cygpathコマンドは、Windows形式のパス表記とUNIX(msys)形式のパス表記を相互に変換するためのmsys2専用コマンド。
又、重要なWindowsシステムディレクトリの位置を、Windows形式あるいはUNIX形式のいずれかで取得することもできる。

$ cygpath (-d|-m|-u|-w|-t 種類) [-c ハンドル] [-f ファイル名] [オプション] パス名

$ cygpath [-ADHPSW] [-d|-m|-u-w]

出力形式の変換を行う際は、msys2のマウント情報に従ってディレクトリの読み替えを行う。なお、cygpathコマンドは指定したファイルが存在するか否かの確認はせず、与えられたファイル名やパスをオプションにしたがって変換するだけ。

$ cygpath -u "C:\work\t-kernel"
/c/work/t-kernel

$ cygpath -w "/c/work/t-kernel"
C:\work\t-kernel

$ cygpath -W
/c/Windows

$BD変数をUNIX形式に変換

BD := $(shell /usr/bin/cygpath -u $(BD))

$BD変数をUNIX形式に変換

BD=`cygpath -u $(BD)`

$ cygpath --version
cygpath (cygwin) 2.7.0
Path Conversion Utility
Copyright (C) 1998 - 2017 Cygwin Authors
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cygpath --help
Usage: cygpath (-d|-m|-u|-w|-t TYPE) [-f FILE] [OPTION]... NAME...
       cygpath [-c HANDLE]
       cygpath [-ADHOPSW]
       cygpath [-F ID]

Convert Unix and Windows format paths, or output system path information

Output type options:

  -d, --dos             print DOS (short) form of NAMEs (C:\PROGRA~1\)
  -m, --mixed           like --windows, but with regular slashes (C:/WINNT)
  -M, --mode            report on mode of file (binmode or textmode)
  -u, --unix            (default) print Unix form of NAMEs (/cygdrive/c/winnt)
  -w, --windows         print Windows form of NAMEs (C:\WINNT)
  -t, --type TYPE       print TYPE form: 'dos', 'mixed', 'unix', or 'windows'

Path conversion options:

  -a, --absolute        output absolute path
  -l, --long-name       print Windows long form of NAMEs (with -w, -m only)
  -p, --path            NAME is a PATH list (i.e., '/bin:/usr/bin')
  -U, --proc-cygdrive   Emit /proc/cygdrive path instead of cygdrive prefix
                        when converting Windows path to UNIX path.
  -s, --short-name      print DOS (short) form of NAMEs (with -w, -m only)
  -C, --codepage CP     print DOS, Windows, or mixed pathname in Windows
                        codepage CP.  CP can be a numeric codepage identifier,
                        or one of the reserved words ANSI, OEM, or UTF8.
                        If this option is missing, cygpath defaults to the
                        character set defined by the current locale.

System information:

  -A, --allusers        use `All Users' instead of current user for -D, -O, -P
  -D, --desktop         output `Desktop' directory and exit
  -H, --homeroot        output `Profiles' directory (home root) and exit
  -O, --mydocs          output `My Documents' directory and exit
  -P, --smprograms      output Start Menu `Programs' directory and exit
  -S, --sysdir          output system directory and exit
  -W, --windir          output `Windows' directory and exit
  -F, --folder ID       output special folder with numeric ID and exit

Other options:

  -f, --file FILE       read FILE for input; use - to read from STDIN
  -o, --option          read options from FILE as well (for use with --file)
  -c, --close HANDLE    close HANDLE (for use in captured process)
  -i, --ignore          ignore missing argument
  -h, --help            output usage information and exit
  -V, --version         output version information and exit

1. NTEmacs + cygwin or msys2 で exec-path-from-shell
2. cygpath の便利なオプション