Chapter 3. Using Cygwin

The Cygwin DLL supports both POSIX- and Win32-style paths. Directory delimiters may be either forward slashes or backslashes. Paths using backslashes or starting with a drive letter are always handled as Win32 paths. POSIX paths must only use forward slashes as delimiter, otherwise they are treated as Win32 paths and file access might fail in surprising ways.

POSIX operating systems (such as Linux) do not have the concept of drive letters. Instead, all absolute paths begin with a slash (instead of a drive letter such as "c:") and all file systems appear as subdirectories (for example, you might buy a new disk and make it be the /disk2 directory).

Because many programs written to run on UNIX systems assume the existence of a single unified POSIX file system structure, Cygwin maintains a special internal POSIX view of the Win32 file system that allows these programs to successfully run under Windows. Cygwin uses this mapping to translate from POSIX to Win32 paths as necessary.

The /etc/fstab file is used to map Win32 drives and network shares into Cygwin's internal POSIX directory tree. This is a similar concept to the typical UNIX fstab file. The mount points stored in /etc/fstab are globally set for all users. Sometimes there's a requirement to have user specific mount points. The Cygwin DLL supports user specific fstab files. These are stored in the directory /etc/fstab.d and the name of the file is the Cygwin username of the user, as it's created from the Windows account database or stored in the /etc/passwd file (see the section called “Mapping Windows accounts to POSIX accounts”). The structure of the user specific file is identical to the system-wide fstab file.

The file fstab contains descriptive information about the various file systems. fstab is only read by programs, and not written; it is the duty of the system administrator to properly create and maintain this file. Each filesystem is described on a separate line; fields on each line are separated by tabs or spaces. Lines starting with '#' are comments.

The first field describes the block special device or remote filesystem to be mounted. On Cygwin, this is the native Windows path which the mount point links in. As path separator you MUST use a slash. Usage of a backslash might lead to unexpected results. UNC paths (using slashes, not backslashes) are allowed. If the path contains spaces these can be escaped as '\040'.

The second field describes the mount point for the filesystem. If the name of the mount point contains spaces these can be escaped as '\040'.

The third field describes the type of the filesystem. Cygwin supports any string here, since the file system type is usually not evaluated. So it doesn't matter if you write FAT into this field even if the filesystem is NTFS. Cygwin figures out the filesystem type and its capabilities by itself.

The only two exceptions are the file system types cygdrive and usertemp. The cygdrive type is used to set the cygdrive prefix. For a description of the cygdrive prefix see the section called “The cygdrive path prefix”, for a description of the usertemp file system type see the section called “The usertemp file system type”

The fourth field describes the mount options associated with the filesystem. It is formatted as a comma separated list of options. It contains at least the type of mount (binary or text) plus any additional options appropriate to the filesystem type. The list of the options, including their meaning, follows.

  acl       - Cygwin uses the filesystem's access control lists (ACLs) to
              implement real POSIX permissions (default).  This flag only
	      affects filesystems supporting ACLs (NTFS, for instance) and
	      is ignored otherwise.
  auto      - Ignored.
  binary    - Files default to binary mode (default).
  bind      - Allows to remount part of the file hierarchy somewhere else.
              In contrast to other entries, the first field in the fstab
	      line specifies an absolute POSIX path.  This path is remounted
	      to the POSIX path specified as the second path.  The conversion
	      to a Win32 path is done on the fly.  Only the root path and
	      paths preceding the bind entry in the fstab file are used to
	      convert the POSIX path in the first field to an absolute Win32
	      path.  Note that symlinks are ignored while performing this path
	      conversion.
  cygexec   - Treat all files below mount point as cygwin executables.
  dos       - Always convert leading spaces and trailing dots and spaces to
	      characters in the UNICODE private use area.  This allows to use
	      broken filesystems which only allow DOS filenames, even if they
	      are not recognized as such by Cygwin.
  exec      - Treat all files below mount point as executable.
  ihash     - Always fake inode numbers rather than using the ones returned
	      by the filesystem.  This allows to use broken filesystems which
	      don't return unambiguous inode numbers, even if they are not
	      recognized as such by Cygwin.
  noacl     - Cygwin ignores filesystem ACLs and only fakes a subset of
	      permission bits based on the DOS readonly attribute.  This
	      behaviour is the default on FAT and FAT32.  The flag is
	      ignored on NFS filesystems.
  nosuid    - No suid files are allowed (currently unimplemented).
  notexec   - Treat all files below mount point as not executable.
  nouser    - Mount is a system-wide mount.
  override  - Force the override of an immutable mount point (currently "/").
  posix=0   - Switch off case sensitivity for paths under this mount point
	      (default for the cygdrive prefix).
  posix=1   - Switch on case sensitivity for paths under this mount point
	      (default for all other mount points).
  sparse    - Switch on support for sparse files.  This option only makes
              sense on NTFS and then only if you really need sparse files.
	      Cygwin does not try to create sparse files by default for
	      performance reasons.
  text      - Files default to CRLF text mode line endings.
  user      - Mount is a user mount.

While normally the execute permission bits are used to evaluate executability, this is not possible on filesystems which don't support permissions at all (like FAT/FAT32), or if ACLs are ignored on filesystems supporting them (see the aforementioned acl mount option). In these cases, the following heuristic is used to evaluate if a file is executable: Files ending in certain extensions (.exe, .com, .lnk) are assumed to be executable. Files whose first two characters are "#!", "MZ", or ":\n" are also considered to be executable. The exec option is used to instruct Cygwin that the mounted file is "executable". If the exec option is used with a directory then all files in the directory are executable. This option allows other files to be marked as executable and avoids the overhead of opening each file to check for "magic" bytes. The cygexec option is very similar to exec, but also prevents Cygwin from setting up commands and environment variables for a normal Windows program, adding another small performance gain. The opposite of these options is the notexec option, which means that no files should be marked as executable under that mount point.

A correct root directory is quite essential to the operation of Cygwin. A default root directory is evaluated at startup so a fstab entry for the root directory is not necessary. If it's wrong, nothing will work as expected. Therefore, the root directory evaluated by Cygwin itself is treated as an immutable mount point and can't be overridden in /etc/fstab... unless you think you really know what you're doing. In this case, use the override flag in the options field in the /etc/fstab file. Since this is a dangerous thing to do, do so at your own risk.

/usr/bin and /usr/lib are by default also automatic mount points generated by the Cygwin DLL similar to the way the root directory is evaluated. /usr/bin points to the directory the Cygwin DLL is installed in, /usr/lib is supposed to point to the /lib directory. This choice is safe and usually shouldn't be changed. An fstab entry for them is not required.

nouser mount points are not overridable by a later call to mount. Mount points given in /etc/fstab are by default nouser mount points, unless you specify the option user. This allows the administrator to set certain paths so that they are not overridable by users. In contrast, all mount points in the user specific fstab file are user mount points.

The fifth and sixth field are ignored. They are so far only specified to keep a Linux-like fstab file layout.

Note that you don't have to specify an fstab entry for the root dir, unless you want to have the root dir pointing to somewhere entirely different (hopefully you know what you're doing), or if you want to mount the root dir with special options (for instance, as text mount).

Example entries:

Whenever Cygwin generates a Win32 path from a POSIX one, it uses the longest matching prefix in the mount table. Thus, if C: is mounted as /c and also as /, then Cygwin would translate C:/foo/bar to /c/foo/bar. This translation is normally only used when trying to derive the POSIX equivalent current directory. Otherwise, the handling of MS-DOS filenames bypasses the mount table.

If you want to see the current set of mount points valid in your session, you can invoke the Cygwin tool mount without arguments:

  bash$ mount
  f:/cygwin/bin on /usr/bin type ntfs (binary,auto)
  f:/cygwin/lib on /usr/lib type ntfs (binary,auto)
  f:/cygwin on / type ntfs (binary,auto)
  e:/src on /usr/src type vfat (binary)
  c: on /cygdrive/c type ntfs (binary,posix=0,user,noumount,auto)
  e: on /cygdrive/e type vfat (binary,posix=0,user,noumount,auto)

You can also use the mount command to add new mount points, and the umount to delete them. However, since they are only stored in memory, these mount points will disappear as soon as your last Cygwin process ends. See mount(1) and umount(1) for more information.

Apart from the unified POSIX tree starting at the / directory, UNC pathnames starting with two slashes and a server name (//machine/share/...) are supported as well. They are handled as POSIX paths if only containing forward slashes. There's also a virtual directory // which allows to enumerate the fileservers known to the local machine with ls. Same goes for the UNC paths of the type //machine, which allow to enumerate the shares provided by the server machine. For often used UNC paths it makes sense to add them to the mount table (see the section called “The Cygwin Mount Table” so they are included in the unified POSIX path tree.

As already outlined in the section called “File Access”, you can access arbitary drives on your system by using the cygdrive path prefix. The default value for this prefix is /cygdrive, and a path to any drive can be constructed by using the cygdrive prefix and appending the drive letter as subdirectory, like this:

  bash$ ls -l /cygdrive/f/somedir

This lists the content of the directory F:\somedir.

The cygdrive prefix is a virtual directory under which all drives on a system are subsumed. The mount options of the cygdrive prefix is used for all file access through the cygdrive prefixed drives. For instance, assuming the cygdrive mount options are binary,posix=0, then any file /cygdrive/x/file will be opened in binary mode by default (mount option binary), and the case of the filename doesn't matter (mount option posix=0).

The cygdrive prefix flags are also used for all UNC paths starting with two slashes, unless they are accessed through a mount point. For instance, consider these /etc/fstab entries:

  //server/share /mysrv    ntfs     posix=1,acl   0 0
  none           /cygdrive cygdrive posix=0,noacl 0 0

Assume there's a file \\server\share\foo on the share. When accessing it as /mysrv/foo, then the flags posix=1,acl of the /mysrv mount point are used. When accessing it as //server/share/foo, then the flags for the cygdrive prefix, posix=0,noacl are used.

The cygdrive prefix may be changed in the fstab file as outlined above. Please note that you must not use the cygdrive prefix for any other mount point. For instance this:

  none /cygdrive cygdrive binary 0 0
  D:   /cygdrive/d somefs text 0 0

will not make file access using the /mnt/d path prefix suddenly using textmode. If you want to mount any drive explicitly in another mode than the cygdrive prefix, use a distinct path prefix:

  none /cygdrive cygdrive binary 0 0
  D:   /mnt/d somefs text 0 0

To simplify scripting, Cygwin also provides a /proc/cygdrive symlink, which allows to use a fixed path in scripts, even if the actual cygdrive prefix has been changed, or is different between different users. So, in scripts, conveniently use the /proc/cygdrive symlink to successfully access files independently from the current cygdrive prefix:

  $ mount -p
  Prefix              Type         Flags
  /mnt                user         binmode
  $ cat > x.sh <<EOF
  cd /proc/cygdrive/c/Windows/System32/Drivers/etc
  ls -l hosts
  EOF
  $ sh -c ./x.sh
  -rwxrwx---+ 1 SYSTEM SYSTEM 826 Sep  4 22:43 hosts

On Windows, the environment variable TEMP specifies the location of the temp folder. It serves the same purpose as the /tmp/ directory in Unix systems. In contrast to /tmp/, it is by default a different folder for every Windows user. By using the special purpose usertemp file system, that temp folder can be mapped to /tmp/. This is particularly useful in setups where the administrator wants to write-protect the entire Cygwin directory. The usertemp file system can be configured in /etc/fstab like this:

  none /tmp usertemp binary,posix=0 0 0

Symbolic links are supported by Windows only on NTFS and have a lot of quirks making them (almost) unusable in a POSIX context. POSIX applications are rightfully expecting to use symlinks and the symlink(2) system call, so Cygwin has worked around the Windows shortcomings.

Cygwin creates symbolic links potentially in multiple different ways.

All of the above symlink types are recognized and used as symlinks under all circumstances. However, if the default plain file symlink type is lacking its DOS SYSTEM bit, or if the shortcut file is lacking the DOS READONLY attribute, they are not recognized as symlink.

Apart from these types, there's also a Windows native type, so called directory junctions. They are recognized as symlink but never generated by Cygwin. Filesystem junctions on the other hand are not handled as symlinks, otherwise they would not be recognized as filesystem borders by commands like find -xdev.

Using native Win32 paths in Cygwin, while often possible, is generally inadvisable. Those paths circumvent all internal integrity checking and bypass the information given in the Cygwin mount table.

The following paths are treated as native Win32 paths by the Cygwin DLL (but not necessarily by Cygwin applications):

When accessing files using native Win32 paths as above, Cygwin uses a default setting for the mount flags. All paths using DOS notation will be treated as case insensitive, and permissions are just faked as if the underlying drive is a FAT drive. This also applies to NTFS and other filesystems which usually are capable of case sensitivity and storing permissions.

Special care must be taken if your application uses Win32 file API functions like CreateFile to access files using relative pathnames, or if your application uses functions like CreateProcess or ShellExecute to start other applications.

When a Cygwin application is started, the Windows idea of the current working directory (CWD) is not necessarily the same as the Cygwin CWD. There are a couple of restrictions in the Win32 API, which disallow certain directories as Win32 CWD:

As long as the Cygwin CWD is usable as Windows CWD, the Cygwin and Windows CWDs are in sync within a process. However, if the Cygwin process changes its working directory into one of the directories which are unusable as Windows CWD, we're in trouble. If the process uses the Win32 API to access a file using a relative pathname, the resulting absolute path would not match the expectations of the process. In the worst case, the wrong files are deleted.

To workaround this problem, Cygwin sets the Windows CWD to a special directory in this case. This special directory points to a virtual filesystem within the native NT namespace (\??\PIPE\). Since it's not a real filesystem, the deliberate effect is that a call to, for instance, CreateFile ("foo", ...); will fail, as long as the processes CWD doesn't work as Windows CWD.

So, in general, don't use the Win32 file API in Cygwin applications. If you really need to access files using the Win32 API, or if you really have to use CreateProcess to start applications, rather than the POSIX exec(3) family of functions, you have to make sure that the Cygwin CWD is set to some directory which is valid as Win32 CWD.

The cygpath program provides the ability to translate between Win32 and POSIX pathnames in shell scripts. See cygpath(1) for the details.

The HOME, PATH, and LD_LIBRARY_PATH environment variables are automatically converted from Win32 format to POSIX format (e.g. from c:/cygwin\bin to /bin, if there was a mount from that Win32 path to that POSIX path) when a Cygwin process first starts.

Symbolic links can also be used to map Win32 pathnames to POSIX. For example, the command ln -s //pollux/home/joe/data /data would have about the same effect as creating a mount point from //pollux/home/joe/data to /data using mount, except that symbolic links cannot set the default file access mode. Other differences are that the mapping is distributed throughout the file system and proceeds by iteratively walking the directory tree instead of matching the longest prefix in a kernel table. Note that symbolic links will only work on network drives that are properly configured to support the "system" file attribute. Many do not do so by default (the Unix Samba server does not by default, for example).