Linux open existing file

How to open, create, edit, and view a file in Linux

One thing GNU/Linux does as well as any other operating system is give you the tools you need to create and edit text files. Ask ten Linux users to name their favorite text editor, and you might get ten different answers. On this page, we cover a few of the many text editors available for Linux.

GUI text editors

This section discusses text editing applications for the Linux windowing system, X Windows, more commonly known as X11 or X.

If you are coming from Microsoft Windows, you are no doubt familiar with the classic Windows text editor, Notepad. Linux offers many similar programs, including NEdit, gedit, and geany. Each of these programs are free software, and they each provide roughly the same functionality. It’s up to you to decide which one feels best and has the best interface for you. All three of these programs support syntax highlighting, which helps with editing source code or documents written in a markup language such as HTML or CSS.

NEdit

NEdit, which is short for the Nirvana Editor, is a straightforward text editor that is very similar to Notepad. It uses a Motif-style interface.

The NEdit homepage is located at https://sourceforge.net/projects/nedit/. If you are on a Debian or Ubuntu system, you can install NEdit with the following command:

For more information, see our NEdit information page.

Geany

Geany is a text editor that is a lot like Notepad++ for Windows. It provides a tabbed interface for working with multiple open files at once and has nifty features like displaying line numbers in the margin. It uses the GTK+ interface toolkit.

The Geany homepage is located at http://www.geany.org/. On Debian and Ubuntu systems, you can install Geany by running the command:

Gedit

Gedit is the default text editor of the GNOME desktop environment. It’s a great, text editor that can be used on about any Linux system.

The Gedit homepage is located at https://wiki.gnome.org/Apps/Gedit. On Debian and Ubuntu systems, Gedit can be installed by running the following command:

Terminal-based text editors

If you are working from the Linux command line interface and you need a text editor, you have many options. Here are some of the most popular:

pico started out as the editor built into the text-based e-mail program pine, and it was eventually packaged as a stand-alone program for editing text files. («pico» is a scientific prefix for very small things.)

The modern version of pine is called alpine, but pico is still called pico. You can find more information about how to use it in our pico command documentation.

On Debian and Ubuntu Linux systems, you can install pico using the command:

nano is the GNU version of pico and is essentially the same program under a different name.

On Debian and Ubuntu Linux systems, nano can be installed with the command:

vim, which stands for «vi improved,» is a text editor used by millions of computing professionals all over the world. Its controls are a little confusing at first, but once you get the hang of them, vim makes executing complex editing tasks fast and easy. For more information, see our in-depth vim guide.

On Debian and Ubuntu Linux systems, vim can be installed using the command:

emacs

emacs is a complex, highly customizable text editor with a built-in interpreter for the Lisp programming language. It is used religiously by some computer programmers, especially those who write computer programs in Lisp dialects such as Scheme. For more information, see our emacs information page.

On Debian and Ubuntu Linux systems, emacs can be installed using the command:

Redirecting command output into a text file

When at the Linux command line, you sometimes want to create or make changes to a text file without actually running a text editor. Here are some commands you might find useful.

Creating an empty file with the touch command

To create an empty file, it’s common to use the command touch. The touch command updates the atime and mtime attributes of a file as if the contents of the file had been changed — without actually changing anything. If you touch a file that doesn’t exist, the system creates the file without putting any data inside.

For instance, the command:

The above command creates a new, empty file called myfile.txt if that file does not already exist.

Redirecting text into a file

Sometimes you need to stick the output of a command into a file. To accomplish this quickly and easily, you can use the > symbol to redirect the output to a file.

For instance, the echo command is used to «echo» text as output. By default, this goes to the standard output — the screen. So the command:

The above command prints that text on your screen and return you to the command prompt. However, you can use > to redirect this output to a file. For instance:

The above command puts the text «Example text» into the file myfile.txt. If myfile.txt does not exist, it is created. If it already exists, its contents will be overwritten, destroying the previous contents and replacing them.

Be careful when redirecting output to a file using >. It will overwrite the previous contents of the file if it already exists. There is no undo for this operation, so make sure you want to completely replace the file’s contents before you run the command.

Here’s an example using another command:

The above command executes ls with the -l option, which gives a detailed list of files in the current directory. The > operator redirects the output to the file directory.txt, instead of printing it to the screen. If directory.txt does not exist, it is created first. If it already exists, its contents will be replaced.

Redirecting to the end of a file

The redirect operator >> is similar to >, but instead of overwriting the file contents, it appends the new data to the end of the file. For instance, the command:

Источник

open(3) — Linux man page

Prolog

Synopsis

Description

The open() function shall establish the connection between a file and a file descriptor. It shall create an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file.

The open() function shall return a file descriptor for the named file that is the lowest file descriptor not currently open for that process. The open file description is new, and therefore the file descriptor shall not share it with any other process in the system. The FD_CLOEXEC file descriptor flag associated with the new file descriptor shall be cleared.

The file offset used to mark the current position within the file shall be set to the beginning of the file.

The file status flags and file access modes of the open file description shall be set according to the value of oflag.

Values for oflag are constructed by a bitwise-inclusive OR of flags from the following list, defined in . Applications shall specify exactly one of the first three values (file access modes) below in the value of oflag: O_RDONLY Open for reading only. O_WRONLY Open for writing only. O_RDWR Open for reading and writing. The result is undefined if this flag is applied to a FIFO.

Any combination of the following may be used: O_APPEND If set, the file offset shall be set to the end of the file prior to each write. O_CREAT If the file exists, this flag has no effect except as noted under O_EXCL below. Otherwise, the file shall be created; the user ID of the file shall be set to the effective user ID of the process; the group ID of the file shall be set to the group ID of the file’s parent directory or to the effective group ID of the process; and the access permission bits (see ) of the file mode shall be set to the value of the third argument taken as type mode_t modified as follows: a bitwise AND is performed on the file-mode bits and the corresponding bits in the complement of the process’ file mode creation mask. Thus, all bits in the file mode whose corresponding bit in the file mode creation mask is set are cleared. When bits other than the file permission bits are set, the effect is unspecified. The third argument does not affect whether the file is open for reading, writing, or for both. Implementations shall provide a way to initialize the file’s group ID to the group ID of the parent directory. Implementations may, but need not, provide an implementation-defined way to initialize the file’s group ID to the effective group ID of the calling process. O_DSYNC Write I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. O_EXCL If O_CREAT and O_EXCL are set, open() shall fail if the file exists. The check for the existence of the file and the creation of the file if it does not exist shall be atomic with respect to other threads executing open() naming the same filename in the same directory with O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are set, and path names a symbolic link, open() shall fail and set errno to [EEXIST], regardless of the contents of the symbolic link. If O_EXCL is set and O_CREAT is not set, the result is undefined. O_NOCTTY If set and path identifies a terminal device, open() shall not cause the terminal device to become the controlling terminal for the process. O_NONBLOCK When opening a FIFO with O_RDONLY or O_WRONLY set: * If O_NONBLOCK is set, an open() for reading-only shall return without delay. An open() for writing-only shall return an error if no process currently has the file open for reading. * If O_NONBLOCK is clear, an open() for reading-only shall block the calling thread until a thread opens the file for writing. An open() for writing-only shall block the calling thread until a thread opens the file for reading.

When opening a block special or character special file that supports non-blocking opens:

* If O_NONBLOCK is set, the open() function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific. * If O_NONBLOCK is clear, the open() function shall block the calling thread until the device is ready or available before returning.

Otherwise, the behavior of O_NONBLOCK is unspecified. O_RSYNC Read I/O operations on the file descriptor shall complete at the same level of integrity as specified by the O_DSYNC and O_SYNC flags. If both O_DSYNC and O_RSYNC are set in oflag, all I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. If both O_SYNC and O_RSYNC are set in flags, all I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. O_SYNC Write I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. O_TRUNC If the file exists and is a regular file, and the file is successfully opened O_RDWR or O_WRONLY, its length shall be truncated to 0, and the mode and owner shall be unchanged. It shall have no effect on FIFO special files or terminal device files. Its effect on other file types is implementation-defined. The result of using O_TRUNC with O_RDONLY is undefined.

If O_CREAT is set and the file did not previously exist, upon successful completion, open() shall mark for update the st_atime, st_ctime, and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory.

If O_TRUNC is set and the file did previously exist, upon successful completion, open() shall mark for update the st_ctime and st_mtime fields of the file.

If both the O_SYNC and O_DSYNC flags are set, the effect is as if only the O_SYNC flag was set.

If path refers to a STREAMS file, oflag may be constructed from O_NONBLOCK OR’ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values are not applicable to STREAMS devices and shall have no effect on them. The value O_NONBLOCK affects the operation of STREAMS drivers and certain functions applied to file descriptors associated with STREAMS files. For STREAMS drivers, the implementation of O_NONBLOCK is device-specific.

If path names the master side of a pseudo-terminal device, then it is unspecified whether open() locks the slave side so that it cannot be opened. Conforming applications shall call unlockpt() before opening the slave side.

The largest value that can be represented correctly in an object of type off_t shall be established as the offset maximum in the open file description.

Return Value

Upon successful completion, the function shall open the file and return a non-negative integer representing the lowest numbered unused file descriptor. Otherwise, -1 shall be returned and errno set to indicate the error. No files shall be created or modified if the function returns -1.

Errors

The open() function shall fail if: EACCES Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by oflag are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created, or O_TRUNC is specified and write permission is denied. EEXIST O_CREAT and O_EXCL are set, and the named file exists. EINTR A signal was caught during open(). EINVAL The implementation does not support synchronized I/O for this file. EIO The path argument names a STREAMS file and a hangup or error occurred during the open(). EISDIR The named file is a directory and oflag includes O_WRONLY or O_RDWR. ELOOP A loop exists in symbolic links encountered during resolution of the path argument. EMFILE file descriptors are currently open in the calling process. ENAMETOOLONG The length of the path argument exceeds or a pathname component is longer than . ENFILE The maximum allowable number of files is currently open in the system. ENOENT O_CREAT is not set and the named file does not exist; or O_CREAT is set and either the path prefix does not exist or the path argument points to an empty string. ENOSR The path argument names a STREAMS-based file and the system is unable to allocate a STREAM. ENOSPC The directory or file system that would contain the new file cannot be expanded, the file does not exist, and O_CREAT is specified. ENOTDIR A component of the path prefix is not a directory. ENXIO O_NONBLOCK is set, the named file is a FIFO, O_WRONLY is set, and no process has the file open for reading. ENXIO The named file is a character special or block special file, and the device associated with this special file does not exist. EOVERFLOW The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t. EROFS The named file resides on a read-only file system and either O_WRONLY, O_RDWR, O_CREAT (if the file does not exist), or O_TRUNC is set in the oflag argument.

The open() function may fail if: EAGAIN The path argument names the slave side of a pseudo-terminal device that is locked. EINVAL The value of the oflag argument is not valid. ELOOP More than symbolic links were encountered during resolution of the path argument. ENAMETOOLONG As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded . ENOMEM The path argument names a STREAMS file and the system is unable to allocate resources. ETXTBSY The file is a pure procedure (shared text) file that is being executed and oflag is O_WRONLY or O_RDWR.

The following sections are informative.

Examples

Opening a File for Writing by the Owner

The following example opens the file /tmp/file, either by creating it (if it does not already exist), or by truncating its length to 0 (if it does exist). In the former case, if the call creates a new file, the access permission bits in the file mode of the file are set to permit reading and writing by the owner, and to permit reading only by group members and others.

If the call to open() is successful, the file is opened for writing.

Opening a File Using an Existence Check

The following example uses the open() function to try to create the LOCKFILE file and open it for writing. Since the open() function specifies the O_EXCL flag, the call fails if the file already exists. In that case, the program assumes that someone else is updating the password file and exits.

Opening a File for Writing

The following example opens a file for writing, creating the file if it does not already exist. If the file does exist, the system truncates the file to zero bytes.

Application Usage

Rationale

Except as specified in this volume of IEEE Std 1003.1-2001, the flags allowed in oflag are not mutually-exclusive and any number of them may be used simultaneously.

Some implementations permit opening FIFOs with O_RDWR. Since FIFOs could be implemented in other ways, and since two file descriptors can be used to the same effect, this possibility is left as undefined.

See getgroups() about the group of a newly created file.

The use of open() to create a regular file is preferable to the use of creat(), because the latter is redundant and included only for historical reasons.

The use of the O_TRUNC flag on FIFOs and directories (pipes cannot be open()-ed) must be permissible without unexpected side effects (for example, creat() on a FIFO must not remove data). Since terminal special files might have type-ahead data stored in the buffer, O_TRUNC should not affect their content, particularly if a program that normally opens a regular file should open the current controlling terminal instead. Other file types, particularly implementation-defined ones, are left implementation-defined.

IEEE Std 1003.1-2001 permits [EACCES] to be returned for conditions other than those explicitly listed.

The O_NOCTTY flag was added to allow applications to avoid unintentionally acquiring a controlling terminal as a side effect of opening a terminal file. This volume of IEEE Std 1003.1-2001 does not specify how a controlling terminal is acquired, but it allows an implementation to provide this on open() if the O_NOCTTY flag is not set and other conditions specified in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 11, General Terminal Interface are met. The O_NOCTTY flag is an effective no-op if the file being opened is not a terminal device.

In historical implementations the value of O_RDONLY is zero. Because of that, it is not possible to detect the presence of O_RDONLY and another option. Future implementations should encode O_RDONLY and O_WRONLY as bit flags so that:

In general, the open() function follows the symbolic link if path names a symbolic link. However, the open() function, when called with O_CREAT and O_EXCL, is required to fail with [EEXIST] if path names an existing symbolic link, even if the symbolic link refers to a nonexistent file. This behavior is required so that privileged applications can create a new file in a known location without the possibility that a symbolic link might cause the file to be created in a different location.

For example, a privileged application that must create a file with a predictable name in a user-writable directory, such as the user’s home directory, could be compromised if the user creates a symbolic link with that name that refers to a nonexistent file in a system directory. If the user can influence the contents of a file, the user could compromise the system by creating a new system configuration or spool file that would then be interpreted by the system. The test for a symbolic link which refers to a nonexisting file must be atomic with the creation of a new file.

The POSIX.1-1990 standard required that the group ID of a newly created file be set to the group ID of its parent directory or to the effective group ID of the creating process. FIPS 151-2 required that implementations provide a way to have the group ID be set to the group ID of the containing directory, but did not prohibit implementations also supporting a way to set the group ID to the effective group ID of the creating process. Conforming applications should not assume which group ID will be used. If it matters, an application can use chown() to set the group ID after the file is created, or determine under what conditions the implementation will set the desired group ID.

Future Directions

See Also

chmod(), close(), creat(), dup(), fcntl(), lseek(), read(), umask(), unlockpt(), write(), the Base Definitions volume of IEEE Std 1003.1-2001, , ,

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