l
login shell — see below interactive mode — see below
-s
the shell reads commands from standard input; all non-option
arguments are positional parameters
In addition to the above, the options described in the set builtin
command can also be used on the command line.
If neither the -c nor the -s options are specified, the first
non-option argument specifies the name of a file the shell reads
commands from; if there are no non-option arguments, the shell reads
commands from standard input. The name of the shell (i.e., the contents
of the $0) parameter is determined as follows: if the -c option is used
and there is a non-option argument, it is used as the name; if commands
are being read from a file, the file is used as the name; otherwise the
name the shell was called with (i.e., argv[0]) is used.
A shell is interactive if the -i option is used or if both standard
input and standard error are attached to a tty. An interactive shell
has job control enabled (if available), ignores the INT, QUIT and TERM
signals, and prints prompts before reading input (see PS1 and PS2
parameters).
A shell is privileged if the -p option is used or if the real user-id
or group-id does not match the effective user-id or group-id (see
getuid(2), getgid(2)). A privileged shell does not process
$HOME/.profile. Instead the file /etc/suid_profile is processed.
Clearing the privileged option causes the shell to set its effective
user-id (group-id) to its real user-id (group-id).
If the basename of the name the shell is called with (i.e., argv[0])
starts with - or if the -l option is used, the shell is assumed to be a
login shell and the shell reads and executes the contents of
/etc/profile and $HOME/.profile if they exist and are readable.
The exit status of the shell is 127 if the command file specified on
the command line could not be opened, or non-zero if a fatal syntax
error occurred during the execution of a script. In the absence of
fatal errors, the exit status is that of the last command executed, or
zero, if no command is executed.
Command Syntax
The shell begins parsing its input by breaking it into words. Words,
which are sequences of characters, are delimited by unquoted
white-space characters (space, tab and newline) or meta-characters (<,
>, |, ;, &, ( and )). Aside from delimiting words, spaces and tabs are
ignored, while newlines usually delimit commands. The meta-characters
are used in building the following tokens: <, <&, <<, >, >&, >>, etc.
are used to specify redirections (see Input/Output Redirection below);
| is used to create pipelines; ; is used to separate commands; & is
used to create asynchronous pipelines; && and || are used to specify
conditional execution; ;; is used in case statements; and lastly, ( ...
) are used to create subshells.
White-space and meta-characters can be quoted individually using
backslash (\), or in groups using double (") or single (') quotes. Note
that the following characters are also treated specially by the shell
and must be quoted if they are to represent themselves: \, ", ', #, $,
`, ~, {, }, *, ? and [. The first three of these are the above
mentioned quoting characters (see Quoting below); #, if used at the
beginning of a word, introduces a comment — everything after the # up
to the nearest newline is ignored; $ is used to introduce parameter,
command and arithmetic substitutions (see Substitution below); `
introduces an old-style command substitution (see Substitution below);
~ begins a directory expansion (see Tilde Expansion below); { and }
delimit csh(1) style alternations (see Brace Expansion below); and,
finally, *, ? and [ are used in file name generation (see File Name
Patterns below).
As words and tokens are parsed, the shell builds commands, of which
there are two basic types: simple-commands, typically programs that are
executed, and compound-commands, such as for and if statements,
grouping constructs and function definitions.
A simple-command consists of some combination of parameter assignments
(see Parameters below), input/output redirections (see Input/Output
Redirections below), and command words; the only restriction is that
parameter assignments come before any command words. The command words,
if any, define the command that is to be executed and its arguments.
The command may be a shell built-in command, a function or an external
command, i.e., a separate executable file that is located using the
PATH parameter (see Command Execution below). Note that all command
constructs have an exit status: for external commands, this is related
to the status returned by wait(2) (if the command could not be found,
the exit status is 127, if it could not be executed, the exit status is
126); the exit status of other command constructs (built-in commands,
functions, compound-commands, pipelines, lists, etc.) are all well
defined and are described where the construct is described. The exit
status of a command consisting only of parameter assignments is that of
the last command substitution performed during the parameter assignment
or zero if there were no command substitutions.
Commands can be chained together using the | token to form pipelines,
in which the standard output of each command but the last is piped (see
pipe(2)) to the standard input of the following command. The exit
status of a pipeline is that of its last command. A pipeline may be
prefixed by the ! reserved word which causes the exit status of the
pipeline to be logically complemented: if the original status was 0 the
complemented status will be 1, and if the original status was not 0,
then the complemented status will be 0.
Lists of commands can be created by separating pipelines by any of the
following tokens: &&, ||, &, |& and ;. The first two are for
conditional execution: cmd1 && cmd2 executes cmd2 only if the exit
status of cmd1 is zero; || is the opposite — cmd2 is executed only if
the exit status of cmd1 is non-zero. && and || have equal precedence
which is higher than that of &, |& and ;, which also have equal
precedence. The & token causes the preceding command to be executed
asynchronously, that is, the shell starts the command, but does not
wait for it to complete (the shell does keep track of the status of
asynchronous commands — see Job Control below). When an asynchronous
command is started when job control is disabled (i.e., in most
scripts), the command is started with signals INT and QUIT ignored and
with input redirected from /dev/null (however, redirections specified
in the asynchronous command have precedence). Note that a command must
follow the && and || operators, while a command need not follow &, |&
and ;. The exit status of a list is that of the last command executed,
with the exception of asynchronous lists, for which the exit status is
0.
Compound commands are created using the following reserved words —
these words are only recognized if they are unquoted and if they are
used as the first word of a command (i.e., they can't be preceded by
parameter assignments or redirections):
case else
do esac if time [[
done fi in until {
elif for select while }
Note: Some shells (but not this one) execute control structure commands
in a subshell when one or more of their file descriptors are
redirected, so any environment changes inside them may fail. To be
portable, the exec statement should be used instead to redirect file
descriptors before the control structure.
In the following compound command descriptions, command lists (denoted
as list) that are followed by reserved words must end with a
semi-colon, a newline or a (syntactically correct) reserved word. For
example,
{ echo foo; echo bar; }
{ echo foo; echo bar<newline>}
{ { echo foo; echo bar; } } are all valid, but
{ echo foo; echo bar } is not.
( list )
Execute list in a subshell. There is no implicit way to pass
environment changes from a subshell back to its parent.
{ list }
Compound construct; list is executed, but not in a subshell. Note
that { and } are reserved words, not meta-characters.
case word in [ [(] pattern [| pattern] ... ) list ;; ] ... esac
The case statement attempts to match word against the specified
patterns; the list associated with the first successfully matched
pattern is executed. Patterns used in case statements are the same
as those used for file name patterns except that the restrictions
regarding . and / are dropped. Note that any unquoted space before
and after a pattern is stripped; any space with a pattern must be
quoted. Both the word and the patterns are subject to parameter,
command, and arithmetic substitution as well as tilde substitution.
For historical reasons, open and close braces may be used instead
of in and esac (e.g., case $foo { *) echo bar; }). The exit status
of a case statement is that of the executed list; if no list is
executed, the exit status is zero.
for name [ in word ... term ] do list done
where term is either a newline or a ;. For each word in the
specified word list, the parameter name is set to the word and list
is executed. If in is not used to specify a word list, the
positional parameters ("$1", "$2", etc.) are used instead. For
historical reasons, open and close braces may be used instead of do
and done (e.g., for i; { echo $i; }). The exit status of a for
statement is the last exit status of list; if list is never
executed, the exit status is zero.
if list then list [elif list then list] ... [else list] fi
If the exit status of the first list is zero, the second list is
executed; otherwise the list following the elif, if any, is
executed with similar consequences. If all the lists following the
if and elifs fail (i.e., exit with non-zero status), the list
following the else is executed. The exit status of an if statement
is that of non-conditional list that is executed; if no
non-conditional list is executed, the exit status is zero.
until list do list done
This works like while, except that the body is executed only while
the exit status of the first list is non-zero.
while list do list done
A while is a prechecked loop. Its body is executed as often as the
exit status of the first list is zero. The exit status of a while
statement is the last exit status of the list in the body of the
loop; if the body is not executed, the exit status is zero.
name () command
Defines the function name. See Functions below. Note that
redirections specified after a function definition are performed
whenever the function is executed, not when the function definition
is executed.
time [ -p ] [ pipeline ]
The time reserved word is described in the Command Execution
section.
Quoting
Quoting is used to prevent the shell from treating characters or words
specially. There are three methods of quoting: First, \ quotes the
following character, unless it is at the end of a line, in which case
both the \ and the newline are stripped. Second, a single quote (')
quotes everything up to the next single quote (this may span lines).
Third, a double quote (") quotes all characters, except $, ` and \, up
to the next unquoted double quote. $ and ` inside double quotes have
their usual meaning (i.e., parameter, command or arithmetic
substitution) except no field splitting is carried out on the results
of double-quoted substitutions. If a \ inside a double-quoted string is
followed by \, $, ` or ", it is replaced by the second character; if it
is followed by a newline, both the \ and the newline are stripped;
otherwise, both the \ and the character following are unchanged.
Note: see POSIX Mode below for a special rule regarding sequences of
the form "...`...\"...`..".
Substitution
The first step the shell takes in executing a simple-command is to
perform substitutions on the words of the command. There are three
kinds of substitution: parameter, command and arithmetic. Parameter
substitutions, which are described in detail in the next section, take
the form $name or ${...}; command substitutions take the form
$(command) or `command`; and arithmetic substitutions take the form
$((expression)).
If a substitution appears outside of double quotes, the results of the
substitution are generally subject to word or field splitting according
to the current value of the IFS parameter. The IFS parameter specifies
a list of characters which are used to break a string up into several
words; any characters from the set space, tab and newline that appear
in the IFS characters are called IFS white space. Sequences of one or
more IFS white space characters, in combination with zero or one
non-IFS white space characters delimit a field. As a special case,
leading and trailing IFS white space is stripped (i.e., no leading or
trailing empty field is created by it); leading or trailing non-IFS
white space does create an empty field. Example: if IFS is set to
`<space>:', the sequence of characters
`<space>A<space>:<space><space>B::D' contains four fields: `A', `B', `'
and `D'. Note that if the IFS parameter is set to the null string, no
field splitting is done; if the parameter is unset, the default value
of space, tab and newline is used.
The results of substitution are, unless otherwise specified, also
subject to brace expansion and file name expansion (see the relevant
sections below).
A command substitution is replaced by the output generated by the
specified command, which is run in a subshell. For $(command)
substitutions, normal quoting rules are used when command is parsed,
however, for the `command` form, a \ followed by any of $, ` or \ is
stripped (a \ followed by any other character is unchanged). As a
special case in command substitutions, a command of the form < file is
interpreted to mean substitute the contents of file ($(< foo) has the
same effect as $(cat foo), but it is carried out more efficiently
because no process is started).
NOTE: $(command) expressions are currently parsed by finding the
matching parenthesis, regardless of quoting. This will hopefully be
fixed soon.
Arithmetic substitutions are replaced by the value of the specified
expression. For example, the command echo $((2+3*4)) prints 14. See
Arithmetic Expressions for a description of an expression.
Parameters
Parameters are shell variables; they can be assigned values and their
values can be accessed using a parameter substitution. A parameter name
is either one of the special single punctuation or digit character
parameters described below, or a letter followed by zero or more
letters or digits (`_' counts as a letter). The later form can be
treated as arrays by appending an array index of the form: [expr] where
expr is an arithmetic expression. Array indices are currently limited
to the range 0 through 1023, inclusive. Parameter substitutions take
the form $name, ${name} or ${name[expr]}, where name is a parameter
name. If substitution is performed on a parameter (or an array
parameter element) that is not set, a null string is substituted unless
the nounset option (set -o nounset or set -u) is set, in which case an
error occurs.
Parameters can be assigned values in a number of ways. First, the shell
implicitly sets some parameters like #, PWD, etc.; this is the only way
the special single character parameters are set. Second, parameters are
imported from the shell's environment at startup. Third, parameters can
be assigned values on the command line, for example, `FOO=bar' sets the
parameter FOO to bar; multiple parameter assignments can be given on a
single command line and they can be followed by a simple-command, in
which case the assignments are in effect only for the duration of the
command (such assignments are also exported, see below for implications
of this). Note that both the parameter name and the = must be unquoted
for the shell to recognize a parameter assignment. The fourth way of
setting a parameter is with the export and readonly commands; see their
descriptions in the Command Execution section. Fifth, for and select
loops set parameters as well as the getopts, read and set -A commands.
Lastly, parameters can be assigned values using assignment operators
inside arithmetic expressions (see Arithmetic Expressions below) or
using the ${name=value} form of parameter substitution (see below).
Parameters with the export attribute (set using the export command, or
by parameter assignments followed by simple commands) are put in the
environment (see environ(5)) of commands run by the shell as name=value
pairs. The order in which parameters appear in the environment of a
command is unspecified. When the shell starts up, it extracts
parameters and their values from its environment and automatically sets
the export attribute for those parameters.
Modifiers can be applied to the ${name} form of parameter substitution:
${name:-word}
if name is set and not null, it is substituted, otherwise word is
substituted.
${name:+word}
if name is set and not null, word is substituted, otherwise nothing
is substituted.
${name:=word}
if name is set and not null, it is substituted, otherwise it is
assigned word and the resulting value of name is substituted.
${name:?word}
if name is set and not null, it is substituted, otherwise word is
printed on standard error (preceded by name:) and an error occurs
(normally causing termination of a shell script, function or
.-script). If word is omitted the string `parameter null or not
set' is used instead.
In the above modifiers, the : can be omitted, in which case the
conditions only depend on name being set (as opposed to set and not
null). If word is needed, parameter, command, arithmetic and tilde
substitution are performed on it; if word is not needed, it is not
evaluated.
The following forms of parameter substitution can also be used:
${#name}
The number of positional parameters if name is *, @ or is not
specified, or the length of the string value of parameter name.
${#name[*]}, ${#name[@]}
The number of elements in the array name.
${name#pattern}, ${name##pattern}
If pattern matches the beginning of the value of parameter name,
the matched text is deleted from the result of substitution. A
single # results in the shortest match, two #'s results in the
longest match.
${name%pattern}, ${name%%pattern}
Like ${..#..} substitution, but it deletes from the end of the
value.
The following special parameters are implicitly set by the shell and
cannot be set directly using assignments:
!
Process id of the last background process started. If no background
processes have been started, the parameter is not set.
#
The number of positional parameters (i.e., $1, $2, etc.).
$
The process ID of the shell, or the PID of the original shell if it
is a subshell.
-
The concatenation of the current single letter options (see set
command below for list of options).
?
The exit status of the last non-asynchronous command executed. If
the last command was killed by a signal, $? is set to 128 plus the
signal number.
0
The name the shell was invoked with (that is, argv[0]), or the
command-name if it was invoked with the -c option and the
command-name was supplied, or the file argument, if it was
supplied. If the posix option is not set, $0 is the name of the
current function or script.
1 ... 9
The first nine positional parameters that were supplied to the
shell, function or .-script. Further positional parameters may be
accessed using ${number}.
*
All positional parameters (except parameter 0), i.e., $1 $2 $3....
If used outside of double quotes, parameters are separate words
(which are subjected to word splitting); if used within double
quotes, parameters are separated by the first character of the IFS
parameter (or the empty string if IFS is null).
@
Same as $*, unless it is used inside double quotes, in which case a
separate word is generated for each positional parameter - if there
are no positional parameters, no word is generated ("$@" can be
used to access arguments, verbatim, without loosing null arguments
or splitting arguments with spaces).
The following parameters are set and/or used by the shell:
"CDPATH"
Search path for the cd built-in command. Works the same way as PATH
for those directories not beginning with / in cd commands. Note
that if CDPATH is set and does not contain . nor an empty path,
the current directory is not searched.
COLUMNS
Set to the number of columns on the terminal or window. Currently
set to the cols value as reported by stty(1) if that value is
non-zero. This parameter is used by the interactive line editing
modes, and by select, set -o and kill -l commands to format
information in columns.
ERRNO
Integer value of the shell's errno variable — indicates the reason
the last system call failed.
Not implemented yet.
EXECSHELL
If set, this parameter is assumed to contain the shell that is to
be used to execute commands that execve(2) fails to execute and
which do not start with a `#! shell' sequence.
FCEDIT
The editor used by the fc command (see below).
FPATH
Like PATH, but used when an undefined function is executed to
locate the file defining the function. It is also searched when a
command can't be found using PATH. See Functions below for more
information.
HOME
The default directory for the cd command and the value substituted
for an unqualified ~ (see Tilde Expansion below).
IFS
Internal field separator, used during substitution and by the read
command, to split values into distinct arguments; normally set to
space, tab and newline. See Substitution above for details.
Note: this parameter is not imported from the environment when the
shell is started.
POSH_VERSION
The version of posh (readonly).
LINENO
The line number of the function or shell script that is currently
being executed.
LINES
Set to the number of lines on the terminal or window.
Not implemented yet.
OLDPWD
The previous working directory. Unset if cd has not successfully
changed directories since the shell started, or if the shell
doesn't know where it is.
OPTARG
When using getopts, it contains the argument for a parsed option,
if it requires one.
OPTIND
The index of the last argument processed when using getopts.
Assigning 1 to this parameter causes getopts to process arguments
from the beginning the next time it is invoked.
PATH
A colon separated list of directories that are searched when
looking for commands and .'d files. An empty string resulting from
a leading or trailing colon, or two adjacent colons is treated as a
`.', the current directory.
PPID
The process ID of the shell's parent (readonly).
PS1
The primary prompt for interactive shells. The prompt is printed
verbatim (i.e., no substitutions are done). Default is ‘$ ’ for
non-root users, `# ' for root..
PS2
Secondary prompt string, by default `> ', used when more input is
needed to complete a command.
PS4
Used to prefix commands that are printed during execution tracing
(see set -x command below). The prompt is printed verbatim (i.e.,
no substitutions are done). Default is `+ '.
PWD
The current working directory. Maybe unset or null if shell doesn't
know where it is.
REPLY
Default parameter for the read command if no names are given. Also
used in select loops to store the value that is read from standard
input.
TMPDIR
The directory shell temporary files are created in. If this
parameter is not set, or does not contain the absolute path of a
writable directory, temporary files are created in /tmp.
Tilde Expansion
Tilde expansion, which is done in parallel with parameter substitution,
is done on words starting with an unquoted ~. The characters following
the tilde, up to the first /, if any, are assumed to be a login name.
If the login name is empty, + or -, the value of the HOME, PWD, or
OLDPWD parameter is substituted, respectively. Otherwise, the password
file is searched for the login name, and the tilde expression is
substituted with the user's home directory. If the login name is not
found in the password file or if any quoting or parameter substitution
occurs in the login name, no substitution is performed.
In parameter assignments (those preceding a simple-command or those
occurring in the arguments of alias, export, and readonly, tilde
expansion is done after any unquoted colon (:), and login names are
also delimited by colons.
The home directory of previously expanded login names are cached and
re-used. The alias -d command may be used to list, change and add to
this cache (e.g., `alias -d fac=/usr/local/facilities; cd ~fac/bin').
File Name Patterns
A file name pattern is a word containing one or more unquoted ? or *
characters or [..] sequences. Once brace expansion has been performed,
the shell replaces file name patterns with the sorted names of all the
files that match the pattern (if no files match, the word is left
unchanged). The pattern elements have the following meaning:
"?"
matches any single character.
*
matches any sequence of characters.
[..]
matches any of the characters inside the brackets. Ranges of
characters can be specified by separating two characters by a -,
e.g., [a0-9] matches the letter a or any digit. In order to
represent itself, a - must either be quoted or the first or last
character in the character list. Similarly, a ] must be quoted or
the first character in the list if it is represent itself instead
of the end of the list. Also, a ! appearing at the start of the
list has special meaning (see below), so to represent itself it
must be quoted or appear later in the list.
[!..]
like [..], except it matches any character not inside the brackets.
Note that posh currently never matches . and .., but the original ksh,
Bourne sh and bash do, so this may have to change (too bad).
Note that none of the above pattern elements match either a period (.)
at the start of a file name or a slash (/), even if they are explicitly
used in a [..] sequence; also, the names . and .. are never matched,
even by the pattern .*.
The POSIX character classes (i.e., [:class-name:] inside a [..]
expression) are not yet implemented.
Input/Output Redirection
When a command is executed, its standard input, standard output and
standard error (file descriptors 0, 1 and 2, respectively) are normally
inherited from the shell. Three exceptions to this are commands in
pipelines, for which standard input and/or standard output are those
set up by the pipeline, asynchronous commands created when job control
is disabled, for which standard input is initially set to be from
/dev/null, and commands for which any of the following redirections
have been specified:
"> file"
standard output is redirected to file. If file does not exist, it
is created; if it does exist, is a regular file and the noclobber
option is set, an error occurs, otherwise the file is truncated.
Note that this means the command cmd < foo > foo will open foo for
reading and then truncate it when it opens it for writing, before
cmd gets a chance to actually read foo.
">| file"
same as >, except the file is truncated, even if the noclobber
option is set.
">> file"
same as >, except the file an existing file is appended to instead
of being truncated. Also, the file is opened in append mode, so
writes always go to the end of the file (see open(2)).
"< file"
standard input is redirected from file, which is opened for
reading.
"<> file"
same as <, except the file is opened for reading and writing.
"<< marker"
after reading the command line containing this kind of redirection
(called a here document), the shell copies lines from the command
source into a temporary file until a line matching marker is read.
When the command is executed, standard input is redirected from the
temporary file. If marker contains no quoted characters, the
contents of the temporary file are processed as if enclosed in
double quotes each time the command is executed, so parameter,
command and arithmetic substitutions are performed, along with
backslash (\) escapes for $, `, \ and \newline. If multiple here
documents are used on the same command line, they are saved in
order.
"<<- marker"
same as <<, except leading tabs are stripped from lines in the here
document.
"<& fd"
standard input is duplicated from file descriptor fd. fd can be a
single digit, indicating the number of an existing file descriptor,
the letter p, indicating the file descriptor associated with the
output of the current co-process, or the character -, indicating
standard input is to be closed.
">& fd"
same as <&, except the operation is done on standard output.
In any of the above redirections, the file descriptor that is
redirected (i.e., standard input or standard output) can be explicitly
given by preceding the redirection with a single digit. Parameter,
command and arithmetic substitutions, tilde substitutions and (if the
shell is interactive) file name generation are all performed on the
file, marker and fd arguments of redirections. Note however, that the
results of any file name generation are only used if a single file is
matched; if multiple files match, the word with the unexpanded file
name generation characters is used.
For simple-commands, redirections may appear anywhere in the command,
for compound-commands (if statements, etc.), any redirections must
appear at the end. Redirections are processed after pipelines are
created and in the order they are given, so
cat /foo/bar 2>&1 > /dev/null | cat -n will print an error with a line
number prepended to it.
Arithmetic Expressions
Integer arithmetic expressions can be used inside $((..)) expressions,
inside array references (e.g., name[expr]), as numeric arguments to the
test command, and as the value of an assignment to an integer
parameter.
Expression may contain alpha-numeric parameter identifiers, array
references, and integer constants and may be combined with the
following C operators (listed and grouped in increasing order of
precedence).
Unary operators:
+ - ! ~ ++ --
Binary operators:
,
= *= /= %= += -= <<= >>= &= ^= |=
||
&&
|
^
&
== !=
< <= >= >
<< >>
+ -
* / %
Ternary operator:
?: (precedence is immediately higher than assignment)
Grouping operators:
( )
Integer constants may be specified with arbitrary bases using the
notation base#number, where base is a decimal integer specifying the
base, and number is a number in the specified base.
The operators are evaluated as follows:
"unary +"
result is the argument (included for completeness).
"unary -"
negation.
"!"
logical not; the result is 1 if argument is zero, 0 if not.
"~"
arithmetic (bit-wise) not.
"++"
increment; must be applied to a parameter (not a literal or other
expression) - the parameter is incremented by 1. When used as a
prefix operator, the result is the incremented value of the
parameter, when used as a postfix operator, the result is the
original value of the parameter.
"++"
similar to ++, except the parameter is decremented by 1.
","
separates two arithmetic expressions; the left hand side is
evaluated first, then the right. The result is value of the
expression on the right hand side.
"="
assignment; variable on the left is set to the value on the right.
"*= /= %= += -= <<= >>= &= ^= |="
assignment operators; <var> <op>= <expr> is the same as <var> =
<var> <op> ( <expr> ).
"||"
logical or; the result is 1 if either argument is non-zero, 0 if
not. The right argument is evaluated only if the left argument is
zero.
"&&"
logical and; the result is 1 if both arguments are non-zero, 0 if
not. The right argument is evaluated only if the left argument is
non-zero.
"|"
arithmetic (bit-wise) or.
"^"
arithmetic (bit-wise) exclusive-or.
"&"
arithmetic (bit-wise) and.
"=="
equal; the result is 1 if both arguments are equal, 0 if not.
"!="
not equal; the result is 0 if both arguments are equal, 1 if not.
"<"
less than; the result is 1 if the left argument is less than the
right, 0 if not.
"<= >= >"
less than or equal, greater than or equal, greater than. See <.
"<< >>"
shift left (right); the result is the left argument with its bits
shifted left (right) by the amount given in the right argument.
"+ - * /"
addition, subtraction, multiplication, and division.
"%"
remainder; the result is the remainder of the division of the left
argument by the right. The sign of the result is unspecified if
either argument is negative.
"<arg1> ? <arg2> : <arg3>"
if <arg1> is non-zero, the result is <arg2>, otherwise <arg3>.
Functions
Functions are defined using the Bourne/POSIX shell name() syntax.
Functions are like .-scripts in that they are executed in the current
environment, however, unlike .-scripts, shell arguments (i.e.,
positional parameters, $1, etc.) are never visible inside them. When
the shell is determining the location of a command, functions are
searched after special built-in commands, and before regular and
non-regular built-ins, and before the PATH is searched.
An existing function may be deleted using unset -f function-name.
Since functions are executed in the current shell environment,
parameter assignments made inside functions are visible after the
function completes.
The exit status of a function is that of the last command executed in
the function. A function can be made to finish immediately using the
return command; this may also be used to explicitly specify the exit
status.
Command Execution
After evaluation of command line arguments, redirections and parameter
assignments, the type of command is determined: a special built-in, a
function, a regular built-in or the name of a file to execute found
using the PATH parameter. The checks are made in the above order.
Special built-in commands differ from other commands in that the PATH
parameter is not used to find them, an error during their execution can
cause a non-interactive shell to exit and parameter assignments that
are specified before the command are kept after the command completes.
Just to confuse things, if the posix option is turned off (see set
command below) some special commands are very special in that no field
splitting, file globbing, brace expansion nor tilde expansion is
performed on arguments that look like assignments. Regular built-in
commands are different only in that the PATH parameter is not used to
find them.
Job Control
Job control refers to the shell's ability to monitor and control jobs,
which are processes or groups of processes created for commands or
pipelines. At a minimum, the shell keeps track of the status of the
background (i.e., asynchronous) jobs that currently exist; this
information can be displayed using the jobs command. If job control is
fully enabled (using set -m or set -o monitor), as it is for
interactive shells, the processes of a job are placed in their own
process group, foreground jobs can be stopped by typing the suspend
character from the terminal (normally ^Z), jobs can be restarted in
either the foreground or background, using the fg and bg commands,
respectively, and the state of the terminal is saved or restored when a
foreground job is stopped or restarted, respectively.
Note that only commands that create processes (e.g., asynchronous
commands, subshell commands, and non-built-in, non-function commands)
can be stopped; commands like read cannot be.
When a job is created, it is assigned a job-number. For interactive
shells, this number is printed inside [..], followed by the process-ids
of the processes in the job when an asynchronous command is run. A job
may be referred to in bg, fg, jobs, kill and wait commands either by
the process id of the last process in the command pipeline (as stored
in the $! parameter) or by prefixing the job-number with a percent
sign (%). Other percent sequences can also be used to refer to jobs:
%+ %+ The most recently stopped job,
or, if there are no stopped
jobs, the oldest
running
job.
%%, % %%, % Same as %+.
%- %- The job that
would be the %+
job, if the
later did not
exist.
%n %n The job with
job-number n.
%?string %?string The job
containing the
string string
(an error occurs
if multiple jobs
are
matched).
%string %string The job starting with
string string (an error
occurs if multiple jobs
are
matched).
When a job changes state (e.g., a background job finishes or foreground
job is stopped), the shell prints the following status information:
[number] flag status command where
" number"
is the job-number of the job.
" flag"
is + or - if the job is the %+ or %- job, respectively, or space if
it is neither.
" status"
indicates the current state of the job and can be
"Running"
the job has neither stopped or exited (note that running does not
necessarily mean consuming CPU time — the process could be blocked
waiting for some event).
"Done [(number)]"
the job exited. number is the exit status of the job, which is
omitted if the status is zero.
"Stopped [(signal)]"
the job was stopped by the indicated signal (if no signal is given,
the job was stopped by SIGTSTP).
"signal-description [(core dumped)]"
the job was killed by a signal (e.g., Memory fault, Hangup, etc. —
use kill -l for a list of signal descriptions). The (core dumped)
message indicates the process created a core file.
" command"
is the command that created the process. If there are multiple
processes in the job, then each process will have a line showing
its command and possibly its status, if it is different from the
status of the previous process.
When an attempt is made to exit the shell while there are jobs in the
stopped state, the shell warns the user that there are stopped jobs and
does not exit. If another attempt is immediately made to exit the
shell, the stopped jobs are sent a HUP signal and the shell exits.
Similarly, if the nohup option is not set and there are running jobs
when an attempt is made to exit a login shell, the shell warns the user
and does not exit. If another attempt is immediately made to exit the
shell, the running jobs are sent a HUP signal and the shell exits.
BUILTIN UTILITIESposh implements the following builtin utilities:
· .
· :
· [
· break
· builtin
· continue
· eval
· exec
· exit
· false
· return
· set
· shift
· times
· trap
· wait
· read
· test
· true
· umask
· unset
· cd
· command
· echo
· export
· getopts
· kill
· local
· pwd
· readonly
FILES
~/.profile /etc/profile /etc/suid_profile
BUGS
Any bugs in posh should be reported via the Debian BTS. Legitimate bugs
are inconsistencies between manpage and behavior, and inconsistencies
between behavior and Debian policy (currently SUSv3 compliance with the
following exceptions: echo -n, binary -a and -o to test, local
scoping).
VERSION
This page documents the Policy-compliant Ordinary SHell.
AUTHORS
This shell is based on pdksh.
SEE ALSOawk(1), ksh(1), dash(1), ed(1), getconf(1), getopt(1), sed(1), stty(1),
vi(1), dup(2), execve(2), getgid(2), getuid(2), open(2), pipe(2),
wait(2), getopt(3), rand(3), signal(3), system(3), environ(5)User commands 01/24/2011 POSH(1)
