The getopt module is a parser for command line options whose API isdesigned to be familiar to users of the C getopt() function. Users whoare unfamiliar with the C getopt() function or who would like to writeless code and get better help and error messages should consider using theargparse module instead.
Parses command line options and parameter list. args is the argument list tobe parsed, without the leading reference to the running program. Typically, thismeans sys.argv[1:]. shortopts is the string of option letters that thescript wants to recognize, with options that require an argument followed by acolon (':'; i.e., the same format that Unix getopt() uses).
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This function works like getopt(), except that GNU style scanning mode isused by default. This means that option and non-option arguments may beintermixed. The getopt() function stops processing options as soon as anon-option argument is encountered.
The variable optind is the index of the next element of the argv[] vector to be processed. It shall be initializedto 1 by the system, and getopt() shall update it when it finishes with each element of argv[]. If the applicationsets optind to zero before calling getopt(), the behavior is unspecified. When an element of argv[] containsmultiple option characters, it is unspecified how getopt() determines which options have already been processed.
The getopt() function shall return the next option character (if one is found) from argv that matches a characterin optstring, if there is one that matches. If the option takes an argument, getopt() shall set the variableoptarg to point to the option-argument as follows:
If the option was the last character in the string pointed to by an element of argv, then optarg shall contain thenext element of argv, and optind shall be incremented by 2. If the resulting value of optind is greater thanargc, this indicates a missing option-argument, and getopt() shall return an error indication.
If getopt() encounters an option character that is not contained in optstring, it shall return the ( '?' ) character. If it detects a missing option-argument, it shall return the character ( ':' ) if the first character of optstring was a , or a character ('?' ) otherwise. In either case, getopt() shall set the variable optopt to the option character that causedthe error. If the application has not set the variable opterr to 0 and the first character of optstring is not a, getopt() shall also print a diagnostic message to stderr in the format specified for the getopts utility, unless the stderr stream has wide orientation, in which case thebehavior is undefined.
If the application has not set the variable opterr to 0, the first character of optstring is not a ,and a write error occurs while getopt() is printing a diagnostic message to stderr, then the error indicator forstderr shall be set; but getopt() shall still succeed and the value of errno after getopt() isunspecified.
The getopt() function is only required to support option characters included in Utility Syntax Guideline 3. Manyhistorical implementations of getopt() support other characters as options. This is an allowed extension, but applicationsthat use extensions are not maximally portable. Note that support for multi-byte option characters is only possible when suchcharacters can be represented as type int.
Applications which use wide-character output functions with stderr should ensure that any calls to getopt() do notwrite to stderr, either by setting opterr to 0 or by ensuring the first character of optstring is always a.
While ferror(stderr) may be used to detect failures to write a diagnostic to stderr when getopt()returns '?', the value of errno is unspecified in such a condition. Applications desiring more control overhandling write failures should set opterr to 0 and independently perform output to stderr, rather than relying ongetopt() to do the output.
The description has been written to make it clear that getopt(), like the getopts utility, deals with option-arguments whether separated from the option by characters or not. Note that the requirements on getopt() and getopts are more stringent than the Utility Syntax Guidelines.
The special significance of a as the first character of optstring makes getopt() consistent with thegetopts utility. It allows an application to make a distinction between a missingargument and an incorrect option letter without having to examine the option letter. It is true that a missing argument can only bedetected in one case, but that is a case that has to be considered.
The .mw-parser-output .monospaced{font-family:monospace,monospace}getopt function was written to be a standard mechanism that all programs could use to parse command-line options so that there would be a common interface on which everyone could depend. As such, the original authors picked out of the variations support for single character options,multiple options specified together, and options with arguments (-a arg or -aarg), all controllable by an option string.
getopt dates back to at least 1980[1] and was first published by AT&T at the 1985 UNIFORUM conference in Dallas, Texas, with the intent for it to be available in the public domain.[2] Versions of it were subsequently picked up by other flavors of Unix (4.3BSD, Linux, etc.). It is specified in the POSIX.2 standard as part of the unistd.h header file. Derivatives of getopt have been created for many programming languages to parse command-line options.
The conventional (POSIX and BSD) handling is that the options end when the first non-option argument is encountered, and that getopt would return -1 to signal that. In the glibc extension, however, options are allowed anywhere for ease of use; getopt implicitly permutes the argument vector so it still leaves the non-options in the end. Since POSIX already has the convention of returning -1 on -- and skipping it, one can always portably use it as an end-of-options signifier.[4]
A GNU extension, getopt_long, allows parsing of more readable, multicharacter options, which are introduced by two dashes instead of one. The choice of two dashes allows multicharacter options (--inum) to be differentiated from single character options specified together (-abc). The GNU extension also allows an alternative format for options with arguments: --name=arg.[4] This interface proved popular, and has been taken up (sans the permution) by many BSD distributions including FreeBSD as well as Solaris.[5] An alternative way to support long options is seen in Solaris and Korn Shell (extending optstring), but it was not as popular.[6]
Another common advanced extension of getopt is resetting the state of argument parsing; this is useful as a replacement of the options-anyware GNU extension, or as a way to "layer" a set of command-line interface with different options at different levels. This is achieved in BSD systems using an optreset variable, and on GNU systems by setting optind to 0.[4]
getopt itself returns an integer that is either an option character or -1 for end-of-options.[10] The idiom is to use a while-loop to go through options, and to use a switch-case statement to pick and act on options. See the example section of this article.
The GNU extension getopt_long interface is similar, although it belongs to a different header file and takes an extra option for defining the "short" names of long options and some extra controls. If a short name is not defined, getopt will put an index referring to the option structure in the longindex pointer instead.[10]
The first attempt at porting was the program getopt, implemented by Unix System Laboratories (USL). This version was unable to deal with quoting and shell metacharacters, as it shows no attempts at quoting. It has been inherited to FreeBSD.[11]
In 1986, USL decided that being unsafe around metacharacters and whitespace was no longer acceptable, and they created the builtin getopts command for Unix SVR3 Bourne Shell instead. The advantage of building the command into the shell is that it now has access to the shell's variables, so values could be written safely without quoting. It uses the shell's own variables to track the position of current and argument positions, OPTIND and OPTARG, and returns the option name in a shell variable.
In 1995, getopts was included in the Single UNIX Specification version 1 / X/Open Portability Guidelines Issue 4.[12] Now a part of the POSIX Shell standard, getopts have spread far and wide in many other shells trying to be POSIX-compliant.
getopt was basically forgotten until util-linux came out with an enhanced version that fixed all of old getopt's problems by escaping. It also supports GNU's long option names.[13] On the other hand, long options have been implemented rarely in the getopts command in other shells, ksh93 being an exception.
getopt is a concise description of the common POSIX command argument structure, and it is replicated widely by programmers seeking to provide a similar interface, both to themselves and to the user on the command-line.
bar is not an option argument in the eyes of getopt. Rather, GNU getopt rearranges the positional arguments so that at the end of the processing, you have argv[3] being "hello" and argv[4] being "bar". Basically, when you're done getopting, you still have positional arguments [optind, argc) to process:
If you have GNU getopt() and do not have POSIXLY_CORRECT set, then it will process option arguments anywhere on the command line. In that case, you'd have one option, -r with the argument value foo, and two non-option arguments (hello and bar). ff782bc1db
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