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258 lines
12 KiB
258 lines
12 KiB
// Copyright 2012 Jesse van den Kieboom. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package flags provides an extensive command line option parser.
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The flags package is similar in functionality to the go built-in flag package
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but provides more options and uses reflection to provide a convenient and
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succinct way of specifying command line options.
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Supported features
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The following features are supported in go-flags:
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Options with short names (-v)
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Options with long names (--verbose)
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Options with and without arguments (bool v.s. other type)
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Options with optional arguments and default values
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Option default values from ENVIRONMENT_VARIABLES, including slice and map values
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Multiple option groups each containing a set of options
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Generate and print well-formatted help message
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Passing remaining command line arguments after -- (optional)
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Ignoring unknown command line options (optional)
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Supports -I/usr/include -I=/usr/include -I /usr/include option argument specification
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Supports multiple short options -aux
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Supports all primitive go types (string, int{8..64}, uint{8..64}, float)
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Supports same option multiple times (can store in slice or last option counts)
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Supports maps
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Supports function callbacks
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Supports namespaces for (nested) option groups
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Additional features specific to Windows:
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Options with short names (/v)
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Options with long names (/verbose)
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Windows-style options with arguments use a colon as the delimiter
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Modify generated help message with Windows-style / options
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Windows style options can be disabled at build time using the "forceposix"
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build tag
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Basic usage
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The flags package uses structs, reflection and struct field tags
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to allow users to specify command line options. This results in very simple
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and concise specification of your application options. For example:
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type Options struct {
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Verbose []bool `short:"v" long:"verbose" description:"Show verbose debug information"`
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}
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This specifies one option with a short name -v and a long name --verbose.
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When either -v or --verbose is found on the command line, a 'true' value
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will be appended to the Verbose field. e.g. when specifying -vvv, the
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resulting value of Verbose will be {[true, true, true]}.
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Slice options work exactly the same as primitive type options, except that
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whenever the option is encountered, a value is appended to the slice.
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Map options from string to primitive type are also supported. On the command
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line, you specify the value for such an option as key:value. For example
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type Options struct {
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AuthorInfo string[string] `short:"a"`
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}
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Then, the AuthorInfo map can be filled with something like
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-a name:Jesse -a "surname:van den Kieboom".
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Finally, for full control over the conversion between command line argument
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values and options, user defined types can choose to implement the Marshaler
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and Unmarshaler interfaces.
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Available field tags
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The following is a list of tags for struct fields supported by go-flags:
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short: the short name of the option (single character)
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long: the long name of the option
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required: if non empty, makes the option required to appear on the command
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line. If a required option is not present, the parser will
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return ErrRequired (optional)
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description: the description of the option (optional)
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long-description: the long description of the option. Currently only
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displayed in generated man pages (optional)
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no-flag: if non-empty, this field is ignored as an option (optional)
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optional: if non-empty, makes the argument of the option optional. When an
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argument is optional it can only be specified using
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--option=argument (optional)
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optional-value: the value of an optional option when the option occurs
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without an argument. This tag can be specified multiple
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times in the case of maps or slices (optional)
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default: the default value of an option. This tag can be specified
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multiple times in the case of slices or maps (optional)
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default-mask: when specified, this value will be displayed in the help
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instead of the actual default value. This is useful
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mostly for hiding otherwise sensitive information from
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showing up in the help. If default-mask takes the special
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value "-", then no default value will be shown at all
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(optional)
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env: the default value of the option is overridden from the
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specified environment variable, if one has been defined.
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(optional)
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env-delim: the 'env' default value from environment is split into
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multiple values with the given delimiter string, use with
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slices and maps (optional)
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value-name: the name of the argument value (to be shown in the help)
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(optional)
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choice: limits the values for an option to a set of values.
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This tag can be specified multiple times (optional)
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hidden: if non-empty, the option is not visible in the help or man page.
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base: a base (radix) used to convert strings to integer values, the
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default base is 10 (i.e. decimal) (optional)
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ini-name: the explicit ini option name (optional)
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no-ini: if non-empty this field is ignored as an ini option
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(optional)
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group: when specified on a struct field, makes the struct
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field a separate group with the given name (optional)
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namespace: when specified on a group struct field, the namespace
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gets prepended to every option's long name and
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subgroup's namespace of this group, separated by
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the parser's namespace delimiter (optional)
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command: when specified on a struct field, makes the struct
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field a (sub)command with the given name (optional)
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subcommands-optional: when specified on a command struct field, makes
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any subcommands of that command optional (optional)
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alias: when specified on a command struct field, adds the
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specified name as an alias for the command. Can be
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be specified multiple times to add more than one
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alias (optional)
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positional-args: when specified on a field with a struct type,
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uses the fields of that struct to parse remaining
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positional command line arguments into (in order
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of the fields). If a field has a slice type,
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then all remaining arguments will be added to it.
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Positional arguments are optional by default,
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unless the "required" tag is specified together
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with the "positional-args" tag. The "required" tag
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can also be set on the individual rest argument
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fields, to require only the first N positional
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arguments. If the "required" tag is set on the
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rest arguments slice, then its value determines
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the minimum amount of rest arguments that needs to
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be provided (e.g. `required:"2"`) (optional)
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positional-arg-name: used on a field in a positional argument struct; name
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of the positional argument placeholder to be shown in
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the help (optional)
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Either the `short:` tag or the `long:` must be specified to make the field eligible as an
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option.
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Option groups
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Option groups are a simple way to semantically separate your options. All
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options in a particular group are shown together in the help under the name
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of the group. Namespaces can be used to specify option long names more
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precisely and emphasize the options affiliation to their group.
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There are currently three ways to specify option groups.
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1. Use NewNamedParser specifying the various option groups.
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2. Use AddGroup to add a group to an existing parser.
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3. Add a struct field to the top-level options annotated with the
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group:"group-name" tag.
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Commands
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The flags package also has basic support for commands. Commands are often
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used in monolithic applications that support various commands or actions.
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Take git for example, all of the add, commit, checkout, etc. are called
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commands. Using commands you can easily separate multiple functions of your
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application.
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There are currently two ways to specify a command.
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1. Use AddCommand on an existing parser.
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2. Add a struct field to your options struct annotated with the
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command:"command-name" tag.
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The most common, idiomatic way to implement commands is to define a global
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parser instance and implement each command in a separate file. These
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command files should define a go init function which calls AddCommand on
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the global parser.
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When parsing ends and there is an active command and that command implements
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the Commander interface, then its Execute method will be run with the
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remaining command line arguments.
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Command structs can have options which become valid to parse after the
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command has been specified on the command line, in addition to the options
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of all the parent commands. I.e. considering a -v flag on the parser and an
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add command, the following are equivalent:
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./app -v add
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./app add -v
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However, if the -v flag is defined on the add command, then the first of
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the two examples above would fail since the -v flag is not defined before
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the add command.
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Completion
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go-flags has builtin support to provide bash completion of flags, commands
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and argument values. To use completion, the binary which uses go-flags
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can be invoked in a special environment to list completion of the current
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command line argument. It should be noted that this `executes` your application,
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and it is up to the user to make sure there are no negative side effects (for
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example from init functions).
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Setting the environment variable `GO_FLAGS_COMPLETION=1` enables completion
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by replacing the argument parsing routine with the completion routine which
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outputs completions for the passed arguments. The basic invocation to
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complete a set of arguments is therefore:
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GO_FLAGS_COMPLETION=1 ./completion-example arg1 arg2 arg3
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where `completion-example` is the binary, `arg1` and `arg2` are
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the current arguments, and `arg3` (the last argument) is the argument
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to be completed. If the GO_FLAGS_COMPLETION is set to "verbose", then
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descriptions of possible completion items will also be shown, if there
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are more than 1 completion items.
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To use this with bash completion, a simple file can be written which
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calls the binary which supports go-flags completion:
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_completion_example() {
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# All arguments except the first one
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args=("${COMP_WORDS[@]:1:$COMP_CWORD}")
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# Only split on newlines
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local IFS=$'\n'
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# Call completion (note that the first element of COMP_WORDS is
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# the executable itself)
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COMPREPLY=($(GO_FLAGS_COMPLETION=1 ${COMP_WORDS[0]} "${args[@]}"))
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return 0
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}
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complete -F _completion_example completion-example
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Completion requires the parser option PassDoubleDash and is therefore enforced if the environment variable GO_FLAGS_COMPLETION is set.
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Customized completion for argument values is supported by implementing
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the flags.Completer interface for the argument value type. An example
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of a type which does so is the flags.Filename type, an alias of string
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allowing simple filename completion. A slice or array argument value
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whose element type implements flags.Completer will also be completed.
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*/
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package flags
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