You don’t have write permissions for the /usr/bin directory.といわれたら
macOSにおいてgemでいろんなユーティリティソフトをインストールする際に出くわすことがあるらしい。
解決策はオプション -n /usr/local/bin を付けよ、というものであるが、この機会にgem installについて調べておく。
gem installのオプション
まずこれを知るためにはHelpドキュメントを読まなくてはならない。特定のコマンドに関するヘルプを得るにはgem help commandとする。今回はinstallなので
$ gem help install
とすればよい。全文は後で記すとして-n, –bindir DIRは”Directory where executables are located”ということなので実行ファイルをどこに置くかを指定するためのオプションだった。
他の使えそうなオプション
全文の中にある（installコマンドの）オプションからいくつか使えそうなものを持ってきた。
-v, –version VERSION：特定のバージョンのGemをインストールする
-i, –install-dir DIR：Gemのリポジトリをインストールするディレクトリを指定する
–[no-]prerelease：プリリリースのGemを入れたり、入れなかったりを指定する
共通オプションの中にも
-h, –help：このコマンドに対するヘルプを取得。gem help commandと同じ
-V, –[no-]verbose：出力を饒舌にするか、否か
-q, –quiet：コマンド進行のメーターを出さない
–silent：RubyGems出力を出さない
–config-file FILE：コンフィグファイルを使う（複数台で同じ事をやるには良さそう）
–backtrace：エラーのバックトレースを見せる
という便利そうなものがある。
gem installのヘルプドキュメント全文
$ gem help install
Usage: gem install GEMNAME [GEMNAME ...] [options] -- --build-flags [options]
Options:
--platform PLATFORM Specify the platform of gem to install
-v, --version VERSION Specify version of gem to install
--[no-]prerelease Allow prerelease versions of a gem
to be installed. (Only for listed gems)
Deprecated Options:
--[no-]rdoc Generate RDoc for installed gems
Use --document instead
--[no-]ri Generate ri data for installed gems.
Use --document instead
-u, --[no-]update-sources Update local source cache
Install/Update Options:
-i, --install-dir DIR Gem repository directory to get installed
gems
-n, --bindir DIR Directory where executables are
located
--[no-]document [TYPES] Generate documentation for installed gems
List the documentation types you wish to
generate. For example: rdoc,ri
--build-root DIR Temporary installation root. Useful for building
packages. Do not use this when installing remote gems.
--vendor Install gem into the vendor directory.
Only for use by gem repackagers.
-N, --no-document Disable documentation generation
-E, --[no-]env-shebang Rewrite the shebang line on installed
scripts to use /usr/bin/env
-f, --[no-]force Force gem to install, bypassing dependency
checks
-w, --[no-]wrappers Use bin wrappers for executables
Not available on dosish platforms
-P, --trust-policy POLICY Specify gem trust policy
--ignore-dependencies Do not install any required dependent gems
--[no-]format-executable Make installed executable names match Ruby.
If Ruby is ruby18, foo_exec will be
foo_exec18
--[no-]user-install Install in user's home directory instead
of GEM_HOME.
--development Install additional development
dependencies
--development-all Install development dependencies for all
gems (including dev deps themselves)
--conservative Don't attempt to upgrade gems already
meeting version requirement
--minimal-deps Don't upgrade any dependencies that already
meet version requirements
--[no-]post-install-message Print post install message
-g, --file [FILE] Read from a gem dependencies API file and
install the listed gems
--without GROUPS Omit the named groups (comma separated)
when installing from a gem dependencies
file
--default Add the gem's full specification to
specifications/default and extract only its bin
--explain Rather than install the gems, indicate which would
be installed
--[no-]lock Create a lock file (when used with -g/--file)
--[no-]suggestions Suggest alternates when gems are not found
Local/Remote Options:
-l, --local Restrict operations to the LOCAL domain
-r, --remote Restrict operations to the REMOTE domain
-b, --both Allow LOCAL and REMOTE operations
-B, --bulk-threshold COUNT Threshold for switching to bulk
synchronization (default 1000)
--clear-sources Clear the gem sources
-s, --source URL Append URL to list of remote gem sources
-p, --[no-]http-proxy [URL] Use HTTP proxy for remote operations
Common Options:
-h, --help Get help on this command
-V, --[no-]verbose Set the verbose level of output
-q, --quiet Silence command progress meter
--silent Silence RubyGems output
--config-file FILE Use this config file instead of default
--backtrace Show stack backtrace on errors
--debug Turn on Ruby debugging
--norc Avoid loading any .gemrc file
Arguments:
GEMNAME name of gem to install
Summary:
Install a gem into the local repository
Description:
The install command installs local or remote gem into a gem repository.
For gems with executables ruby installs a wrapper file into the executable
directory by default. This can be overridden with the --no-wrappers option.
The wrapper allows you to choose among alternate gem versions using
_version_.
For example `rake _0.7.3_ --version` will run rake version 0.7.3 if a newer
version is also installed.
Gem Dependency Files
====================
RubyGems can install a consistent set of gems across multiple environments
using `gem install -g` when a gem dependencies file (gem.deps.rb, Gemfile or
Isolate) is present. If no explicit file is given RubyGems attempts to find
one in the current directory.
When the RUBYGEMS_GEMDEPS environment variable is set to a gem dependencies
file the gems from that file will be activated at startup time. Set it to a
specific filename or to "-" to have RubyGems automatically discover the gem
dependencies file by walking up from the current directory.
NOTE: Enabling automatic discovery on multiuser systems can lead to
execution of arbitrary code when used from directories outside your control.
Extension Install Failures
==========================
If an extension fails to compile during gem installation the gem
specification is not written out, but the gem remains unpacked in the
repository. You may need to specify the path to the library's headers and
libraries to continue. You can do this by adding a -- between RubyGems'
options and the extension's build options:
$ gem install some_extension_gem
[build fails]
Gem files will remain installed in \
/path/to/gems/some_extension_gem-1.0 for inspection.
Results logged to /path/to/gems/some_extension_gem-1.0/gem_make.out
$ gem install some_extension_gem -- --with-extension-lib=/path/to/lib
[build succeeds]
$ gem list some_extension_gem
*** LOCAL GEMS ***
some_extension_gem (1.0)
$
If you correct the compilation errors by editing the gem files you will need
to write the specification by hand. For example:
$ gem install some_extension_gem
[build fails]
Gem files will remain installed in \
/path/to/gems/some_extension_gem-1.0 for inspection.
Results logged to /path/to/gems/some_extension_gem-1.0/gem_make.out
$ [cd /path/to/gems/some_extension_gem-1.0]
$ [edit files or what-have-you and run make]
$ gem spec ../../cache/some_extension_gem-1.0.gem --ruby > \
../../specifications/some_extension_gem-1.0.gemspec
$ gem list some_extension_gem
*** LOCAL GEMS ***
some_extension_gem (1.0)
$
Command Alias
==========================
You can use `i` command instead of `install`.
$ gem i GEMNAME
Defaults:
--both --version '>= 0' --document --no-force
--install-dir /Library/Ruby/Gems/2.3.0 --lock