fcposix - Online in the Cloud

PROGRAM:

NAME

fc — process the command history list

SYNOPSIS

fc [−r] [−e editor] [first [last]]

fc −l [−nr] [first [last]]

fc −s [old=new] [first]

DESCRIPTION

The fc utility shall list, or shall edit and re-execute, commands previously entered to an
interactive sh.

The command history list shall reference commands by number. The first number in the list
is selected arbitrarily. The relationship of a number to its command shall not change
except when the user logs in and no other process is accessing the list, at which time the
system may reset the numbering to start the oldest retained command at another number
(usually 1). When the number reaches an implementation-defined upper limit, which shall be
no smaller than the value in HISTSIZE or 32767 (whichever is greater), the shell may wrap
the numbers, starting the next command with a lower number (usually 1). However, despite
this optional wrapping of numbers, fc shall maintain the time-ordering sequence of the
commands. For example, if four commands in sequence are given the numbers 32766, 32767, 1
(wrapped), and 2 as they are executed, command 32767 is considered the command previous to
1, even though its number is higher.

When commands are edited (when the −l option is not specified), the resulting lines shall
be entered at the end of the history list and then re-executed by sh. The fc command that
caused the editing shall not be entered into the history list. If the editor returns a
non-zero exit status, this shall suppress the entry into the history list and the command
re-execution. Any command line variable assignments or redirection operators used with fc
shall affect both the fc command itself as well as the command that results; for example:

fc −s −− −1 2>/dev/null

reinvokes the previous command, suppressing standard error for both fc and the previous
command.

OPTIONS

The fc utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section 12.2,
Utility Syntax Guidelines.

The following options shall be supported:

−e editor Use the editor named by editor to edit the commands. The editor string is a
utility name, subject to search via the PATH variable (see the Base Definitions
volume of POSIX.1‐2008, Chapter 8, Environment Variables). The value in the
FCEDIT variable shall be used as a default when −e is not specified. If FCEDIT
is null or unset, ed shall be used as the editor.

−l (The letter ell.) List the commands rather than invoking an editor on them. The
commands shall be written in the sequence indicated by the first and last
operands, as affected by −r, with each command preceded by the command number.

−n Suppress command numbers when listing with −l.

−r Reverse the order of the commands listed (with −l) or edited (with neither −l
nor −s).

−s Re-execute the command without invoking an editor.

OPERANDS

The following operands shall be supported:

first, last
Select the commands to list or edit. The number of previous commands that can be
accessed shall be determined by the value of the HISTSIZE variable. The value of
first or last or both shall be one of the following:

[+]number A positive number representing a command number; command numbers can
be displayed with the −l option.

−number A negative decimal number representing the command that was executed
number of commands previously. For example, −1 is the immediately
previous command.

string A string indicating the most recently entered command that begins with
that string. If the old=new operand is not also specified with −s, the
string form of the first operand cannot contain an embedded <equals-
sign>.

When the synopsis form with −s is used:

* If first is omitted, the previous command shall be used.

For the synopsis forms without −s:

* If last is omitted, last shall default to the previous command when −l is
specified; otherwise, it shall default to first.

* If first and last are both omitted, the previous 16 commands shall be listed
or the previous single command shall be edited (based on the −l option).

* If first and last are both present, all of the commands from first to last
shall be edited (without −l) or listed (with −l). Editing multiple commands
shall be accomplished by presenting to the editor all of the commands at one
time, each command starting on a new line. If first represents a newer
command than last, the commands shall be listed or edited in reverse
sequence, equivalent to using −r. For example, the following commands on
the first line are equivalent to the corresponding commands on the second:

fc −r 10 20 fc 30 40
fc 20 10 fc −r 40 30

* When a range of commands is used, it shall not be an error to specify first
or last values that are not in the history list; fc shall substitute the
value representing the oldest or newest command in the list, as appropriate.
For example, if there are only ten commands in the history list, numbered 1
to 10:

fc −l
fc 1 99

shall list and edit, respectively, all ten commands.

old=new Replace the first occurrence of string old in the commands to be re-executed by
the string new.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of fc:

FCEDIT This variable, when expanded by the shell, shall determine the default value for
the −e editor option's editor option-argument. If FCEDIT is null or unset, ed
shall be used as the editor.

HISTFILE Determine a pathname naming a command history file. If the HISTFILE variable is
not set, the shell may attempt to access or create a file .sh_history in the
directory referred to by the HOME environment variable. If the shell cannot
obtain both read and write access to, or create, the history file, it shall use
an unspecified mechanism that allows the history to operate properly.
(References to history ``file'' in this section shall be understood to mean this
unspecified mechanism in such cases.) An implementation may choose to access
this variable only when initializing the history file; this initialization shall
occur when fc or sh first attempt to retrieve entries from, or add entries to,
the file, as the result of commands issued by the user, the file named by the
ENV variable, or implementation-defined system start-up files. In some
historical shells, the history file is initialized just after the ENV file has
been processed. Therefore, it is implementation-defined whether changes made to
HISTFILE after the history file has been initialized are effective.
Implementations may choose to disable the history list mechanism for users with
appropriate privileges who do not set HISTFILE; the specific circumstances under
which this occurs are implementation-defined. If more than one instance of the
shell is using the same history file, it is unspecified how updates to the
history file from those shells interact. As entries are deleted from the history
file, they shall be deleted oldest first. It is unspecified when history file
entries are physically removed from the history file.

HISTSIZE Determine a decimal number representing the limit to the number of previous
commands that are accessible. If this variable is unset, an unspecified default
greater than or equal to 128 shall be used. The maximum number of commands in
the history list is unspecified, but shall be at least 128. An implementation
may choose to access this variable only when initializing the history file, as
described under HISTFILE. Therefore, it is unspecified whether changes made to
HISTSIZE after the history file has been initialized are effective.

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.

LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments and input files).

LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.

NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS

Default.

STDOUT

When the −l option is used to list commands, the format of each command in the list shall
be as follows:

"%d\t%s\n", <line number>, <command>

If both the −l and −n options are specified, the format of each command shall be:

"\t%s\n", <command>

If the <command> consists of more than one line, the lines after the first shall be
displayed as:

"\t%s\n", <continued-command>

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

0 Successful completion of the listing.

>0 An error occurred.

Otherwise, the exit status shall be that of the commands executed by fc.

CONSEQUENCES OF ERRORS

Default.

The following sections are informative.

APPLICATION USAGE

Since editors sometimes use file descriptors as integral parts of their editing,
redirecting their file descriptors as part of the fc command can produce unexpected
results. For example, if vi is the FCEDIT editor, the command:

fc −s | more

does not work correctly on many systems.

Users on windowing systems may want to have separate history files for each window by
setting HISTFILE as follows:

HISTFILE=$HOME/.sh_hist$$

EXAMPLES

None.

RATIONALE

This utility is based on the fc built-in of the KornShell.

An early proposal specified the −e option as [−e editor [old= new ]], which is not
historical practice. Historical practice in fc of either [−e editor] or [−e − [ old= new
]] is acceptable, but not both together. To clarify this, a new option −s was introduced
replacing the [−e −]. This resolves the conflict and makes fc conform to the Utility
Syntax Guidelines.

HISTFILE Some implementations of the KornShell check for the superuser and do not create
a history file unless HISTFILE is set. This is done primarily to avoid creating
unlinked files in the root file system when logging in during single-user mode.
HISTFILE must be set for the superuser to have history.

HISTSIZE Needed to limit the size of history files. It is the intent of the standard
developers that when two shells share the same history file, commands that are
entered in one shell shall be accessible by the other shell. Because of the
difficulties of synchronization over a network, the exact nature of the
interaction is unspecified.

The initialization process for the history file can be dependent on the system start-up
files, in that they may contain commands that effectively preempt the settings the user
has for HISTFILE and HISTSIZE. For example, function definition commands are recorded in
the history file. If the system administrator includes function definitions in some system
start-up file called before the ENV file, the history file is initialized before the user
can influence its characteristics. In some historical shells, the history file is
initialized just after the ENV file has been processed. Because of these situations, the
text requires the initialization process to be implementation-defined.

Consideration was given to omitting the fc utility in favor of the command line editing
feature in sh. For example, in vi editing mode, typing "<ESC>v" is equivalent to:

EDITOR=vi fc

However, the fc utility allows the user the flexibility to edit multiple commands
simultaneously (such as fc 10 20) and to use editors other than those supported by sh for
command line editing.

In the KornShell, the alias r (``re-do'') is preset to fc −e − (equivalent to the POSIX fc
−s). This is probably an easier command name to remember than fc (``fix command''), but
it does not meet the Utility Syntax Guidelines. Renaming fc to hist or redo was
considered, but since this description closely matches historical KornShell practice
already, such a renaming was seen as gratuitous. Users are free to create aliases
whenever odd historical names such as fc, awk, cat, grep, or yacc are standardized by
POSIX.

Command numbers have no ordering effects; they are like serial numbers. The −r option and
−number operand address the sequence of command execution, regardless of serial numbers.
So, for example, if the command number wrapped back to 1 at some arbitrary point, there
would be no ambiguity associated with traversing the wrap point. For example, if the
command history were:

32766: echo 1
32767: echo 2
1: echo 3

the number −2 refers to command 32767 because it is the second previous command,
regardless of serial number.

FUTURE DIRECTIONS

None.

Use fcposix online using onworks.net services