ksh - Public domain Korn shell
ksh [+-abCefhikmnprsuvxX] [+-o option] [ [ -c command-string [command-
name] | -s | file ] [argument ...] ]
ksh is a command interpreter that is intended for both interactive and
shell script use. Its command language is a superset of the sh(1)
shell language.
Shell Startup [Toc] [Back]
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
-r restricted mode -- see below
In addition to the above, the options described in the set built-in
command can also be used on the command line.
If neither the -c nor the -s options are specified, the first nonoption
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). For non-interactive shells, the trackall option is on by
default (see set command below).
A shell is restricted if the -r option is used or if either the basename
of the name the shell is invoked with or the SHELL parameter match
the pattern *r*sh (e.g., rsh, rksh, rpdksh, etc.). The following
restrictions come into effect after the shell processes any profile and
$ENV files:
o the cd command is disabled
o the SHELL, ENV and PATH parameters can't be changed
o command names can't be specified with absolute or relative paths
o the -p option of the command built-in can't be used
o redirections that create files can't be used (i.e., >, >|, >>,
<>)
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
nor the ENV parameter (see below), 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/pro-
file and $HOME/.profile if they exist and are readable.
If the ENV parameter is set when the shell starts (or, in the case of
login shells, after any profiles are processed), its value is subjected
to parameter, command, arithmetic and tilde substitution and the
resulting file (if any) is read and executed. If ENV parameter is not
set (and not null) and pdksh was compiled with the DEFAULT_ENV macro
defined, the file named in that macro is included (after the above mentioned
substitutions have been performed).
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 [Toc] [Back]
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 create co-processes (see
Co-Processes below); ; is used to separate commands; & is used to create
asynchronous pipelines; && and || are used to specify conditional
execution; ;; is used in case statements; (( .. )) are used in arithmetic
expressions; 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). The |& operator starts a co-process which is
special kind of asynchronous process (see Co-Processes below). 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 function then !
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 semicolon,
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.
select name [ in word ... term ] do list done
where term is either a newline or a ;. The select statement
provides an automatic method of presenting the user with a menu
and selecting from it. An enumerated list of the specified
words is printed on standard error, followed by a prompt (PS3,
normally `#? '). A number corresponding to one of the enumerated
words is then read from standard input, name is set to the
selected word (or is unset if the selection is not valid), REPLY
is set to what was read (leading/trailing space is stripped),
and list is executed. If a blank line (i.e., zero or more IFS
characters) is entered, the menu is re-printed without executing
list. When list completes, the enumerated list is printed if
REPLY is null, the prompt is printed and so on. This process is
continues until an end-of-file is read, an interrupt is received
or a break statement is executed inside the loop. If in word
... is omitted, the positional parameters are used (i.e., "$1",
"$2", etc.). For historical reasons, open and close braces may
be used instead of do and done (e.g., select i; { echo $i; }).
The exit status of a select statement is zero if a break statement
is used to exit the loop, non-zero otherwise.
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.
function name { list }
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.
name () command
Mostly the same as function. See Functions below.
time [ -p ] [ pipeline ]
The time reserved word is described in the Command Execution
section.
(( expression ))
The arithmetic expression expression is evaluated; equivalent to
let "expression". See Arithmetic Expressions and the let command
below.
[[ expression ]]
Similar to the test and [ ... ] commands (described later), with
the following exceptions:
o Field splitting and file name generation are not performed
on arguments.
o The -a (and) and -o (or) operators are replaced with &&
and ||, respectively.
o Operators (e.g., -f, =, !, etc.) must be unquoted.
o The second operand of != and = expressions are patterns
(e.g., the comparison in
[[ foobar = f*r ]]
succeeds).
o There are two additional binary operators: < and > which
return true if their first string operand is less than,
or greater than, their second string operand, respectively.
o The single argument form of test, which tests if the
argument has non-zero length, is not valid - explicit
operators must be always be used, e.g., instead of
[ str ]
use
[[ -n str ]]
o Parameter, command and arithmetic substitutions are performed
as expressions are evaluated and lazy expression
evaluation is used for the && and || operators. This
means that in the statement
[[ -r foo && $(< foo) = b*r ]]
the $(< foo) is evaluated if and only if the file foo
exists and is readable.
Quoting [Toc] [Back]
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 "...`...\"...`..".
Aliases [Toc] [Back]
There are two types of aliases: normal command aliases and tracked
aliases. Command aliases are normally used as a short hand for a long
or often used command. The shell expands command aliases (i.e., substitutes
the alias name for its value) when it reads the first word of
a command. An expanded alias is re-processed to check for more
aliases. If a command alias ends in a space or tab, the following word
is also checked for alias expansion. The alias expansion process stops
when a word that is not an alias is found, when a quoted word is found
or when an alias word that is currently being expanded is found.
The following command aliases are defined automatically by the shell:
autoload='typeset -fu'
functions='typeset -f'
hash='alias -t'
history='fc -l'
integer='typeset -i'
local='typeset'
login='exec login'
newgrp='exec newgrp'
nohup='nohup '
r='fc -e -'
stop='kill -STOP'
suspend='kill -STOP $$'
type='whence -v'
Tracked aliases allow the shell to remember where it found a particular
command. The first time the shell does a path search for a command
that is marked as a tracked alias, it saves the full path of the command.
The next time the command is executed, the shell checks the
saved path to see that it is still valid, and if so, avoids repeating
the path search. Tracked aliases can be listed and created using alias
-t. Note that changing the PATH parameter clears the saved paths for
all tracked aliases. If the trackall option is set (i.e., set -o
trackall or set -h), the shell tracks all commands. This option is set
automatically for non-interactive shells. For interactive shells, only
the following commands are automatically tracked: cat, cc, chmod, cp,
date, ed, emacs, grep, ls, mail, make, mv, pr, rm, sed, sh, vi and who.
Substitution [Toc] [Back]
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 nonIFS
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 [Toc] [Back]
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 indicies 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, readonly and typeset 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 or typeset
-x commands, 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 (i.e., argv[0]), or the com-
mand-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:
_ (underscore)
When an external command is executed by the shell, this parameter
is set in the environment of the new process to the path of
the executed command. In interactive use, this parameter is
also set in the parent shell to the last word of the previous
command. When MAILPATH messages are evaluated, this parameter
contains the name of the file that changed (see MAILPATH parameter
below).
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 [Toc] [Back]
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.
EDITOR If the VISUAL parameter is not set, this parameter controls the
command line editing mode for interactive shells. See VISUAL
parameter below for how this works.
ENV If this parameter is found to be set after any profile files are
executed, the expanded value is used as a shell start-up file.
It typically contains function and alias definitions.
ERRNO Integer value of the shell's errno variable -- indicates the
reason the last system call failed.
Not implemented yet.
EXECSHELL [Toc] [Back]
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.
HISTFILE [Toc] [Back]
The name of the file used to store history. When assigned to,
history is loaded from the specified file. Also, several invocations
of the shell running on the same machine will share history
if their HISTFILE parameters all point at the same file.
NOTE: if HISTFILE isn't set, no history file is used. This is
different from the original Korn shell, which uses
$HOME/.sh_history; in future, pdksh may also use a default history
file.
HISTSIZE [Toc] [Back]
The number of commands normally stored for history, default 128.
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.
KSH_VERSION [Toc] [Back]
The version of shell and the date the version was created (readonly).
See also the version commands in Emacs Editing Mode and
Vi Editing Mode sections, below.
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.
MAIL If set, the user will be informed of the arrival of mail in the
named file. This parameter is ignored if the MAILPATH parameter
is set.
MAILCHECK [Toc] [Back]
How often, in seconds, the shell will check for mail in the
file(s) specified by MAIL or MAILPATH. If 0, the shell checks
before each prompt. The default is 600 (10 minutes).
MAILPATH [Toc] [Back]
A list of files to be checked for mail. The list is colon separated,
and each file may be followed by a ? and a message to be
printed if new mail has arrived. Command, parameter and arithmetic
substitution is performed on the message, and, during substitution,
the parameter $_ contains the name of the file. The
default message is you have mail in $_.
OLDPWD The previous working directory. Unset if cd has not successfully
changed directories since the shell started, or if the
shell doesn't know where it is.
OPTARG When using getopts, it contains the argument for a parsed
option, if it requires one.
OPTIND The index of the last argument processed when using getopts.
Assigning 1 to this parameter causes getopts to process arguments
from the beginning the next time it is invoked.
PATH A colon separated list of directories that are searched when
looking for commands and .'d files. An empty string resulting
from a leading or trailing colon, or two adjacent colons is
treated as a `.', the current directory.
POSIXLY_CORRECT [Toc] [Back]
If set, this parameter causes the posix option to be enabled.
See POSIX Mode below.
PPID The process ID of the shell's parent (readonly).
PS1 PS1 is the primary prompt for interactive shells. Parameter,
command and arithmetic substitutions are performed, and ! is
replaced with the current command number (see fc command below).
A literal ! can be put in the prompt by placing !! in PS1. Note
that since the command line editors try to figure out how long
the prompt is (so they know how far it is to edge of the
screen), escape codes in the prompt tend to mess things up. You
can tell the shell not to count certain sequences (such as
escape codes) by prefixing your prompt with a non-printing character
(such as control-A) followed by a carriage return and then
delimiting the escape codes with this non-printing character.
If you don't have any non-printing characters, you're out of
luck... BTW, don't blame me for this hack; it's in the original
ksh. Default is `$ ' for non-root users, `# ' for root..
PS2 Secondary prompt string, by default `> ', used when more input
is needed to complete a command.
PS3 Prompt used by select statement when reading a menu selection.
Default is `#? '.
PS4 Used to prefix commands that are printed during execution tracing
(see set -x command below). Parameter, command and arithmetic
substitutions are performed before it is printed. Default
is `+ '.
PWD The current working directory. Maybe unset or null if shell
doesn't know where it is.
RANDOM A simple random number generator. Every time RANDOM is referenced,
it is assigned the next number in a random number series.
The point in the series can be set by assigning a number to RAN-
DOM (see rand(3)).
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.
SECONDS [Toc] [Back]
The number of seconds since the shell started or, if the parameter
has been assigned an integer value, the number of seconds
since the assignment plus the value that was assigned.
TMOUT If set to a positive integer in an interactive shell, it specifies
the maximum number of seconds the shell will wait for input
after printing the primary prompt (PS1). If the time is
exceeded, the shell exits.
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.
VISUAL If set, this parameter controls the command line editing mode
for interactive shells. If the last component of the path specified
in this parameter contains the string vi, emacs or gmacs,
the vi, emacs or gmacs (Gosling emacs) editing mode is enabled,
respectively.
Tilde Expansion [Toc] [Back]
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 OLD-
PWD 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, readonly, and typeset),
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').
Brace Expansion (alternation) [Toc] [Back]
Brace expressions, which take the form
prefix{str1,...,strN}suffix
are expanded to N words, each of which is the concatenation of prefix,
stri and suffix (e.g., `a{c,b{X,Y},d}e' expands to four word: ace,
abXe, abYe, and ade). As noted in the example, brace expressions can
be nested and the resulting words are not sorted. Brace expressions
must contain an unquoted comma (,) for expansion to occur (i.e., {} and
{foo} are not expanded). Brace expansion is carried out after parameter
substitution and before file name generation.
File Name Patterns [Toc] [Back]
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.
*(pattern| ... |pattern)
matches any string of characters that matches zero or more occurances
of the specified patterns. Example: the pattern
*(foo|bar) matches the strings `', `foo', `bar', `foobarfoo',
etc..
+(pattern| ... |pattern)
matches any string of characters that matches one or more occurances
of the specified patterns. Example: the pattern
+(foo|bar) matches the strings `foo', `bar', `foobarfoo', etc..
?(pattern| ... |pattern)
matches the empty string or a string that matches one of the
specified patterns. Example: the pattern ?(foo|bar) only
matches the strings `', `foo' and `bar'.
@(pattern| ... |pattern)
matches a string that matches one of the specified patterns.
Example: the pattern @(foo|bar) only matches the strings `foo'
and `bar'.
!(pattern| ... |pattern)
matches any string that does not match one of the specified patterns.
Examples: the pattern !(foo|bar) matches all strings
except `foo' and `bar'; the pattern !(*) matches no strings; the
pattern !(?)* matches all strings (think about it).
Note that pdksh 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 .*.
If the markdirs option is set, any directories that result from file
name generation are marked with a trailing /.
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. Note that in restricted shells,
redirections which can create files cannot be 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 [Toc] [Back]
Integer arithmetic expressions can be used with the let command, 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 paramter 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 nonzero,
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 nonzero,
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>.
Co-Processes [Toc] [Back]
A co-process, which is a pipeline created with the |& operator, is an
asynchronous process that the shell can both write to (using print -p)
and read from (using read -p). The input and output of the co-process
can also be manipulated using >&p and <&p redirections, respectively.
Once a co-process has been started, another can't be started until the
co-process exits, or until the co-process input has been redirected
using an exec n>&p redirection. If a co-process's input is redirected
in this way, the next co-process to be started will share the output
with the first co-process, unless the output of the initial co-process
has been redirected using an exec n<&p redirection.
Some notes concerning co-processes:
o the only way to close the co-process input (so the co-process
reads an end-of-file) is to redirect the input to a numbered
file descriptor and then close that file descriptor (e.g., exec
3>&p;exec 3>&-).
o in order for co-processes to share a common output, the shell
must keep the write portion of the output pipe open. This means
th
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