ksh(1)

NAME

ksh, rksh — shell, the standard/restricted command programming language

SYNOPSIS

ksh [-aefhikmnoprstuvx] [+aefhikmnoprstuvx] [-o option]... [+o option]... [-c string] [arg]...

rksh [-aefhikmnoprstuvx] [+aefhikmnoprstuvx] [-o option]... [+o option]... [-c string] [arg]...

DESCRIPTION

ksh is a command programming language that executes commands read from a terminal or a file. rksh is a restricted version of the command interpreter ksh, used to set up login names and execution environments whose capabilities are more controlled than those of the standard shell. See Invoking ksh and Special Commands sections later in this entry for details about command line options and arguments, particularly the set command.

Definitions

metacharacter: One of the following characters:
; & ( ) | < > newline space tab
blank: A tab or space character.
identifier: A sequence of letters, digits, or underscores starting with a letter or underscore. Identifiers are used as names for functions and named parameters.
word: A sequence of characters separated by one or more non-quoted metacharacters .
command: A sequence of characters in the syntax of the shell language. The shell reads each command and carries out the desired action, either directly or by invoking separate utilities.
special command: A command that is carried out by the shell without creating a separate process. Often called ``built-in commands''. Except for documented side effects, most special commands can be implemented as separate utilities.
#: The # character is interpreted as the beginning of a comment. See Quoting below.

Commands

A simple-command is a sequence of blank-separated words that can be preceded by a parameter assignment list. (See Environment below). 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 exec(2)). The value of a simple-command is its exit status if it terminates normally, or (octal) 200+status if it terminates abnormally (see signal(5) for a list of status values).

A pipeline is a sequence of one or more commands separated by |. The standard output of each command except the last is connected by a pipe (see pipe(2)) to the standard input of the next command. 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 in the pipeline.

A list is a sequence of one or more pipelines separated by ;, &, &&, or ||, and optionally terminated by ;, &, or |&. Of these five symbols, ;, &, and |& have equal precedence. && and || have a higher but also equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline; an ampersand (&) causes asynchronous execution of the preceding pipeline (that is, the shell does not wait for that pipeline to finish). The symbol |& causes asynchronous execution of the preceding command or pipeline with a two-way pipe established to the parent shell (known as a co-process). The standard input and output of the spawned command can be written to and read from by the parent shell using the -p option of the special commands read and print described later. The symbol && (||) causes the list following it to be executed only if the preceding pipeline returns a zero (nonzero) value. An arbitrary number of newlines can appear in a list, instead of semicolons, to delimit commands.

A 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.

for identifier [in word ...] do list done: Each time for is executed, identifier is set to the next word taken from the in word list. If in word ... is omitted, for executes the do list once for each positional parameter set (see Parameter Substitution below). Execution ends when there are no more words in the list.
select identifier [in word...] do list done: A select command prints on standard error (file descriptor 2), the set of words, each preceded by a number. If in word ... is omitted, the positional parameters are used instead (see Parameter Substitution below). The PS3 prompt is printed and a line is read from the standard input. If this line starts with the number of one of the listed words, the value of the parameter identifier is set to the word corresponding to this number. If this line is empty, the selection list is printed again. Otherwise the value of the parameter identifier is set to null. The contents of the line read from standard input is saved in the parameter REPLY. The list is executed for each selection until a break or end-of-file (eof) is encountered.
case word in [[ (] pattern [ | pattern] ... ) list ;; ] ... esac: A case command executes the list associated with the first pattern that matches word. The form of the patterns is identical to that used for file name generation (see File Name Generation below).
if list then list [ elif list then list] ... [ else list] fi: The list following if is executed and, if it returns a zero exit status, the list following the first then is executed. Otherwise, the list following elif is executed and, if its value is zero, the list following the next then is executed. Failing that, the else list is executed. If no else list or then list is executed, if returns a zero exit status.
while list do list done
until list do list done: A while command repeatedly executes the while list, and if the exit status of the last command in the list is zero, executes the do list; otherwise the loop terminates. If no commands in the do list are executed, while returns a zero exit status; until can be used in place of while to negate the loop termination test.
(list): Execute list in a separate environment. If two adjacent open parentheses are needed for nesting, a space must be inserted to avoid arithmetic evaluation as described below.
{ list;}: Execute list, but not in a separate environment. Note that { is a keyword and requires a trailing blank to be recognized.
[[ expression ]]: Evaluates expression and returns a zero exit status when expression is true. See Conditional Expressions below, for a description of expression. Note that [[ and ]] are keywords and require blanks between them and expression.
function identifier {list;}
identifier () {list;}: Define a function referred to by identifier. The body of the function is the list of commands between { and } (see Functions below).
time pipeline: pipeline is executed and the elapsed time, user time, and system time are printed on standard error. Note that the time keyword can appear anywhere in the pipeline to time the entire pipeline. To time a particular command in a pipeline, see time(1).

The following keywords are recognized only as the first word of a command and when not quoted:

if then else elif fi case esac for while 
until do done { } function select time [[ ]] 

Comments

A word beginning with # causes that word and all subsequent characters up to a newline to be ignored.

Aliasing

The first word of each command is replaced by the text of an alias, if an alias for this word has been defined. An alias name consists of any number of characters excluding metacharacters, quoting characters, file expansion characters, parameter and command substitution characters, and =. The replacement string can contain any valid shell script, including the metacharacters listed above. The first word of each command in the replaced text, other than any that are in the process of being replaced, is tested for additional aliases. If the last character of the alias value is a blank, the word following the alias is also checked for alias substitution.

Aliases can be used to redefine special built-in commands, but cannot be used to redefine the keywords listed above. Aliases can be created, listed, and exported with the alias command and can be removed with the unalias command. Exported aliases remain in effect for subshells but must be reinitialized for separate invocations of the shell (see Invoking ksh below).

Aliasing is performed when scripts are read, not while they are executed. Therefore, for it to take effect, alias must be executed before the command referring to the alias is read.

Aliases are frequently used as a shorthand for full path names. An option to the aliasing facility allows the value of the alias to be automatically set to the full path name of the corresponding command. These aliases are called tracked aliases. The value of a tracked alias is defined the first time the identifier is read and becomes undefined each time the PATH variable is reset. These aliases remain tracked so that the next reference redefines the value. Several tracked aliases are compiled into the shell. The -h option of the set command converts each command name that is an identifier into a tracked alias.

The following exported aliases are compiled into the shell but can be unset or redefined:

autoload='typeset -fu' 
false='let 0' 
functions='typeset -f' 
hash='alias -t -' 
history='fc -l' 
integer='typeset -i' 
nohup='nohup ' 
r='fc -e -' 
stop='kill -STOP' 
suspend='kill -STOP $$' 
true=':' 
type='whence -v' 

Tilde Substitution

After alias substitution is performed, each word is checked to see if it begins with an unquoted ~. If it does, the word up to a / is checked to see if it matches a user name in the /etc/passwd file. If a match is found, the ~ and the matched login name are replaced by the login directory of the matched user. This is called a tilde substitution. If no match is found, the original text is left unchanged. A ~, alone or before a /, is replaced by the value of the HOME parameter. A ~ followed by a + or - is replaced by the value of the parameter PWD and OLDPWD, respectively. In addition, tilde substitution is attempted when the value of a parameter assignment begins with a ~.

Command Substitution

The standard output from a command enclosed in parenthesis preceded by a dollar sign ($(command)) or a pair of back single quotes (accent grave) (`command`) can be used as part or all of a word; trailing newlines are removed. In the second (archaic) form, the string between the quotes is processed for special quoting characters before the command is executed (see Quoting below). The command substitution $(cat file) can be replaced by the equivalent but faster $(<file).

Command substitution of most special commands (built-ins) that do not perform I/O redirection are carried out without creating a separate process. However, command substitution of a function creates a separate process to execute the function and all commands (built-in or otherwise) in that function.

An arithmetic expression enclosed in double parentheses preceded by a dollar sign ($((expression))) is replaced by the value of the arithmetic expression within the double parentheses (see Arithmetic Evaluation below for a description of arithmetic expressions).

Parameter Substitution

A parameter is an identifier, one or more digits, or any of the characters *, @, #, ?, -, $, and !. A named parameter (a parameter denoted by an identifier) has a value and zero or more attributes. Named parameters can be assigned values and attributes by using the typeset special command. Attributes supported by ksh are described later with the typeset special command. Exported parameters pass values and attributes to the environment.

The shell supports a limited one-dimensional array facility. An element of an array parameter is referenced by a subscript. A subscript is denoted by a [ followed by an arithmetic expression (see Arithmetic Evaluation below) followed by a ]. To assign values to an array, use set -A name value .... The value of all subscripts must be in the range of 0 through 1023. Arrays need not be declared. Any reference to a named parameter with a valid subscript is legal and an array is created if necessary. Referencing an array without a subscript is equivalent to referencing the first element.

The value of a named parameter can also be assigned by writing:

name=value 
[name=value]...

If the -i integer attribute is set for name, the value is subject to arithmetic evaluation as described below.

Positional parameters, parameters denoted by a number, can be assigned values with the set special command. Parameter $0 is set from argument zero when the shell is invoked.

The character $ is used to introduce substitutable parameters.

${parameter}

Substitute the value of the parameter, if any. Braces are required when parameter is followed by a letter, digit, or underscore that should not be interpreted as part of its name or when a named parameter is subscripted.

If parameter is one or more digits, it is a positional parameter. A positional parameter of more than one digit must be enclosed in braces. If parameter is * or @, all the positional parameters, starting with $1, are substituted (separated by a field separator character). If an array identifier with subscript * or @ is used, the value for each element is substituted (separated by a field separator character). The shell reads all the characters from ${ to the matching } as part of the same word even if it contains braces or metacharacters.

${#parameter}

If parameter is * or @, the number of positional parameters is substituted. Otherwise, the length of the value of the parameter is substituted.

${#identifier[*]}

Substitute the number of elements in the array identifier.

${parameter:-word}

If parameter is set and is non-null, substitute its value; otherwise substitute word.

${parameter:=word}

If parameter is not set or is null, set it to word; then substitute the value of the parameter. Positional parameters cannot be assigned in this way.

${parameter:?word}

If parameter is set and is non-null, substitute its value; otherwise, print word and exit from the shell. If word is omitted, a standard message is printed.

${parameter:+word}

If parameter is set and is non-null, substitute word; otherwise substitute nothing.

${parameter#pattern}

${parameter##pattern}

If the shell pattern matches the beginning of the value of parameter, the value of this substitution is the value of the parameter with the matched portion deleted; otherwise the value of this parameter is substituted. In the former case, the smallest matching pattern is deleted; in the latter case, the largest matching pattern is deleted.

${parameter%pattern}

${parameter%%pattern}

If the shell pattern matches the end of the value of parameter, the value of parameter with the matched part is deleted; otherwise substitute the value of parameter. In the former, the smallest matching pattern is deleted; in the latter, the largest matching pattern is deleted.

In the above, word is not evaluated unless it is used as the substituted string. Thus, in the following example, pwd is executed only if d is not set or is null:
echo ${d:-$(pwd)}
If the colon (:) is omitted from the above expressions, the shell only checks to determine whether or not parameter is set.

The following parameters are set automatically by the shell:

#: The number of positional parameters in decimal.
-: Options supplied to the shell on invocation or by the set command.
?: The decimal value returned by the last executed command.
$: The process number of this shell.
_: Initially, the value of _ is an absolute pathname of the shell or script being executed as passed in the environment. Subsequently it is assigned the last argument of the previous command. This parameter is not set for commands which are asynchronous. This parameter is also used to hold the name of the matching MAIL file when checking for mail.
!: The process number of the last background command invoked.
COLUMNS: If this variable is set, its value is used to define the width of the edit window for the shell edit modes and for printing select lists. In a windowed environment, if the shell detects that the window size has changed, the shell updates the value of COLUMNS.
ERRNO: The value of errno as set by the most recently failed system call. This value is system dependent and is intended for debugging purposes.
LINENO: The line number of the current line within the script or function being executed.
LINES: If this variable is set, the value is used to determine the column length for printing select lists. select lists print vertically until about two-thirds of LINES lines are filled. In a windowed environment, if the shell detects that the window size has changed, the shell updates the value of LINES.
OLDPWD: The previous working directory set by the cd command.
OPTARG: The value of the last option argument processed by the getopts special command.
OPTIND: The index of the last option argument processed by the getopts special command.
PPID: The process number of the parent of the shell.
PWD: The present working directory set by the cd command.
RANDOM: Each time this parameter is evaluated, a random integer, uniformly distributed between 0 and 32767, is generated. The sequence of random numbers can be initialized by assigning a numeric value to RANDOM.
REPLY: This parameter is set by the select statement and by the read special command when no arguments are supplied.
SECONDS: Each time this parameter is referenced, the number of seconds since shell invocation is returned. If this parameter is assigned a value, the value returned upon reference is the value that was assigned plus the number of seconds since the assignment.

The following parameters are used by the shell:

CDPATH: The search path for the cd command.
EDITOR: If the value of this variable ends in emacs, gmacs, or vi and the VISUAL variable is not set, the corresponding option is turned on (see set in Special Commands below).
ENV: If this parameter is set, parameter substitution is performed on the value to generate the path name of the script to be executed when the shell is invoked (see Invoking ksh below). This file is typically used for alias and function definitions.
FCEDIT: The default editor name for the fc command.
FPATH: The search path for function definitions. This path is searched when a function with the -u attribute is referenced and when a command is not found. If an executable file is found, then it is read and executed in the current environment.
IFS: Internal field separators, normally space, tab, and newline that are used to separate command words resulting from command or parameter substitution, and for separating words with the special command read. The first character of the IFS parameter is used to separate arguments for the "$*" substitution (see Quoting below).
HISTFILE: If this parameter is set when the shell is invoked, its value is the path name of the file that is used to store the command history. The default value is $HOME/.sh_history. If the user has appropriate privileges and no HISTFILE is given, then no history file is used (see Command Re-entry below).
HISTSIZE: If this parameter is set when the shell is invoked, the number of previously entered commands accessible to this shell will be greater than or equal to this number. The default is 128.
HOME: The default argument (home directory) for the cd command.
MAIL: If this parameter is set to the name of a mail file and the MAILPATH parameter is not set, the shell informs the user of arrival of mail in the specified file.
MAILCHECK: This variable specifies how often (in seconds) the shell checks for changes in the modification time of any of the files specified by the MAILPATH or MAIL parameters. The default value is 600 seconds. When the time has elapsed the shell checks before issuing the next prompt.
MAILPATH: A list of file names separated by colons (:). If this parameter is set, the shell informs the user of any modifications to the specified files that have occurred within the last MAILCHECK seconds. Each file name can be followed by a ? and a message to be printed, in which case the message undergoes parameter and command substitution with the parameter $_ defined as the name of the changed file. The default message is you have mail in $_.
PATH: The search path for commands (see Execution below). The user cannot change PATH if executing rksh (except in the .profile file).
PS1: The value of this parameter is expanded for parameter substitution, to define the primary prompt string which, by default, is $ followed by a space character. The character ! in the primary prompt string is replaced by the command number (see Command Re-entry below). To include a ! in the prompt, use !!.
PS2: Secondary prompt string, by default > followed by a space character.
PS3: Selection prompt string used within a select loop, by default #? followed by a space character.
PS4: The value of this variable is expanded for parameter substitution and precedes each line of an execution trace. If PS4 is unset, the execution trace prompt is + followed by a space character.
SHELL: The path name of the shell is kept in the environment. When invoked, the shell is restricted if the value of this variable contains an r in the basename.
TMOUT: If set to a value greater than zero, the shell terminates if a command is not entered within the prescribed number of seconds after issuing the PS1 prompt.
VISUAL: Invokes the corresponding option when the value of this variable ends in emacs, gmacs, or vi (see set in Special Commands below).

The shell gives default values to PATH, PS1, PS2, MAILCHECK, TMOUT, and IFS. HOME, SHELL, ENV, and MAIL are never set automatically by the shell (although HOME, SHELL, and MAIL are set by login(1)).

Blank Interpretation

After parameter and command substitution, the results of substitution are scanned for field separator characters (found in IFS), and split into distinct arguments where such characters are found. ksh retains explicit null arguments ( or '') but removes implicit null arguments (those resulting from parameters that have no values).

File Name Generation

Following substitution, each command word is processed as a pattern for file name expansion unless the -f option has been set. The form of the patterns is the Pattern Matching Notation defined by regexp(5). The word is replaced with sorted file names matching the pattern. If no file name is found that matches the pattern, the word is left unchanged.

In addition to the notation described in regexp(5), ksh recognizes composite patterns made up of one or more pattern lists separated from each other with a |. Composite patterns can be formed with one or more of the following:

?(pattern-list): Optionally matches any one of the given patterns.
*(pattern-list): Matches zero or more occurrences of the given patterns.
+(pattern-list): Matches one or more occurrences of the given patterns.
@(pattern-list): Matches exactly one of the given patterns.
!(pattern-list): Matches anything, except one of the given patterns.

Quoting

Each of the metacharacters listed above (See Definitions above) has a special meaning to the shell and causes termination of a word unless quoted. A character can be quoted (i.e., made to stand for itself) by preceding it with a \. The pair \newline is ignored. All characters enclosed between a pair of single quote marks (''), are quoted.

A single quote cannot appear within single quotes. Inside double quote marks (" ..." ), parameter and command substitution occurs and \ quotes the characters \, `, " , and $. $* and $@ have identical meanings when not quoted or when used as a parameter assignment value or as a file name. However, when used as a command argument, "$*" is equivalent to " $1d$2d... " , where d is the first character of the IFS parameter, whereas "$@" is equivalent to "$1" "$2" ....

Inside back single quote (accent grave) marks (`..`), \ quotes the characters \, `, and $.

If the back single quotes occur within double quotes, \ also quotes the character " .

The special meaning of keywords or aliases can be removed by quoting any character of the keyword. The recognition of function names or special command names listed below cannot be altered by quoting them.

Arithmetic Evaluation

The ability to perform integer arithmetic is provided with the special command let. Evaluations are performed using long arithmetic. Constants take the form [base#]n, where base is a decimal number between two and thirty-six representing the arithmetic base and n is a number in that base. If base is omitted, base 10 is used.

An arithmetic expression uses the same syntax, precedence, and associativity of expression of the C language. All the integral operators, other than ++, --, ?:, and , are supported. Variables can be referenced by name within an arithmetic expression without using the parameter substitution syntax. When a variable is referenced, its value is evaluated as an arithmetic expression.

An internal integer representation of a variable can be specified with the -i option of the typeset special command. Arithmetic evaluation is performed on the value of each assignment to a variable with the -i attribute. If you do not specify an arithmetic base, the first assignment to the variable determines the arithmetic base. This base is used when parameter substitution occurs.

Since many of the arithmetic operators require quoting, an alternative form of the let command is provided. For any command beginning with ((, all characters until the matching )) are treated as a quoted expression. More precisely, ((...)) is equivalent to let "...".

Prompting

When used interactively, the shell prompts with the value of 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 (the value of PS2) is issued.

Conditional Expressions.

A conditional expression is used with the [[ compound command to test attributes of files and to compare strings. Word splitting and file name generation are not performed on the words between [[ and ]]. Each expression can be constructed from one or more of the following unary or binary expressions:

-a file: True if file exists.
-b file: True if file exists and is a block special file.
-c file: True if file exists and is a character special file.
-d file: True if file exists and is a directory.
-f file: True if file exists and is an ordinary file.
-g file: True if file exists and is has its setgid bit set.
-h file: True if file exists and is a a symbolic link.
-k file: True if file exists and is has its sticky bit set.
-n string: True if length of string is nonzero.
-o option: True if option named option is on.
-p file: True if file exists and is a fifo special file or a pipe.
-r file: True if file exists and is readable by current process.
-s file: True if file exists and has size greater than zero.
-t fildes: True if file descriptor number fildes is open and associated with a terminal device.
-u file: True if file exists and is has its setuid bit set.
-w file: True if file exists and is writable by current process.
-x file: True if file exists and is executable by current process. If file exists and is a directory, the current process has permission to search in the directory.
-z string: True if length of string is zero.
-L file: True if file exists and is a symbolic link.
-O file: True if file exists and is owned by the effective user ID of this process.
-G file: True if file exists and its group matches the effective group ID of this process.
-S file: True if file exists and is a socket.
file1 -nt file2: True if file1 exists and is newer than file2.
file1 -ot file2: True if file1 exists and is older than file2.
file1 -ef file2: True if file1 and file2 exist and refer to the same file.
string = pattern: True if string matches pattern.
string != pattern: True if string does not match pattern.
string1 < string2: True if string1 comes before string2 based on the ASCII value of their characters.
string1 > string2: True if string1 comes after string2 based on the ASCII value of their characters.
exp1 -eq exp2: True if exp1 is equal to exp2.
exp1 -ne exp2: True if exp1 is not equal to exp2.
exp1 -lt exp2: True if exp1 is less than exp2.
exp1 -gt exp2: True if exp1 is greater than exp2.
exp1 -le exp2: True if exp1 is less than or equal to exp2.
exp1 -ge exp2: True if exp1 is greater than or equal to exp2.

A compound expression can be constructed from these primitives by using any of the following, listed in decreasing order of precedence.

(expression): True, if expression is true. Used to group expressions.
! expression: True if expression is false.
expression1 && expression2: True, if expression1 and expression2 are both true.
expression1 || expression2: True, if either expression1 or expression2 is true.

Input/Output

Before a command is executed, its input and output can be redirected using a special notation interpreted by the shell. The following can appear anywhere in a simple-command or can precede or follow a command and are not passed on to the invoked command. Command and parameter substitution occurs before word or digit is used, except as noted below. File name generation occurs only if the pattern matches a single file and blank interpretation is not performed.

<word: Use file word as standard input (file descriptor 0).
>word: Use file word as standard output (file descriptor 1). If the file does not exist, it is created. If the file exists, and the noclobber option is on, an error occurs; otherwise, the file is truncated to zero length. Note that the noclobber test is only applied to regular files, not to named pipes or other file types.
>|word: Sames as >, except that it overrides the noclobber option.
>>word: Use file word as standard output. If the file exists, output is appended to it (by first searching for the end-of-file); otherwise, the file is created.
<>word: Open file word for reading and writing as standard input. If the file does not exist it is created.
<<[-]word: The shell input is read up to a line that matches word, or to an end-of-file. No parameter substitution, command substitution, or file name generation is performed on word. The resulting document, called a here-document, becomes the standard input. If any character of word is quoted, no interpretation is placed upon the characters of the document. Otherwise, parameter and command substitution occurs, \newline is ignored, and \ must be used to quote the characters \, $, `, and the first character of word. If - is appended to <<, all leading tabs are stripped from word and from the document.
<&digit: The standard input is duplicated from file descriptor digit (see dup(2)).
>&digit: The standard output is duplicated to file descriptor digit (see dup(2)).
<&-: The standard input is closed.
>&-: The standard output is closed.
<&p: The input from the co-process is moved to standard input.
>&p: The output to the co-process is moved to standard output.

If one of the above is preceded by a digit, the file descriptor number cited is that specified by the digit (instead of the default 0 or 1). For example:

... 2>&1

means file descriptor 2 is to be opened for writing as a duplicate of file descriptor 1.

Redirection order is significant because the shell evaluates redirections referencing file descriptors in terms of the currently open file associated with the specified file descriptor at the time of evaluation. For example:

...  1>fname 2>&1

first assigns file descriptor 1 (standard output) to file fname, then assigns file descriptor 2 (standard error) to the file assigned to file descriptor 1; i.e., fname. On the other hand, if the order of redirection is reversed as follows:

...  2>&1 1>fname

file descriptor 2 is assigned to the current standard output (user terminal unless a different assignment is inherited). File descriptor 1 is then reassigned to file fname without changing the assignment of file descriptor 2.

The input and output of a co-process can be moved to a numbered file descriptor allowing other commands to write to them and read from them using the above redirection operators. If the input of the current co-process is moved to a numbered file descriptor, another co-process can be started.

If a command is followed by & and job control is inactive, the default standard input for the command is the empty file /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.

Environment

The environment (see environ(5)) is a list of name-value pairs passed to an executed program much like a normal argument list. The names must be identifiers and the values are character strings.

The shell interacts with the environment in several ways. When invoked, the shell scans the environment and creates a parameter for each name found, gives it the corresponding value, and marks it export. Executed commands inherit the environment. If the user modifies the values of these parameters or creates new ones by using the export or typeset -x commands, the values become part of the environment. The environment seen by any executed command is thus composed of any name-value pairs originally inherited by the shell whose values can be modified by the current shell, plus any additions which must be noted in export or typeset -x commands.

The environment for any simple-command or function can be augmented by prefixing it with one or more parameter assignments. A parameter assignment argument takes the form identifier=value. For example,

TERM=450 cmd args

and

(export TERM; TERM=450; cmd args)

are equivalent (as far as the above execution of cmd is concerned except for special commands listed below that are preceded by a percent sign).

If the -k option is set, all parameter assignment arguments are placed in the environment, even if they occur after the command name. The following echo statement prints a=b c. After the -k option is set, the second echo statement prints only c:

echo a=b c 
set -k 
echo a=b c 

This feature is intended for use with scripts written for early versions of the shell, and its use in new scripts is strongly discouraged. It is likely to disappear someday.

Functions

The function keyword (described in the Commands section above) is used to define shell functions. Shell functions are read and stored internally. Alias names are resolved when the function is read. Functions are executed like commands, with the arguments passed as positional parameters (see Execution below).

Functions execute in the same process as the caller except that command substitution of a function creates a new process. Functions share all files and present working directory with the caller. Traps caught by the caller are reset to their default action inside the function. If a function does not catch or specifically ignore a trap condition, the function terminates and the condition is passed on to the caller. A trap on EXIT set inside a function is executed after the function completes in the environment of the caller. Ordinarily, variables are shared between the calling program and the function. However, the typeset special command used within a function defines local variables whose scope includes the current function and all functions it calls.

The special command return is used to return from function calls. Errors within functions return control to the caller.

Function identifiers can be listed with the +f option of the typeset special command. Function identifiers and the associated text of the functions can be listed with the -f option when used interactively. ksh stores the function definitions in the history file. Hence, ksh will not display the function definitions if the history file is lost or if the nolog option was on when the function was read. Functions can be undefined with the -f option of the unset special command.

Ordinarily, functions are unset when the shell executes a shell script. The -xf option of the typeset command allows a function to be exported to scripts that are executed without reinvoking the shell. Functions that must be defined across separate invocations of the shell should be placed in the ENV file.

Jobs

If the monitor option of the set command is turned on, an interactive shell associates a job with each pipeline. It keeps a table of current jobs, printed by the jobs command, and assigns them small integer numbers. When a job is started asynchronously with &, the shell prints a line resembling:

[1] 1234 

indicating that job number 1 was started asynchronously and had one (top-level) process whose process ID was 1234.

If you are running a job and want to do something else, type the suspend character (usually ^Z (Ctrl-Z)) to send a STOP signal to the current job. The shell then indicates that the job has been `Stopped', and prints another prompt. The state of this job can be manipulated by using the bg command to put it in the background, running other commands (while it is stopped or running in the background), and eventually restarting or returning the job to the foreground by using the fg command. A ^Z takes effect immediately and resembles an interrupt, since pending output and unread input are discarded when ^Z is typed.

A job run in the background stops if it tries to read from the terminal. Background jobs normally are allowed to produce output, but can be disabled by giving the stty tostop command. If the user sets this tty option, background jobs stop when trying to produce output.

There are several ways to refer to jobs in the shell. A job can be referred to by the process ID of any process in the job or by one of the following:

%number: The job with the given number.
%string: Any job whose command line begins with string.
%?string: Any job whose command line contains string.
%%: Current job.
%+: Equivalent to %%.
%-: Previous job.

The shell learns immediately when a process changes state. It informs the user when a job is blocked and prevented from further progress, but only just before it prints a prompt.

When the monitor mode is on, each background job that completes triggers any trap set for CHLD.

Attempting to leave the shell while jobs are running or stopped produces the warning, You have stopped (running) jobs. Use the jobs command to identify them. An immediate attempt to exit again terminates the stopped jobs; the shell does not produce a warning the second time.

Signals

The INT and QUIT signals for an invoked command are ignored if the command is followed by & and the monitor option is off. Otherwise, signals have the values inherited by the shell from its parent, with the exception of signal 11 (but see also the trap command below).

Execution

Substitutions are made each time a command is executed. If the command name matches one of the Special Commands listed below, it is executed within the current shell process. Next, ksh checks the command name to determine whether it matches one of the user-defined functions. If it does, ksh saves the positional parameters and then sets them to the arguments of the function call. The positional parameter 0 is set to the function name. When the function completes or issues a return, ksh restores the positional parameter list and executes any trap set on EXIT within the function. The value of a function is the value of the last command executed.

A function is executed in the current shell process. If a command name is not a special command or a user-defined function, ksh creates a process and attempts to execute the command using exec (see exec(2)).

The shell parameter PATH defines the search path for the directory containing the command. Alternative directory names are separated by a colon (:). The default path is /usr/bin: (specifying /usr/bin and the current directory in that order).

Note that the current directory is specified by a null path name which can appear immediately after the equals sign, between colon delimiters, or at the end of the path list.

The search path is not used if the command name contains a /. Otherwise, each directory in the path is searched for an executable file.

If the file has execute permissions but is not a directory or an executable object code file, it is assumed to be a script file, which is a file of data for an interpreter. If the first two characters of the script file are #!, exec (see exec(2)) expects an interpreter path name to follow. exec then attempts to execute the specified interpreter as a separate process to read the entire script file. If a call to exec fails, /usr/bin/ksh is spawned to interpret the script file. All non-exported aliases, functions, and named parameters are removed in this case.

If the shell command file does not have read permission, or if the setuid and/or setgid bits are set on the file, the shell executes an agent to set up the permissions and execute the shell with the shell command file passed down as an open file. A parenthesized command is also executed in a sub-shell without removing non-exported quantities.

Command Re-entry

The text of the last HISTSIZE (default 128) commands entered from a terminal device is saved in a history file. The file $HOME/.sh_history is used if the HISTFILE variable is not set or writable. A shell can access the commands of all interactive shells that use the same named HISTFILE.

The special command fc is used to list or edit a portion of this file. The portion of the file to be edited or listed can be selected by number or by giving the first character or characters of the command. A single command or range of commands can be specified. If no editor program is specified as an argument to fc, the value of the FCEDIT parameter is used. If FCEDIT is not defined, /usr/bin/ed is used. The edited command is printed and re-executed upon leaving the editor. The editor name - is used to skip the editing phase and to re-execute the command. In this case a substitution parameter of the form old=new can be used to modify the command before execution.

For example, if r is aliased to fc -e -, typing r bad=good c re-executes the most recent command that starts with the letter c and replaces the first occurrence of the string bad with the string good.

The history file will be trimmed when all of the following conditions occurs:

Its size is greater than four kilobytes.
The number of commands in it is more than HISTSIZE.
The file has not been modified in the last ten minutes.
The user has write permission for the directory in which the history file resides.

If any one of the above conditions does not occur, the history file will not be trimmed. When the history file is trimmed, the latest HISTSIZE commands will be available in the history file.

Special Commands

The following simple-commands are executed in the shell process. They permit input/output redirection. Unless otherwise indicated, file descriptor 1 is the default output location and the exit status, when there are no syntax errors, is zero. Commands that are preceded by % or %% are treated specially in the following ways:

1.: Variable assignment lists preceding the command remain in effect when the command completes.
2.: I/O redirections are processed after variable assignments.
3.: Certain errors cause a script that contains them to abort.
4.: Words following a command preceded by %% that are in the format of a variable assignment are expanded with the same rules as a variable assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation are not performed.

The special commands are list here:

% : [arg ...]

The command only expands parameters. A zero exit code is returned.

% . file [arg ...]

Read and execute commands from file and return. The commands are executed in the current shell environment. The search path specified by PATH is used to find the directory containing file. If any arguments arg are given, they become the positional parameters. Otherwise the positional parameters are unchanged. The exit status is the exit status of the last command executed. It is not necessary that the execute permission bit be set for file.

%% alias [-tx] [name[=value] ...]

alias with no arguments prints the list of aliases in the form name=value on standard output. An alias is defined for each name whose value is given. A trailing space in value causes the next word to be checked for alias substitution. The -t option is used to set and list tracked aliases. The value of a tracked alias is the full path name corresponding to the given name. The value of a tracked alias becomes undefined when the value of PATH is reset, but the alias remains tracked. Without the -t option, for each name in the argument list for which no value is given, the name and value of the alias is printed. The -x option is used to set or print exported aliases. An exported alias is defined across sub-shell environments. Alias returns true unless a name is given for which no alias has been defined.

bg [job ...]

Puts the specified jobs into the background. The current job is put in the background if job is unspecified. See Jobs for a description of the format of job.

% break [n]

Exit from the enclosing for, while, until, or select loop, if any. If n is specified, break n levels.

% continue [n]

Resume the next iteration of the enclosing for, while, until, or select loop. If n is specified, resume at the n-th enclosing loop.

cd [-L|-P] [arg]

cd old new

This command can take either of two forms. In the first form it changes the current directory to arg. If arg is - the directory is changed to the previous directory. The -L option (default) preserves logical naming when treating symbolic links. cd -L .. moves the current directory one path component closer to the root directory. The -P option preserves the physical path when treating symbolic links. cd -P .. changes the working directory to the parent directory of the current directory. The shell parameter HOME is the default arg. The parameter PWD is set to the current directory. The shell parameter CDPATH defines the search path for the directory containing arg. Alternative directory names are separated by a colon (:). If CDPATH is null or undefined, the default value is the current directory. 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 arg begins with a /, the search path is not used. Otherwise, each directory in the path is searched for arg. See also cd(1).

The second form of cd substitutes the string new for the string old in the current directory name, PWD and tries to change to this new directory.

The cd command cannot be executed by rksh.

echo [arg ...]

See echo(1) for usage and description.

% eval [arg ...]

Reads the arguments as input to the shell and executes the resulting command(s).

% exec [arg ...]

Parameter assignments remain in effect after the command completes. If arg is given, the command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments can appear and affect the current process. If no arguments are given, the effect of this command is to modify file descriptors as prescribed by the input/output redirection list. In this case, any file descriptor numbers greater than 2 opened with this mechanism are closed when invoking another program.

% exit [n]

Causes the shell to exit with the exit status specified by n. If n is omitted, the exit status is that of the last command executed. An end-of-file also causes the shell to exit, except when a shell has the ignoreeof option set (see set below).

%% export [name [=value] ...]

The given names are marked for automatic export to the environment of subsequently executed commands.

fc [-eename] [-nlr] [first [last]]

fc -e - [old=new] [command]

In the first form, a range of commands from first to last is selected from the last HISTSIZE commands typed at the terminal. The arguments first and last can be specified as a number or string. A given string is used to locate the most recent command. A negative number is used to offset the current command number. The -l option causes the commands to be listed on standard output. Otherwise, the editor program ename is invoked on a file containing these keyboard commands. If ename is not supplied, the value of the parameter FCEDIT (default /usr/bin/ed) is used as the editor. Once editing has ended, the commands (if any) are executed. If last is omitted, only the command specified by first is used. If first is not specified, the default is the previous command for editing and -16 for listing. The -r option reverses the order of the commands and the -n option suppresses command numbers when listing. In the latter, the command is re-executed after the substitution old=new is performed.

fg [job ...]

Brings each job into the foreground in the order specified. If no job is specified, the current job is brought into the foreground. See Jobs for a description of the format of job.

getopts optstring name [arg ...]

Checks arg for legal options. If arg is omitted, the positional parameters are used. An option argument begins with a + or a -. An option not beginning with + or -, or the argument -- ends the options. optstring contains the letters that getopts recognizes. If a letter is followed by a :, that option is expected to have an argument. The options can be separated from the argument by blanks.

getopts places the next option letter it finds inside variable name each time it is invoked with a + preceding it when arg begins with a +. The index of the next arg is stored in OPTIND. The option argument, if any, gets stored in OPTARG.

A leading : in optstring causes getopts to store the letter of an invalid option in OPTARG, and to set name to ? for an unknown option and to : when a required option is missing. Otherwise, getopts prints an error message. The exit status is nonzero when there are no more options. See also getopts(1).

jobs [-lnp] [job ...]

Lists information about each given job; or all active jobs if job is omitted. The -l option lists process ids in addition to the normal information. The -n option only displays jobs that have stopped or exited since last notified. The -p option causes only the process group to be listed. See Jobs for a description of the format of job.

kill [-sig] process ...

Sends either the TERM (terminate) signal or the specified signal to the specified jobs or processes. Signals are given either by number or name (as given in signal(5), stripped of the prefix SIG). The signal names are listed by kill -l. No default exists; merely typing kill does not affect the current job. If the signal being sent is TERM (terminate) or HUP (hangup), the job or process is sent a CONT (continue) signal when stopped. The process argument can be either a process ID or job. If the first argument to kill is a negative integer, it is interpreted as a sig argument and not as a process group. See also kill(1).

let arg ...

Each arg is a separate arithmetic expression to be evaluated. See Arithmetic Evaluation above, for a description of arithmetic expression evaluation. The exit status is 0 if the value of the last expression is nonzero, and 1 otherwise.

% newgrp [arg ...]

Equivalent to exec newgrp arg ....

print[-Rnprsu[n]] [arg ...]

The shell output mechanism. With no options or with option - or -- the arguments are printed on standard output as described by echo(1). Raw mode, -R or -r, ignores the escape conventions of echo. The -R option prints all subsequent arguments and options other than -n. The -p option causes the arguments to be written onto the pipe of the process spawned with |& instead of standard output. The -s option causes the arguments to be written onto the history file instead of standard output. The -u option can be used to specify a one-digit file descriptor unit number n on which the output is to be placed. The default is 1. If the option -n is used, no newline character is added to the output.

pwd [-L|-P]

With no arguments prints the current working directory (equivalent to print -r - $PWD). The -L option (default) preserves the logical meaning of the current directory and -P preserves the physical meaning of the current directory if it is a symbolic link. See the special cd command, cd(1), ln(1)), and pwd(1).

read [-prsu[n]] [name] [?prompt] [name ...]

The shell input mechanism. One line is read and broken up into words using the characters in IFS as separators. In -r raw mode, \ at the end of a line does not signify line continuation. The first word is assigned to the first name, the second word to the second name, etc., with remaining words assigned to the last name. The -p option causes the input line to be taken from the input pipe of a process spawned by the shell using |&. If the -s option is present, the input is saved as a command in the history file. The option -u can be used to specify a one-digit file descriptor unit to read from. The file descriptor can be opened with the exec special command. The default value of n is 0. If name is omitted, REPLY is used as the default name. The return code is 0, unless an end-of-file is encountered. An end-of-file with the -p option causes cleanup for this process so that another process can be spawned. If the first argument contains a ?, the remainder of this word is used as a prompt when the shell is interactive. If the given file descriptor is open for writing and is a terminal device, the prompt is placed on this unit. Otherwise the prompt is issued on file descriptor 2. The return code is 0, unless an end-of-file is encountered. See also read(1).

%% readonly [name[=value] ...]

The given names are marked read-only and these names cannot be changed by subsequent assignment.

% return [n]

Causes a shell function to return to the invoking script with the return status specified by n. If n is omitted, the return status is that of the last command executed. Only the low 8 bits of n are passed back to the caller. If return is invoked while not in a function or executing a script by the . (dot) built-in command, it has the same effect as an exit command.

set [±aefhkmnopstuvx | ±o option] ... [ ±A name] [arg ...]

The following options are used for this command:

-A

Array assignment. Unset the variable name and assign values sequentially from the list arg. If +A is used, the variable name is not unset first.

-a

All subsequent defined parameters are automatically exported.

-e

If the shell is non-interactive and if a command fails, execute the ERR trap, if set, and exit immediately. This mode is disabled while reading profiles.

-f

Disables file name generation.

-h

Each command whose name is an identifier becomes a tracked alias when first encountered.

-k

All parameter assignment arguments (not just those that precede the command name) are placed in the environment for a command.

-m

Background jobs are run in a separate process group and a line is printed upon completion. The exit status of background jobs is reported in a completion message. This option is turned on automatically for interactive shells.

-n

Read commands and check them for syntax errors, but do not execute them. The -n option is ignored for interactive shells.

-o

The -o argument takes any of several option names, but only one option can be specified with each -o option. If none is supplied, the current option settings are printed. The -o argument option names follow:

allexport: Same as -a.
bgnice: All background jobs are run at a lower priority.
errexit: Same as -e.
emacs: Activates an emacs- style in-line editor for command entry.
gmacs: Activates a gmacs- style in-line editor for command entry.
ignoreeof: The shell does not exit on end-of-file. The command exit must be used.
keyword: Same as -k.
markdirs: All directory names resulting from file name generation have a trailing / appended.
monitor: Same as -m.
noclobber: Prevents redirection > from truncating existing regular files. Requires >| to truncate a file when turned on.
noexec: Same as -n.
noglob: Same as -f.
nolog: Do not save function definitions in history file.
nounset: Same as -u.
privileged: Same as -p.
verbose: Same as -v.
trackall: Same as -h.
vi: Activates the insert mode of a vi- style in-line editor until you press the ESC key which puts you in move mode. A return sends the line.
viraw: Each character is processed as it is typed in vi mode.
xtrace: Same as -x.

-p

Disables processing of the $HOME/.profile file and uses the file /etc/suid_profile instead of the ENV file. This mode is on whenever the effective uid (gid) is not equal to the real uid (gid). Turning this off causes the effective uid and gid to be set to the real uid and gid.

-s

Sort the positional parameters.

-t

Exit after reading and executing one command.

-u

Treat unset parameters as an error when substituting.

-v

Print shell input lines as they are read.

-x

Print commands and their arguments as they are executed.

-

Turns off -x and -v options and stops examining arguments for options.