Certain files in Cygwin's /etc directory are read by Cygwin before the mount table has been established. The list of files is /etc/fstab /etc/fstab.d/$USER /etc/passwd /etc/group These file are read using native Windows NT functions which have no notion of Cygwin symlinks or POSIX paths. For that reason there are a few requirements as far as /etc is concerned. To access these files, the Cygwin DLL evaluates it's own full Windows path, strips off the innermost directory component and adds "\etc". Let's assume the Cygwin DLL is installed as C:\cygwin\bin\cygwin1.dll. First the DLL name as well as the innermost directory (bin) is stripped off: C:\cygwin\. Then "etc" and the filename to look for is attached: C:\cygwin\etc\fstab. So the /etc directory must be parallel to the directory in which the cygwin1.dll exists and /etc must not be a Cygwin symlink pointing to another directory. Consequentially none of the files from the above list, including the directory /etc/fstab.d is allowed to be a Cygwin symlink either. However, native NTFS symlinks and reparse points are transparent when accessing the above files so all these files as well as /etc itself may be NTFS symlinks. Last but not least, make sure that these files are world-readable. Every process of any user account has to read these files potentially, so world-readability is essential. The only exception are the user specific files /etc/fstab.d/$USER, which only have to be readable by the $USER user account itself. Filenames invalid under Win32 are not necessarily invalid under Cygwin. There are a few rules which apply to Windows filenames. Most notably, DOS device names like AUX, COM1, LPT1 or PRN (to name a few) cannot be used as filename or extension in a native Win32 application. So filenames like prn.txt or foo.aux are invalid filenames for native Win32 applications. This restriction doesn't apply to Cygwin applications. Cygwin can create and access files with such names just fine. Just don't try to use these files with native Win32 applications. Some characters are disallowed in filenames on Windows filesystems. These forbidden characters are the ASCII control characters from ASCII value 1 to 31, plus the following characters which have a special meaning in the Win32 API: " * : < > ? | \ Cygwin can't fix this, but it has a method to workaround this restriction. All of the above characters, except for the backslash, are converted to special UNICODE characters in the range 0xf000 to 0xf0ff (the "Private use area") when creating or accessing files by adding 0xf000 to the forbidden characters' code points. The backslash has to be exempt from this conversion, because Cygwin accepts Win32 filenames including backslashes as path separators on input. Converting backslashes using the above method would make this impossible. Additionally Win32 filenames can't contain trailing dots and spaces for DOS backward compatibility. When trying to create files with trailing dots or spaces, all of them are removed before the file is created. This restriction only affects native Win32 applications. Cygwin applications can create and access files with trailing dots and spaces without problems. An exception from this rule are some network filesystems (NetApp, NWFS) which choke on these filenames. They return with an error like "No such file or directory" when trying to create such files. Cygwin recognizes these filesystems and works around this problem by applying the same rule as for the other forbidden characters. Leading spaces and trailing dots and spaces will be converted to UNICODE characters in the private use area. This behaviour can be switched on explicitely for a filesystem or a directory tree by using the mount option dos. Windows filesystems use Unicode encoded as UTF-16 to store filename information. If you don't use the UTF-8 character set (see ) then there's a chance that a filename is using one or more characters which have no representation in the character set you're using. In the default "C" locale, Cygwin creates filenames using the UTF-8 charset. This will always result in some valid filename by default, but again might impose problems when switching to a non-"C" or non-"UTF-8" charset. To avoid this scenario altogether, always use UTF-8 as the character set. If you don't want or can't use UTF-8 as character set for whatever reason, you will nevertheless be able to access the file. How does that work? When Cygwin converts the filename from UTF-16 to your character set, it recognizes characters which can't be converted. If that occurs, Cygwin replaces the non-convertible character with a special character sequence. The sequence starts with an ASCII CAN character (hex code 0x18, equivalent Control-X), followed by the UTF-8 representation of the character. The result is a filename containing some ugly looking characters. While it doesn't look nice, it is nice, because Cygwin knows how to convert this filename back to UTF-16. The filename will be converted using your usual character set. However, when Cygwin recognizes an ASCII CAN character, it skips over the ASCII CAN and handles the following bytes as a UTF-8 character. Thus, the filename is symmetrically converted back to UTF-16 and you can access the file. Please be aware that this method is not entirely foolproof. In some character set combinations it might not work for certain native characters. Only by using the UTF-8 charset you can avoid this problem safely. In the Win32 subsystem filenames are only case-preserved, but not case-sensitive. You can't access two files in the same directory which only differ by case, like Abc and aBc. While NTFS (and some remote filesystems) support case-sensitivity, the NT kernel does not support it by default. Rather, you have to tweak a registry setting and reboot. For that reason, case-sensitivity can not be supported by Cygwin, unless you change that registry value. If you really want case-sensitivity in Cygwin, you can switch it on by setting the registry value HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\kernel\obcaseinsensitive to 0 and reboot the machine. When installing Microsoft's Services For Unix (SFU), you're asked if you want to use case-sensitive filenames. If you answer "yes" at this point, the installer will change the aforementioned registry value to 0, too. So, if you have SFU installed, there's some chance that the registry value is already set to case sensitivity. After you set this registry value to 0, Cygwin will be case-sensitive by default on NTFS and NFS filesystems. However, there are limitations: while two programs Abc.exe and aBc.exe can be created and accessed like other files, starting applications is still case-insensitive due to Windows limitations and so the program you try to launch may not be the one actually started. Also, be aware that using two filenames which only differ by case might result in some weird interoperability issues with native Win32 applications. You're using case-sensitivity at your own risk. You have been warned! Even if you use case-sensitivity, it might be feasible to switch to case-insensitivity for certain paths for better interoperability with native Win32 applications (even if it's just Windows Explorer). You can do this on a per-mount point base, by using the "posix=0" mount option in /etc/fstab, or your /etc/fstab.d/$USER file. /cygdrive paths are case-insensitive by default. The reason is that the native Windows %PATH% environment variable is not always using the correct case for all paths in it. As a result, if you use case-sensitivity on the /cygdrive prefix, your shell might claim that it can't find Windows commands like attrib or net. To ease the pain, the /cygdrive path is case-insensitive by default and you have to use the "posix=1" setting explicitly in /etc/fstab or /etc/fstab.d/$USER to switch it to case-sensitivity, or you have to make sure that the native Win32 %PATH% environment variable is using the correct case for all paths throughout. Note that mount points as well as device names and virtual paths like /proc are always case-sensitive! The only exception are the subdirectories and filenames under /proc/registry, /proc/registry32 and /proc/registry64. Registry access is always case-insensitive. Read on for more information. Windows 10 1803 introduced a new feature: NTFS directories can be marked as case-sensitive, independently of the obcaseinsensitive registry key discussed in the previous section. This new per-directory case-sensitivity requires setting a flag in the NTFS filesystem header which is, unfortunately, undocumented. The result is that you have to activate Windows Subsystem for Linux (WSL), a feature available via Programs and Features -> Turn Windows features on or off. You only have to activate WSL, you don't have to install any actual Linux. After turning WSL on and performing the compulsory reboot, case-sensitive directories are activated. Of course, there's a drawback. While these case-sensitive directories work like charm on the local machine, there are massive interoperability problems when trying to access these directories from remote machines at the time of writing this. We opened a bug report for that at Microsoft's WSL issue tracker, if you're interested in the details. If you want case-sensitivity and need interoperability with remote machines, better stick to switching the kernel to case-sensitivity as outlined in With WSL activated and starting with Cygwin 3.0.0, Cygwin's mkdir system call automatically created all directories below the Cygwin installation directory as case-sensitive. With Cygwin 3.0.2, this feature had been disabled again for hopefully obvious reasons. However, you can still use Cygwin's new tool with the -C option to control case-sensitivity of directories on NTFS filesystems. Please keep in mind that switching off case-sensitivity on a directory has a condition attached to it: If the directory contains two files which only differ in case (e. g., foo and FOO), Windows refuses to convert the dir back to case-insensitive. First you have to fix the filename collision, i. e., you have to rename one of these files. While there is no need to create a POSIX /dev directory, the directory is automatically created as part of a Cygwin installation. It's existence is often a prerequisit to run certain applications which create symbolic links, fifos, or UNIX sockets in /dev. Also, the directories /dev/shm and /dev/mqueue are required to exist to use named POSIX semaphores, shared memory, and message queues, so a system without a real /dev directory is functionally crippled. Apart from that, Cygwin automatically simulates POSIX devices internally. The /dev directory is automagically populated with existing POSIX devices by Cygwin in a way comparable with a udev based virtual /dev directory under Linux. Cygwin supports the following character devices commonly found on POSIX systems: /dev/null /dev/zero /dev/full /dev/console Pseudo device name for the current console window of a session. Cygwin's /dev/console is not quite comparable with the console device on UNIX machines. /dev/cons0 Console sessions are numbered from /dev/cons0 upwards. /dev/cons1 Console device names are pseudo device names, only accessible ... from processes within this very console session. This is due to a restriction in Windows. /dev/tty The current controlling tty of a session. /dev/ptmx Pseudo tty master device. /dev/pty0 Pseudo ttys are numbered from /dev/pty0 upwards as they are /dev/pty1 requested. ... /dev/ttyS0 Serial communication devices. ttyS0 == Win32 COM1, /dev/ttyS1 ttyS1 == COM2, etc. ... /dev/pipe /dev/fifo /dev/kmsg Kernel message pipe, for usage with sys logger services. /dev/random Random number generator. /dev/urandom /dev/dsp Default sound device of the system. Cygwin also has several Windows-specific devices: /dev/com1 The serial ports, starting with COM1 which is the same as ttyS0. /dev/com2 Please use /dev/ttySx instead. ... /dev/conin Same as Windows CONIN$. /dev/conout Same as Windows CONOUT$. /dev/clipboard The Windows clipboard, text only /dev/windows The Windows message queue. Block devices are accessible by Cygwin processes using fixed POSIX device names. These POSIX device names are generated using a direct conversion from the POSIX namespace to the internal NT namespace. E.g. the first harddisk is the NT internal device \device\harddisk0\partition0 or the first partition on the third harddisk is \device\harddisk2\partition1. The first floppy in the system is \device\floppy0, the first CD-ROM is \device\cdrom0 and the first tape drive is \device\tape0. The mapping from physical device to the name of the device in the internal NT namespace can be found in various places. For hard disks and CD/DVD drives, the Windows "Disk Management" utility (part of the "Computer Management" console) shows that the mapping of "Disk 0" is \device\harddisk0. "CD-ROM 2" is \device\cdrom2. Another place to find this mapping is the "Device Management" console. Disks have a "Location" number, tapes have a "Tape Symbolic Name", etc. Unfortunately, the places where this information is found is not very well-defined. For external disks (USB-drives, CF-cards in a cardreader, etc) you can use Cygwin to show the mapping. /proc/partitions contains a list of raw drives known to Cygwin. The df command shows a list of drives and their respective sizes. If you match the information between /proc/partitions and the df output, you should be able to figure out which external drive corresponds to which raw disk device name. Apart from tape devices which are not block devices and are by default accessed directly, accessing mass storage devices raw is something you should only do if you know what you're doing and know how to handle the information. Writing to a raw mass storage device you should only do if you really know what you're doing and are aware of the fact that any mistake can destroy important information, for the device, and for you. So, please, handle this ability with care. You have been warned. Last but not least, the mapping from POSIX /dev namespace to internal NT namespace is as follows: POSIX device name Internal NT device name /dev/st0 \device\tape0, rewind /dev/nst0 \device\tape0, no-rewind /dev/st1 \device\tape1 /dev/nst1 \device\tape1 ... /dev/st15 /dev/nst15 /dev/fd0 \device\floppy0 /dev/fd1 \device\floppy1 ... /dev/fd15 /dev/sr0 \device\cdrom0 /dev/sr1 \device\cdrom1 ... /dev/sr15 /dev/scd0 \device\cdrom0 /dev/scd1 \device\cdrom1 ... /dev/scd15 /dev/sda \device\harddisk0\partition0 (whole disk) /dev/sda1 \device\harddisk0\partition1 (first partition) ... /dev/sda15 \device\harddisk0\partition15 (fifteenth partition) /dev/sdb \device\harddisk1\partition0 /dev/sdb1 \device\harddisk1\partition1 [up to] /dev/sddx \device\harddisk127\partition0 /dev/sddx1 \device\harddisk127\partition1 ... /dev/sddx15 \device\harddisk127\partition15 if you don't like these device names, feel free to create symbolic links as they are created on Linux systems for convenience: ln -s /dev/sr0 /dev/cdrom ln -s /dev/nst0 /dev/tape ... Win32 executable filenames end with .exe but the .exe need not be included in the command, so that traditional UNIX names can be used. However, for programs that end in .bat and .com, you cannot omit the extension. As a side effect, the ls filename gives information about filename.exe if filename.exe exists and filename does not. In the same situation the function call stat("filename",..) gives information about filename.exe. The two files can be distinguished by examining their inodes, as demonstrated below. bash$ ls * a a.exe b.exe bash$ ls -i a a.exe 445885548 a 435996602 a.exe bash$ ls -i b b.exe 432961010 b 432961010 b.exe If a shell script myprog and a program myprog.exe coexist in a directory, the shell script has precedence and is selected for execution of myprog. Note that this was quite the reverse up to Cygwin 1.5.19. It has been changed for consistency with the rest of Cygwin. The gcc compiler produces an executable named filename.exe when asked to produce filename. This allows many makefiles written for UNIX systems to work well under Cygwin. Cygwin, like Linux and other similar operating systems, supports the /proc virtual filesystem. The files in this directory are representations of various aspects of your system, for example the command cat /proc/cpuinfo displays information such as what model and speed processor you have. One unique aspect of the Cygwin /proc filesystem is /proc/registry, see next section. The Cygwin /proc is not as complete as the one in Linux, but it provides significant capabilities. The procps package contains several utilities that use it. 2020-11-24 Cygwin Project proc 5 2020-11-24 Cygwin Cygwin User's Manual proc process and system information pseudo-filesystem The proc filesystem is a pseudo-filesystem which provides an interface to Cygwin data structures. It is commonly mounted at /proc. Typically, it is mounted automatically by the system. Underneath /proc, there are the following general groups of files and subdirectories: /proc/[pid] subdirectories Each one of these subdirectories contains files and subdirectories exposing information about the process with the corresponding process id. The /proc/[pid] subdirectories are visible when iterating through /proc with readdir2 (and thus are visible when one uses ls1 to view the contents of /proc). /proc/self When a process accesses this magic symbolic link, it resolves to the process's own /proc/[pid] directory. /proc/[a-z]* Various other files and subdirectories under /proc expose system-wide information. All of the above are described in more detail below. The following list provides details of many of the files and directories under the /proc hierarchy. /proc/[pid] There is a numerical subdirectory for each running process; the subdirectory is named by the process id. Each /proc/[pid] subdirectory contains the pseudo-files and directories described below. The files inside each /proc/[pid] directory are normally owned by the effective user and effective group id of the process. /proc/[pid]/cmdline This read-only file holds the complete command line for the process, unless the process is a zombie. In the latter case, there is nothing in this file: that is, a read on this file will return 0 characters. The command-line arguments appear in this file as a set of strings followed by null bytes ('\0'). /proc/[pid]/ctty This read-only file holds the name of the console or control terminal device for the process, unless the process is detached from any terminal. In the latter case, there is only a newline in this file. /proc/[pid]/cwd This is a symbolic link to the current working directory of the process. To find out the current working directory of process 20, for instance, you can do this: $ cd /proc/20/cwd; /bin/pwd Note that the pwd command is often a shell built-in, and might not work properly. In bash1 , you may use pwd -P. /proc/[pid]/environ This read-only file contains the current environment that may have been changed by the currently executing program. The entries are separated by null bytes ('\0'), and there may be a null byte at the end. Thus, to print out the environment of process 1, you would do: $ cat -A /proc/1/environ If, after an execve2 , the process modifies its environment (e.g., by calling functions such as putenv3 or modifying the environ7 variable directly), this file will reflect those changes. That may not be the case on other systems such as Linux. /proc/[pid]/exe This file is a symbolic link containing the actual pathname of the executed command. This symbolic link can be dereferenced normally; attempting to open it will open the executable. You can even type /proc/[pid]/exe to run another copy of the same executable that is being run by process [pid]. /proc/[pid]/exe is a pointer to the binary which was executed, and appears as a symbolic link. /proc/[pid]/exename This read-only file contains the actual pathname of the executed command. /proc/[pid]/fd/ This is a subdirectory containing one entry for each file which the process has open, named by its file descriptor, and which is a symbolic link to the actual file. Thus, 0 is standard input, 1 standard output, 2 standard error, and so on. For file descriptors for pipes and sockets, the entries will be symbolic links whose content is the file type with the inode. A readlink2 call on this file returns a string in the format: type:[inode] For example, socket:[2248868] will be a socket and its inode is 2248868. Programs that take a filename as a command-line argument, but don't take input from standard input if no argument is supplied, and programs that write to a file named as a command-line argument, but don't send their output to standard output if no argument is supplied, can nevertheless be made to use standard input or standard output by using /proc/[pid]/fd files as command-line arguments. For example, assuming that -i is the flag designating an input file and -o is the flag designating an output file: $ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ... and you have a working filter. /proc/self/fd/N is approximately the same as /dev/fd/N in some Unix and Unix-like systems. Most Linux makedev scripts symbolically link /dev/fd to /proc/self/fd, in fact. Most systems provide symbolic links /dev/stdin, /dev/stdout, and /dev/stderr, which respectively link to the files 0, 1, and 2 in /proc/self/fd. Thus the example command above could be written as: $ foobar -i /dev/stdin -o /dev/stdout ... Note that for file descriptors referring to inodes (pipes and sockets, see above), those inodes still have permission bits and ownership information distinct from those of the /proc/[pid]/fd entry, and that the owner may differ from the user and group ids of the process. An unprivileged process may lack permissions to open them, as in this example: $ echo test | sudo -u nobody cat test $ echo test | sudo -u nobody cat /proc/self/fd/0 cat: /proc/self/fd/0: Permission denied File descriptor 0 refers to the pipe created by the shell and owned by that shell's user, which is not nobody, so cat does not have permission to create a new file descriptor to read from that inode, even though it can still read from its existing file descriptor 0. /proc/[pid]/gid This read-only file contains the primary group id for the process. /proc/[pid]/maps A file containing the currently mapped memory regions and their access permissions. See mmap2 for some further information about memory mappings. The format of the file is: address perms offset dev inode pathname 00010000-00020000 rw-s 00000000 0000:0000 0 [win heap 1 default shared] ... 00080000-00082000 rw-p 00000000 0000:0000 0 [win heap 0 default grow] 00082000-0009A000 ===p 00002000 0000:0000 0 [win heap 0 default grow] 000A0000-000A1000 rw-p 00000000 0000:0000 0 [win heap 2 grow] 000A1000-000BA000 ===p 00001000 0000:0000 0 [win heap 2 grow] 000C0000-000D9000 rw-p 00000000 0000:0000 0 [win heap 0 default grow] 000D9000-001C0000 ===p 00019000 0000:0000 0 [win heap 0 default grow] 00200000-00377000 ===p 00000000 0000:0000 0 00377000-00378000 rw-p 00177000 0000:0000 0 [peb] 00378000-0037A000 rw-p 00178000 0000:0000 0 [teb (tid 8844)] ... 00400000-005F9000 ===p 00000000 0000:0000 0 [stack (tid 8884)] 005F9000-005FC000 rw-g 001F9000 0000:0000 0 [stack (tid 8884)] 005FC000-00600000 rw-p 001FC000 0000:0000 0 [stack (tid 8884)] 00600000-006C7000 r--s 00000000 EE45:4341 281474976741117 /proc/cygdrive/c/Windows/System32/locale.nls ... 100400000-100401000 r--p 00000000 EE45:4341 281474978095037 /usr/bin/sh.exe 100401000-100413000 r-xp 00001000 EE45:4341 281474978095037 /usr/bin/sh.exe 100413000-100414000 rw-p 00013000 EE45:4341 281474978095037 /usr/bin/sh.exe ... 180010000-180020000 rw-s 00000000 0000:0000 0 [procinfo] 180020000-180029000 rw-s 00000000 0000:0000 0 [cygwin-user-shared] 180030000-18003C000 rw-s 00000000 0000:0000 0 [cygwin-shared] 180040000-180041000 r--p 00000000 EE45:4341 2251799814294868 /usr/bin/cygwin1.dll 180041000-18022D000 r-xp 00001000 EE45:4341 2251799814294868 /usr/bin/cygwin1.dll 18022D000-180231000 rwxp 001ED000 EE45:4341 2251799814294868 /usr/bin/cygwin1.dll 180231000-18026A000 rw-p 001F1000 EE45:4341 2251799814294868 /usr/bin/cygwin1.dll ... 800000000-800090000 rw-p 00000000 0000:0000 0 [heap] 800090000-820000000 ===p 00090000 0000:0000 0 [heap] 7FF4FDEB0000-7FF4FDEB5000 r--s 00000000 0000:0000 0 7FF4FDEB5000-7FF4FDFB0000 ===s 00005000 0000:0000 0 7FF4FDFB0000-7FF5FDFD0000 ===p 00000000 0000:0000 0 ... 7FFBEEAC0000-7FFBEEAC1000 r--p 00000000 EE45:4341 844424934724994 /proc/cygdrive/c/Windows/System32/kernel32.dll 7FFBEEAC1000-7FFBEEB36000 r-xp 00001000 EE45:4341 844424934724994 /proc/cygdrive/c/Windows/System32/kernel32.dll 7FFBEEB36000-7FFBEEB68000 r--p 00076000 EE45:4341 844424934724994 /proc/cygdrive/c/Windows/System32/kernel32.dll 7FFBEEB68000-7FFBEEB6A000 rw-p 000A8000 EE45:4341 844424934724994 /proc/cygdrive/c/Windows/System32/kernel32.dll 7FFBEEB6A000-7FFBEEB72000 r--p 000AA000 EE45:4341 844424934724994 /proc/cygdrive/c/Windows/System32/kernel32.dll ... The address field is the address space in the process that the mapping occupies. The perms field is a set of permissions: rread wwrite xexecute ===reserved sshared gguard pprivate The offset field is the offset into the file/whatever; dev is the device (major:minor); inode is the inode on that device. 0 indicates that no inode is associated with the memory region, as would be the case with BSS (uninitialized data). The pathname field will usually be the file that is backing the mapping. There are additional helpful pseudo-paths: [cygwin-shared] Global shared Cygwin process information. [cygwin-user-shared] Global shared Cygwin user information. [peb] Windows Process Environment Block. [procinfo] Cygwin process information. [shared-user-data] Shared user information. [heap] The process's heap. [stack] The initial process's (also known as the main thread's) stack. [stack (tid <tid>)] A thread's stack (where the <tid> is a thread id). [teb (tid <tid>)] Windows Thread Environment Block (where <tid> is a thread id). [win heap <n> default shared exec grow noserial debug] Windows extended heap (where <n> is a heap id) and the rest of the words are heap flags: default default heap flags shared shareable and mapped heap flags exec executable heap flag grow growable heap flag noserial do not serialize heap flag debug debugged heap flag If the pathname field is blank, this is an anonymous mapping as obtained via mmap2 . There is no easy way to coordinate this back to a process's source, short of running it through gdb1 , strace1 , or similar. pathname is shown unescaped except for newline characters, which are replaced with an octal escape sequence. As a result, it is not possible to determine whether the original pathname contained a newline character or the literal \e012 character sequence. If the mapping is file-backed and the file has been deleted, the string " (deleted)" is appended to the pathname. Note that this is ambiguous too. /proc/[pid]/mountinfo This file contains information about mount points in the process's mount namespace (see mount_namespaces7 ). It supplies various information (e.g., propagation state, root of mount for bind mounts, identifier for each mount and its parent) that is missing from the (older) /proc/[pid]/mounts file, and fixes various other problems with that file (e.g., nonextensibility, failure to distinguish per-mount versus per-superblock options). The file contains lines of the form: 36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue (1)(2)(3) (4) (5) (6) (?) (7) (8) (9) (10) The numbers in parentheses are labels for the descriptions below: (1) mount id: a unique id for the mount (may be reused after umount2 ). (2) parent id: the id of the parent mount (or of self for the root of this mount namespace's mount tree). (3) major:minor: the value of st_dev for files on this filesystem (see stat2 ). (4) root: the pathname of the directory in the filesystem which forms the root of this mount. (5) mount point: the pathname of the mount point relative to the process's root directory. (6) mount options: per-mount options (see mount2 ). (?) optional fields: zero or more fields of the form "tag[:value]"; see below. (7) separator: the end of the optional fields is marked by a single hyphen. (8) filesystem type: the filesystem type in the form "type[.subtype]". (9) mount source: filesystem-specific information or "none". (10) super options: per-superblock options (see mount2 ). /proc/[pid]/mounts This file lists all the filesystems currently mounted in the process's mount namespace (see mount_namespaces7 ). The format of this file is documented in fstab5 . /proc/[pid]/pgid This read-only file contains the process group id for the process. /proc/[pid]/ppid This read-only file contains the parent process id for the process. /proc/[pid]/root UNIX and Linux support the idea of a per-process root of the filesystem, set by the chroot2 system call. This file is a symbolic link that points to the process's root directory, and behaves in the same way as exe, and fd/*. /proc/[pid]/sid This read-only file contains the session id for the process. /proc/[pid]/stat Status information about the process. This is used by some implementations of ps1 . The fields, in order, with their proper scanf3 format specifiers, are listed below. (1) pid %d The process id. (2) comm %s The filename of the executable, in parentheses. This is visible whether or not the executable is swapped out. (3) state %c One of the following characters, indicating process state: R Runnable O Running S Sleeping in an interruptible wait D Waiting in uninterruptible disk sleep Z Zombie T Stopped (on a signal) or trace stopped (4) ppid %d The PID of the parent of this process. (5) pgrp %d The process group id of the process. (6) session %d The session id of the process. (7) tty_nr %d The controlling terminal of the process. (The minor device number is contained in the combination of bits 31 to 20 and 7 to 0; the major device number is in bits 15 to 8.) (8) tpgid %d The id of the foreground process group of the controlling terminal of the process. (9) flags %u The kernel flags word of the process. (10) minflt %lu The number of minor faults the process has made which have not required loading a memory page from disk. (11) cminflt %lu The number of minor faults that the process's waited-for children have made. (12) majflt %lu The number of major faults the process has made which have required loading a memory page from disk. (13) cmajflt %lu The number of major faults that the process's waited-for children have made. (14) utime %lu Amount of time that this process has been scheduled in user mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)). (15) stime %lu Amount of time that this process has been scheduled in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)). (16) cutime %ld Amount of time that this process's waited-for children have been scheduled in user mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)). (See also times2 ). (17) cstime %ld Amount of time that this process's waited-for children have been scheduled in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)). (18) priority %ld For processes running a real-time scheduling policy (policy below; see sched_setscheduler2 ), this is the negated scheduling priority, minus one; that is, a number in the range -2 to -100, corresponding to real-time priorities 1 to 99. For processes running under a non-real-time scheduling policy, this is the raw nice value ( setpriority2 ) as represented in the kernel. The kernel stores nice values as numbers in the range 0 (high) to 39 (low), corresponding to the user-visible nice range of -20 to 19. (19) nice %ld The nice value (see setpriority2 ), a value in the range 19 (low priority) to -20 (high priority). (20) num_threads %ld Number of threads in this process. Currently shown as 0. (21) itrealvalue %ld The time in jiffies before the next SIGALRM is sent to the process due to an interval timer. This field is no longer maintained, and is hard coded as 0. (22) starttime %llu The time the process started after system boot. The value is expressed in clock ticks (divide by sysconf(_SC_CLK_TCK)). (23) vsize %lu Virtual memory size in bytes. (24) rss %ld Resident Set Size: number of pages the process has in real memory. This is just the pages which count toward text, data, or stack space. This does not include pages which have not been demand-loaded in, or which are swapped out. (25) rsslim %lu Current soft limit in bytes on the rss of the process; see the description of RLIMIT_RSS in getrlimit2 . /proc/[pid]/statm Provides information about memory usage, measured in pages. The columns are: (1) size total program size (same as VmSize in /proc/[pid]/status) (2) resident resident set size (same as VmRSS in /proc/[pid]/status) (3) shared number of resident shared pages (i.e., backed by a file) (same as RssFile+RssShmem in /proc/[pid]/status) (4) text text (code) (5) lib library (6) data data + stack (7) dt dirty pages (always 0) /proc/[pid]/status Provides much of the information in /proc/[pid]/stat and /proc/[pid]/statm in a format that's easier for humans to parse. Here's an example: $ cat /proc/$$/status Name: bash Umask: 0022 State: S (sleeping) Tgid: 17248 Pid: 17248 PPid: 17200 Uid: 1000 1000 1000 1000 Gid: 100 100 100 100 VmSize: 131168 kB VmLck: 0 kB VmRSS: 13484 kB VmData: 10332 kB VmStk: 136 kB VmExe: 992 kB VmLib: 2104 kB SigPnd: 0000000000000000 SigBlk: 0000000000010000 SigIgn: 0000000000384004 The fields are as follows: Name: Command run by this process. Umask: Process umask, expressed in octal with a leading zero; see umask2 . State: Current state of the process. One of: R runnable O running S sleeping D disk sleep T stopped or tracing stop Z zombie Tgid: Thread group id (i.e., Process id). Pid: Thread id (see gettid2 ). PPid: PID of parent process. Uid, Gid: Real, effective, saved set, and filesystem UIDs (GIDs). VmSize: Virtual memory size. VmLck: Locked memory size (see mlock2 ). VmRSS: Resident set size. VmData, VmStk, VmExe: Size of data, stack, and text segments. VmLib: Shared library code size. SigPnd: Number of signals pending for process as a whole (see pthreads7 and signal7). SigBlk, SigIgn: Masks indicating signals being blocked and ignored (see signal7 ). /proc/[pid]/uid This read-only file contains the user id for the process. /proc/[pid]/winexename This read-only file contains the Windows pathname of the executed command. /proc/[pid]/winpid This read-only file contains the Windows process id for the process. /proc/cpuinfo This is a collection of CPU and system architecture dependent items, for each supported architecture a different list. Two common entries are processor which gives CPU number and bogomips, a system constant that is calculated during kernel initialization. SMP machines have information for each CPU. The lscpu1 command gathers its information from this file. /proc/cygdrive This file is a symbolic link that points to the user's Windows mapped drive mount point, similar to root. /proc/devices Text listing of major numbers and device groups. This can be used by makedev scripts for consistency with the system. /proc/filesystems A text listing of the filesystems which are supported by Cygwin. (See also filesystems5 .) If a filesystem is marked with "nodev", this means that it does not require a block device to be mounted (e.g., virtual filesystem, network filesystem). /proc/loadavg The first three fields in this file are load average figures giving the number of jobs in the run queue (state R) averaged over 1, 5, and 15 minutes. They are the same as the load average numbers given by uptime1 and other programs. The fourth field consists of two numbers separated by a slash (/). The first of these is the number of currently runnable scheduling entities (processes, threads). The value after the slash is the number of scheduling entities that currently exist on the system. /proc/meminfo This file reports statistics about memory usage on the system. It is used by free1 to report the amount of free and used memory (both physical and swap) on the system as well as the shared memory and buffers used by the system. Each line of the file consists of a parameter name, followed by a colon, the value of the parameter, and an option unit of measurement (e.g., "kB"). The list below describes the parameter names and the format specifier required to read the field value. Some fields are displayed only if the system was configured with various options; those dependencies are noted in the list. MemTotal %lu Total usable RAM (i.e., physical RAM minus a few reserved bits and the system binary code). MemFree %lu The sum of LowFree + HighFree. HighTotal %lu Total amount of highmem. HighFree %lu Amount of free highmem. LowTotal %lu Total amount of lowmem. Lowmem is memory which can be used for everything that highmem can be used for, but it is also available for the system's use for its own data structures. Bad things happen when you're out of lowmem. LowFree %lu Amount of free lowmem. SwapTotal %lu Total amount of swap space available. SwapFree %lu Amount of swap space that is currently unused. /proc/misc Text listing of minor device numbers and names of devices with major device number of the misc device group. This can be used by makedev scripts for consistency with the system. /proc/mounts With the introduction of per-process mount namespaces, this file became a link to /proc/self/mounts, which lists the mount points of the process's own mount namespace. The format of this file is documented in fstab5 . /proc/net This directory contains various files and subdirectories containing information about the networking layer. The files contain ASCII structures and are, therefore, readable with cat1 . However, the standard netstat8 suite provides much cleaner access to these files. /proc/net/if_inet6 This file contains information about IP V6 interface adapters, if used. Each line represents an IP V6 interface adapter. fe800000000000002c393d3da6108636 12 40 20 80 {C6B5FBE5-A3AC-4DB0-A308-8EE94E1406A4} fe8000000000000039da016f76bd92bc 13 40 20 20 {E06B8972-0918-41FC-851B-090C446C7D1C} fe8000000000000050ba9cedf1fe1628 0b 40 20 20 {680ED6FD-DFAC-4398-AA85-FB33E17E38EA} fe8000000000000030c5c6a0b30f109d 11 40 20 20 {B9E39F53-1659-4065-BDA5-F41162250E03} 20021840ac2c12343427e3b9ec6fa585 08 40 00 80 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c12342403e3b2c7a5a32f 08 80 00 20 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c1234284e8d0ecb4160cb 08 80 00 20 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c123468cb06ea72f1d678 08 80 00 80 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c12346cb59aca97c36e3b 08 80 00 20 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c123498af9881de1fb828 08 80 00 20 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c1234cd62a3d73a498611 08 80 00 20 {4083A7F8-99CF-4220-8715-6FDF268B002F} 20021840ac2c1234e410c873be09df93 08 80 00 20 {4083A7F8-99CF-4220-8715-6FDF268B002F} fe800000000000003427e3b9ec6fa585 08 40 20 80 {4083A7F8-99CF-4220-8715-6FDF268B002F} 00000000000000000000000000000001 01 80 10 80 {2B5345AC-7502-11EA-AC73-806E6F6E6963} (1) (2)(3)(4)(5) (6) The fields in each line are: (1) The IP V6 address of the interface adapter. (2) The IP V6 interface adapter index. (3) The prefix length of the IP V6 interface address. (4) The scope of the IP V6 interface address. (5) The state of the IP V6 interface address. (6) The DUID/GUID/UUID of the IP V6 interface adapter. The last number exists only for compatibility reasons and is always 1. /proc/partitions Contains the major and minor numbers of each partition as well as the number of 1024-byte blocks and the partition name. /proc/registry Under Cygwin, this directory contains subdirectories for registry paths, keys, and subkeys, and files named for registry values which contain registry data, for the current process. /proc/registry32 Under 64 bit Windows, this directory contains subdirectories for registry paths, keys, and subkeys, and files named for registry values which contain registry data, for 32 bit processes. /proc/registry64 Under 64 bit Windows, this directory contains subdirectories for registry paths, keys, and subkeys, and files named for registry values which contain registry data, for 64 bit processes. /proc/self This directory refers to the process accessing the /proc filesystem, and is identical to the /proc directory named by the process id of the same process. /proc/stat system statistics. Varies with architecture. Common entries include: cpu 10132153 0 3084719 46828483 cpu0 1393280 0 572056 13343292 The amount of time, measured in units of USER_HZ (1/100ths of a second on most architectures, use sysconf(_SC_CLK_TCK) to obtain the right value), that the system ("cpu" line) or the specific CPU ("cpu N" line) spent in various states: (1) user Time spent in user mode. (2) nice Time spent in user mode with low priority (nice). (3) system Time spent in system mode. (4) idle Time spent in the idle task. page 5741 1808 The number of pages the system paged in and the number that were paged out (from disk). swap 1 0 The number of swap pages that have been brought in and out. intr 1462898 The number of interrupts serviced. ctxt 115315 The number of context switches that the system underwent. btime 769041601 boot time, in seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). /proc/swaps Swap areas in use. See also swapon8 . /proc/sys This directory contains a number of files and subdirectories linking to Windows objects, which can be read using these entries. String values may be terminated by either '\0' or '\n'. Integer and long values may be either in decimal or in hexadecimal notation (e.g. 0x3FFF). Multiple integer or long values may be separated by any of the following whitespace characters: ' ', '\t', or '\n'. /proc/sysvipc Subdirectory containing the pseudo-files msg, semand shm. These files list the System V Interprocess Communication (IPC) objects (respectively: message queues, semaphores, and shared memory) that currently exist on the system, providing similar information to that available via ipcs1 . These files are only available if the cygserver Cygwin service is running. These files have headers and are formatted (one IPC object per line) for easy understanding. svipc7 provides further background on the information shown by these files. /proc/uptime This file contains two numbers (values in seconds): the uptime of the system (including time spent in suspend) and the amount of time spent in the idle process. /proc/version This string identifies the Cygwin version that is currently running. For example: CYGWIN_NT-10.0-18363 version 3.1.7-340.x86_64 (corinna@calimero) (gcc version 9.3.0 20200312 (Fedora Cygwin 9.3.0-1) (GCC) ) 2020-08-22 17:48 UTC Many files contain strings (e.g., the environment and command line) that are in the internal format, with subfields terminated by null bytes ('\0'). When inspecting such files, you may find that the results are more readable if you use a command of the following form to display them: $ cat -A file This manual page is incomplete, possibly inaccurate, and is the kind of thing that needs to be updated very often. cat1 , find1 , free1 , ps1 , pstree1 , tr1 , uptime1 , chroot2 , mmap2 , readlink2 , syslog2 , hier7 , arp8 , mount8 , netstat8 , route8 . The /proc/registry filesystem provides read-only access to the Windows registry. It displays each KEY as a directory and each VALUE as a file. As anytime you deal with the Windows registry, use caution since changes may result in an unstable or broken system. There are additionally subdirectories called /proc/registry32 and /proc/registry64. They are identical to /proc/registry on 32 bit host OSes. On 64 bit host OSes, /proc/registry32 opens the 32 bit processes view on the registry, while /proc/registry64 opens the 64 bit processes view. Reserved characters ('/', '\', ':', and '%') or reserved names (. and ..) are converted by percent-encoding: bash$ regtool list -v '\HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices' ... \DosDevices\C: (REG_BINARY) = cf a8 97 e8 00 08 fe f7 ... bash$ cd /proc/registry/HKEY_LOCAL_MACHINE/SYSTEM bash$ ls -l MountedDevices ... -r--r----- 1 Admin SYSTEM 12 Dec 10 11:20 %5CDosDevices%5CC%3A ... bash$ od -t x1 MountedDevices/%5CDosDevices%5CC%3A 0000000 cf a8 97 e8 00 08 fe f7 01 00 00 00 The unnamed (default) value of a key can be accessed using the filename @. If a registry key contains a subkey and a value with the same name foo, Cygwin displays the subkey as foo and the value as foo%val. To circumvent the limitations on shell line length in the native Windows command shells, Cygwin programs, when invoked by non-Cygwin processes, expand their arguments starting with "@" in a special way. If a file pathname exists, the argument @pathname expands recursively to the content of pathname. Double quotes can be used inside the file to delimit strings containing blank space. In the following example compare the behaviors /bin/echo when run from bash and from the Windows command prompt. bash$ /bin/echo 'This is "a long" line' > mylist bash$ /bin/echo @mylist @mylist bash$ cmd c:\> c:\cygwin\bin\echo @mylist This is a long line