25.4 Writing to Files

You can write the contents of a buffer, or part of a buffer, directly to a file on disk using the append-to-file and write-region functions. Don’t use these functions to write to files that are being visited; that could cause confusion in the mechanisms for visiting.

command append-to-file start end filename

This function appends the contents of the region delimited by start and end in the current buffer to the end of file filename. If that file does not exist, it is created. This function returns nil.

An error is signaled if you cannot write or create filename.

When called from Lisp, this function is completely equivalent to:

(write-region start end filename t)

command write-region start end filename \&optional append visit lockname mustbenew

This function writes the region delimited by start and end in the current buffer into the file specified by filename.

If start is nil, then the command writes the entire buffer contents (not just the accessible portion) to the file and ignores end.

If start is a string, then write-region writes or appends that string, rather than text from the buffer. end is ignored in this case.

If append is non-nil, then the specified text is appended to the existing file contents (if any). If append is a number, write-region seeks to that byte offset from the start of the file and writes the data from there.

If mustbenew is non-nil, then write-region asks for confirmation if filename names an existing file. If mustbenew is the symbol excl, then write-region does not ask for confirmation, but instead it signals an error file-already-exists if the file already exists. Although write-region normally follows a symbolic link and creates the pointed-to file if the symbolic link is dangling, it does not follow symbolic links if mustbenew is excl.

The test for an existing file, when mustbenew is excl, uses a special system feature. At least for files on a local disk, there is no chance that some other program could create a file of the same name before Emacs does, without Emacs’s noticing.

If visit is t, then Emacs establishes an association between the buffer and the file: the buffer is then visiting that file. It also sets the last file modification time for the current buffer to filename’s modtime, and marks the buffer as not modified. This feature is used by save-buffer, but you probably should not use it yourself.

If visit is a string, it specifies the file name to visit. This way, you can write the data to one file (filename) while recording the buffer as visiting another file (visit). The argument visit is used in the echo area message and also for file locking; visit is stored in buffer-file-name. This feature is used to implement file-precious-flag; don’t use it yourself unless you really know what you’re doing.

The optional argument lockname, if non-nil, specifies the file name to use for purposes of locking and unlocking, overriding filename and visit for that purpose.

The function write-region converts the data which it writes to the appropriate file formats specified by buffer-file-format and also calls the functions in the list write-region-annotate-functions. See Format Conversion.

Normally, write-region displays the message ‘Wrote filename’ in the echo area. This message is inhibited if visit is neither t nor nil nor a string, or if Emacs is operating in batch mode (see Batch Mode). This feature is useful for programs that use files for internal purposes, files that the user does not need to know about.

variable write-region-inhibit-fsync

If this variable’s value is nil, write-region uses the fsync system call after writing a file. Although this slows Emacs down, it lessens the risk of data loss after power failure. If the value is t, Emacs does not use fsync. The default value is nil when Emacs is interactive, and t when Emacs runs in batch mode. See Files and Storage.

macro with-temp-file file body…

The with-temp-file macro evaluates the body forms with a temporary buffer as the current buffer; then, at the end, it writes the buffer contents into file file. It kills the temporary buffer when finished, restoring the buffer that was current before the with-temp-file form. Then it returns the value of the last form in body.

The current buffer is restored even in case of an abnormal exit via throw or error (see Nonlocal Exits).

See also with-temp-buffer in The Current Buffer.