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SH(1) USER COMMANDS SH(1)
A blank is a tab or a space. An identifier is a sequence of letters, digits, or underscores starting with a letter or underscore. Identifiers are used as components of variable names. A vname is a sequence of one or more identifiers separated by a . and optionally preceded by a .. Vnames are used as function and variable names. A word is a sequence of characters from the character set defined by the current locale, excluding non-quoted metacharacters.
A command is 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. A built-in command is a command that is carried out by the shell itself without creating a separate process. Some commands are built-in purely for convenience and are not documented here. Built-ins that cause side effects in the shell environment and built-ins that are found before performing a path search (see Execution below) are documented here. For historical reasons, some of these built-ins behave differently than other built-ins and are called special built-ins.
A pipeline is a sequence of one or more commands separated by |. The standard output of each command but the last is connected by a pipe(2) to the standard input of the next command. Each command, except possibly the last, 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. Each pipeline can be preceded by the reserved word ! which causes the exit status of the pipeline to become 0 if the exit status of the last command is non-zero, and 1 if the exit status of the last command is 0.
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, which is lower than that of && and | |. The symbols && and | | also have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline; an ampersand (&) causes asynchronous execution of the preceding pipeline (i.e., the shell does not wait for that pipeline to finish). The symbol |& causes asynchronous execution of the preceding pipeline with a two-way pipe established to the parent shell; the standard input and output of the spawned pipeline can be written to and read from by the parent shell by applying the redirection operators <& and >& with arg p to commands and by using -p option of the built-in commands read and print described later. The symbol && ( | | ) causes the list following it to be executed only if the preceding pipeline returns a zero (non-zero) value. One or more new-lines may appear in a list instead of a semicolon, to delimit a command.
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.
The following reserved words are recognized as reserved only when they are the first word of a command and are not quoted:
Aliasing is performed when scripts are read, not while they are executed. Therefore, for an alias to take effect, the alias definition command has to be executed before the command which references the alias is read.
The following aliases are compiled into the shell but can be unset or redefined:
In addition, when expanding a variable assignment, tilde substitution is attempted when the value of the assignment begins with a ~, and when a ~ appears after a :. The : also terminates a ~ login name.
cuts fields 1 and 3 from the files file1 and file2 respectively, pastes the results together, and sends it to the processes process1 and process2, as well as putting it onto the standard output. Note that the file, which is passed as an argument to the command, is a UNIX pipe(2) so programs that expect to lseek(2) on the file will not work.
The shell supports both indexed and associative arrays. An element of an array variable is referenced by a subscript. A subscript for an indexed array is denoted by an arithmeticexpression (see ArithmeticEvaluation below) between a [ and a ]. To assign values to an indexed array, use set -A vname value . . . . The value of all subscripts must be in the range of 0 through 4095. Indexed arrays need not be declared. Any reference to a variable with a valid subscript is legal and an array will be created if necessary.
An associative array is created with the -A option to typeset. A subscript for an associative array is denoted by a string enclosed between [ and ].
Referencing any array without a subscript is equivalent to referencing the array with subscript 0.
The value of a variable may be assigned by writing:
or
Note that no space is allowed before or after the
=.
A nameref is a variable that is a reference to another variable. A nameref is created with the -n attribute of typeset. The value of the variable at the time of the typeset command becomes the variable that will be referenced whenever the nameref variable is used. The name of a nameref cannot contain a .. When a variable or function name contains a ., and the portion of the name up to the first . matches the name of a nameref, the variable referred to is obtained by replacing the nameref portion with the name of the variable referenced by the nameref. A nameref provides a convenient way to refer to the variable inside a function whose name is passed as an argument to a function. For example, if the name of a variable is passed as the first argument to a function, the command
inside the function causes references and assignments to
var
to be references and assignments to the variable whose
name has been passed to the function.
If either of the floating point attributes,
-E,
or
-F,
or the integer attribute,
-i,
is set for
vname,
then the
value
is subject to arithmetic evaluation as described below.
Positional parameters,
parameters denoted by a number,
may be assigned values with the
set
special built-in command.
Parameter
$0
is set from argument zero when the shell
is invoked.
The character $ is used to introduce substitutable parameters.
In the above, word is not evaluated unless it is to be used as the substituted string, so that, in the following example, pwd is executed only if d is not set or is null:
If the colon ( :) is omitted from the above expressions, then the shell only checks whether parameter is set or not.
The following parameters are automatically set by the shell:
The following variables are used by the shell:
The shell gives default values to PATH, PS1, PS2, PS3, PS4, MAILCHECK, FCEDIT, TMOUT and IFS, while HOME, SHELL, ENV, and MAIL are not set at all by the shell (although HOME is set by login(1)). On some systems MAIL and SHELL are also set by login(1).
A pattern-list is a list of one or more patterns separated from each other with a & or |. A & signifies that all patterns must be matched whereas | requires that only one pattern be matched. Composite patterns can be formed with one or more of the following sub-patterns:
Each sub-pattern in a composite pattern is numbered,
starting at 1, by the location of the ( within
the pattern.
The sequence
The special meaning of reserved words or aliases can be removed by quoting any character of the reserved word. The recognition of function names or built-in command names listed below cannot be altered by quoting them.
An arithmetic expression uses the same syntax, precedence, and associativity of expression as the C language. All the C language operators that apply to floating point quantities can be used. In addition, when the value of an arithmetic variable or sub-expression can be represented as a long integer, all C language integer arithmetic operations can be performed. Variables can be referenced by name within an arithmetic expression without using the parameter expansion syntax. When a variable is referenced, its value is evaluated as an arithmetic expression.
The following math library functions can be used with an arithmetic expression:
An internal representation of a variable as a double precision floating point can be specified with the -E [ n ] or -F [ n ] option of the typeset special built-in command. The -E option causes the expansion of the value to be represented using scientific notation when it is expanded. The optional option argument n defines the number of significant figures. The -F option causes the expansion to be represented as a floating decimal number when it is expanded. The optional option argument n defines the number of places after the decimal point in this case.
An internal integer representation of a variable can be specified with the -i [ n ] option of the typeset special built-in command. The optional option argument n specifies an arithmetic base to be used when expanding the variable. If you do not specify an arithmetic base, the first assignment to the variable determines the arithmetic base.
Arithmetic evaluation is performed on the value of each assignment to a variable with the -E, -F, or -i attribute. Assigning a floating point number to a variable whose type is an integer causes the fractional part to be truncated.
The following obsolete arithmetic comparisons are also permitted:
In each of the above expressions, if file is of the form /dev/fd/n, where n is an integer, then the test is applied to the open file whose descriptor number is n.
A compound expression can be constructed from these primitives by using any of the following, listed in decreasing order of precedence.
In each of the following redirections, if file is of the form /dev/tcp/host/port, or /dev/udp/host/port, where host is a hostname or host address, and port is an integer port number, then the redirection attempts to make a tcp or udp connection to the corresponding socket.
If one of the above is preceded by a digit, then the file descriptor number referred to is that specified by the digit (instead of the default 0 or 1). For example:
means file descriptor 2 is to be opened for writing as a duplicate of file descriptor 1.
The order in which redirections are specified is significant. The shell evaluates each redirection in terms of the (file descriptor, file) association at the time of evaluation. For example:
first associates file descriptor 1 with file fname . It then associates file descriptor 2 with the file associated with file descriptor 1 (i.e. fname ). If the order of redirections were reversed, file descriptor 2 would be associated with the terminal (assuming file descriptor 1 had been) and then file descriptor 1 would be associated with file fname .
If a command is followed by & and job control is not active, then 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.
The environment for any simple-command or function may be augmented by prefixing it with one or more variable assignments. A variable assignment argument is a word of the form identifier=value. Thus:
are equivalent (as far as the above execution of cmd is concerned except for special built-in commands listed below - those that are preceded with a dagger).
If the obsolete -k option is set, all variable assignment arguments are placed in the environment, even if they occur after the command name. The following first prints a=b c and then 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.
For historical reasons, there are two ways to define functions, the name( ) syntax and the function name syntax, described in the Commands section above. Shell functions are read in 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 defined by the function name syntax and called by name execute in the same process as the caller and share all files and present working directory with the caller. Traps caught by the caller are reset to their default action inside the function. A trap condition that is not caught or ignored by the function causes the function to terminate and the condition to be passed on to the caller. A trap on EXIT set inside a function is executed in the environment of the caller after the function completes. Ordinarily, variables are shared between the calling program and the function. However, the typeset special built-in command used within a function defines local variables whose scope includes the current function and all functions it calls. Errors within functions return control to the caller.
Functions defined with the name( ) syntax and functions defined with the function name syntax that are invoked with the . special built-in are executed in the caller's environment and share all variables and traps with the caller. Errors within these function executions cause the script that contains them to abort.
The special built-in command return is used to return from function calls.
Function names can be listed with the -f or +f option of the typeset special built-in command. The text of functions, when available, will also be listed with -f. Functions can be undefined with the -f option of the unset special built-in command.
Ordinarily, functions are unset when the shell executes a shell script. Functions that need to be defined across separate invocations of the shell should be placed in a directory and the FPATH variable should contain the name of this directory. They may also be specified in the ENV file.
The variable .sh.name contains the name of the variable for which the discipline function is called, .sh.subscript is the subscript of the variable, and .sh.value will contain the value being assigned inside the .set discipline function. For the set discipline, changing .sh.value will change the value that gets assigned.
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 which looks like:
[1] 1234
indicating that the job which was started asynchronously was job number 1 and had one (top-level) process, whose process id was 1234.
This paragraph and the next require features that are not in all versions of UNIX and may not apply. If you are running a job and wish to do something else you may hit the key ^Z (control-Z) which sends a STOP signal to the current job. The shell will then normally indicate that the job has been `Stopped', and print another prompt. You can then manipulate the state of this job, putting it in the background with the bg command, or run some other commands and then eventually bring the job back into the foreground with the foreground command fg. A ^Z takes effect immediately and is like an interrupt in that pending output and unread input are discarded when it is typed.
A job being run in the background will stop if it tries to read from the terminal. Background jobs are normally allowed to produce output, but this can be disabled by giving the command stty tostop. If you set this tty option, then background jobs will stop when they try to produce output like they do when they try to read input.
There are several ways to refer to jobs in the shell. A job can be referred to by the process id of any process of the job or by one of the following:
The shell learns immediately whenever a process changes state. It normally informs you whenever a job becomes blocked so that no further progress is possible, but only just before it prints a prompt. This is done so that it does not otherwise disturb your work. The notify option of the set command causes the shell to print these job change messages as soon as they occur.
When the monitor option is on, each background job that completes triggers any trap set for CHLD.
When you try to leave the shell while jobs are running or stopped, you will be warned that `You have stopped(running) jobs.' You may use the jobs command to see what they are. If you immediately try to exit again, the shell will not warn you a second time, and the stopped jobs will be terminated. When a login shell receives a HUP signal, it sends a HUP signal to each job that has not been disowned with the disown built-in command described below.
The shell variable PATH defines the search path for the directory containing the command. Alternative directory names are separated by a colon (:). The default path is /bin:/usr/bin: (specifying /bin, /usr/bin, and the current directory in that order). The current directory can be specified by two or more adjacent colons, or by a colon at the beginning or end of the path list. If the command name contains a /, then the search path is not used. Otherwise, each directory in the path is searched for an executable file that is not a directory. If the shell determines that there is a built-in version of a command corresponding to a given pathname, this built-in is invoked in the current process. A process is created and an attempt is made to execute the command via exec(2). If the file has execute permission but is not an a.out file, it is assumed to be a file containing shell commands. A separate shell is spawned to read it. All non-exported variables are removed in this case. If the shell command file doesn't have read permission, or if the setuid and/or setgid bits are set on the file, then the shell executes an agent whose job it is to set up the permissions and execute the shell with the shell command file passed down as an open file. A parenthesized command is executed in a sub-shell without removing non-exported variables.
The editing features require that the user's terminal accept `RETURN' as carriage return without line feed and that a space (` ') must overwrite the current character on the screen.
The editing modes implement a concept where the user is looking through a window at the current line. The window width is the value of COLUMNS if it is defined, otherwise 80. If the window width is too small to display the prompt and leave at least 8 columns to enter input, the prompt is truncated from the left. If the line is longer than the window width minus two, a mark is displayed at the end of the window to notify the user. As the cursor moves and reaches the window boundaries the window will be centered about the cursor. The mark is a > (<, *) if the line extends on the right (left, both) side(s) of the window.
The search commands in each edit mode provide access to the history file. Only strings are matched, not patterns, although a leading ^ in the string restricts the match to begin at the first character in the line.
Each of the edit modes has an operation to list the files or commands that match a partially entered word. When applied to the first word on the line, or the first word after a ;, |, &, or (, and the word does not begin with ~ or contain a /, the list of aliases, functions, and executable commands defined by the PATH variable that could match the partial word is displayed. Otherwise, the list of files that match the given word is displayed. If the partially entered word does not contain any file expansion characters, a * is appended before generating these lists. After displaying the generated list, the input line is redrawn. These operations are called command name listing and file name listing, respectively. There are additional operations, referred to as command name completion and file name completion, which compute the list of matching commands or files, but instead of printing the list, replace the current word with a complete or partial match. For file name completion, if the match is unique, a / is appended if the file is a directory and a space is appended if the file is not a directory. Otherwise, the longest common prefix for all the matching files replaces the word. For command name completion, only the portion of the file names after the last / are used to find the longest command prefix. If only a single name matches this prefix, then the word is replaced with the command name followed by a space.
The variable .sh.edcol is set to the input column number of the cursor at the time of the input. The variable .sh.edmode is set to ESC when in vi insert mode (see below) and is null otherwise. By prepending ${.sh.editmode} to a value assigned to .sh.edchar it will cause the shell to change to control mode if it is not already in this mode.
This trap is not invoked for characters entered as arguments to editing directives, or while reading input for a character search.
The notation for escape sequences is M- followed by a character. For example, M-f (pronounced Meta f) is entered by depressing ESC (ascii 033) followed by `f'. (M-F would be the notation for ESC followed by `SHIFT' (capital) `F'.)
All edit commands operate from any place on the line (not just at the beginning). Neither the `RETURN' nor the `LINE FEED' key is entered after edit commands except when noted.
When in vi mode on most systems, canonical processing is initially enabled and the command will be echoed again if the speed is 1200 baud or greater and it contains any control characters or less than one second has elapsed since the prompt was printed. The ESC character terminates canonical processing for the remainder of the command and the user can then modify the command line. This scheme has the advantages of canonical processing with the type-ahead echoing of raw mode.
If the option viraw is also set, the terminal will always have canonical processing disabled. This mode is implicit for systems that do not support two alternate end of line delimiters, and may be helpful for certain terminals.
The -R option will print 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 unit on which the output will be placed. The default is 1. If the option -n is used, no new-line is added to the output.
As an obsolete feature,
if the first
arg
is
-
then the
-x
and
-v
options are turned off and the next
arg
is treated as the first argument.
Using
+
rather than
-
causes these options to be turned off.
These options can also be used upon invocation of the shell.
The current set of options may be found in
$-.
Unless
-A
is specified,
the remaining arguments are positional
parameters and are assigned, in order, to
$1
$2
The -i attribute cannot be specified along with -R, -L, -Z, or -f.
Using + rather than - causes these options to be turned off. If no vname arguments are given, a list of vnames (and optionally the values ) of the variables is printed. (Using + rather than - keeps the values from being printed.) The -p option causes typeset followed by the option letters to be printed before each name rather than the names of the options. If any option other than -p is given, only those variables which have all of the given options are printed. Otherwise, the vnames and attributes of all variables are printed.
If no option is given, -f is assumed.
The remaining options and arguments are described under the set command above. An optional - as the first argument is ignored.
changing directory (see
cd(1)),
setting or unsetting the value or attributes of
SHELL,
ENV,
or
PATH,
specifying path or
command names containing
/,
redirecting output
(>,
>|,
<>,
and
>>).
adding or deleting built-in commands.
The restrictions above are enforced after .profile and the ENV files are interpreted.
When a command to be executed is found to be a shell procedure, rsh invokes sh to execute it. Thus, it is possible to provide to the end-user shell procedures that have access to the full power of the standard shell, while imposing a limited menu of commands; this scheme assumes that the end-user does not have write and execute permissions in the same directory.
The net effect of these rules is that the writer of the .profile has complete control over user actions, by performing guaranteed setup actions and leaving the user in an appropriate directory (probably not the login directory).
The system administrator often sets up a directory of commands (e.g., /usr/rbin) that can be safely invoked by rsh.
Morris I. Bolsky and David G. Korn, The New KornShell Command and Programming Language, Prentice Hall, 1995.
POSIX - Part 2: Shell and Utilities, IEEE Std 1003.2-1992, ISO/IEC 9945-2, IEEE, 1993.
If a command is executed, and then a command with the same name is installed in a directory in the search path before the directory where the original command was found, the shell will continue to exec the original command. Use the -t option of the alias command to correct this situation.
Some very old shell scripts contain a ^ as a synonym for the pipe character |.
Using the hist built-in command within a compound command will cause the whole command to disappear from the history file.
The built-in command . file reads the whole file before any commands are executed. Therefore, alias and unalias commands in the file will not apply to any commands defined in the file.
Traps are not processed while a job is waiting for a foreground process. Thus, a trap on CHLD won't be executed until the foreground job terminates.
It is a good idea to leave a space after the comma operator in arithmetic expressions to prevent the comma from being interpreted as the decimal point character in certain locales.
March 06, 1997
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