---

   set.1p    ( 1 )

---

Обоснование (Rationale)

The set -- form is listed specifically in the SYNOPSIS even though this usage is implied by the Utility Syntax Guidelines. The explanation of this feature removes any ambiguity about whether the set -- form might be misinterpreted as being equivalent to set without any options or arguments. The functionality of this form has been adopted from the KornShell. In System V, set -- only unsets parameters if there is at least one argument; the only way to unset all parameters is to use shift. Using the KornShell version should not affect System V scripts because there should be no reason to issue it without arguments deliberately; if it were issued as, for example:

set -- "$@"

and there were in fact no arguments resulting from "$@", unsetting the parameters would have no result.

The set + form in early proposals was omitted as being an unnecessary duplication of set alone and not widespread historical practice.

The noclobber option was changed to allow set -C as well as the set -o noclobber option. The single-letter version was added so that the historical "$-" paradigm would not be broken; see Section 2.5.2, Special Parameters.

The description of the -e option is intended to match the behavior of the 1988 version of the KornShell.

The -h flag is related to command name hashing. See hash(1p).

The following set flags were omitted intentionally with the following rationale:

-k The -k flag was originally added by the author of the Bourne shell to make it easier for users of pre-release versions of the shell. In early versions of the Bourne shell the construct set name=value had to be used to assign values to shell variables. The problem with -k is that the behavior affects parsing, virtually precluding writing any compilers. To explain the behavior of -k, it is necessary to describe the parsing algorithm, which is implementation- defined. For example:

set -k; echo name=value

and:

set -k echo name=value

behave differently. The interaction with functions is even more complex. What is more, the -k flag is never needed, since the command line could have been reordered.

-t The -t flag is hard to specify and almost never used. The only known use could be done with here-documents. Moreover, the behavior with ksh and sh differs. The reference page says that it exits after reading and executing one command. What is one command? If the input is date;date, sh executes both date commands while ksh does only the first.

Consideration was given to rewriting set to simplify its confusing syntax. A specific suggestion was that the unset utility should be used to unset options instead of using the non- getopt()-able +option syntax. However, the conclusion was reached that the historical practice of using +option was satisfactory and that there was no compelling reason to modify such widespread historical practice.

The -o option was adopted from the KornShell to address user needs. In addition to its generally friendly interface, -o is needed to provide the vi command line editing mode, for which historical practice yields no single-letter option name. (Although it might have been possible to invent such a letter, it was recognized that other editing modes would be developed and -o provides ample name space for describing such extensions.)

Historical implementations are inconsistent in the format used for -o option status reporting. The +o format without an option- argument was added to allow portable access to the options that can be saved and then later restored using, for instance, a dot script.

Historically, sh did trace the command set +x, but ksh did not.

The ignoreeof setting prevents accidental logouts when the end- of-file character (typically <control>‐D) is entered. A user shall explicitly exit to leave the interactive shell.

The set -m option was added to apply only to the UPE because it applies primarily to interactive use, not shell script applications.

The ability to do asynchronous notification became available in the 1988 version of the KornShell. To have it occur, the user had to issue the command:

trap "jobs -n" CLD

The C shell provides two different levels of an asynchronous notification capability. The environment variable notify is analogous to what is done in set -b or set -o notify. When set, it notifies the user immediately of background job completions. When unset, this capability is turned off.

The other notification ability comes through the built-in utility notify. The syntax is:

notify [%job ... ]

By issuing notify with no operands, it causes the C shell to notify the user asynchronously when the state of the current job changes. If given operands, notify asynchronously informs the user of changes in the states of the specified jobs.

To add asynchronous notification to the POSIX shell, neither the KornShell extensions to trap, nor the C shell notify environment variable seemed appropriate (notify is not a proper POSIX environment variable name).

The set -b option was selected as a compromise.

The notify built-in was considered to have more functionality than was required for simple asynchronous notification.

Historically, some shells applied the -u option to all parameters including $@ and $*. The standard developers felt that this was a misfeature since it is normal and common for $@ and $* to be used in shell scripts regardless of whether they were passed any arguments. Treating these uses as an error when no arguments are passed reduces the value of -u for its intended purpose of finding spelling mistakes in variable names and uses of unset positional parameters.