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Info file emacs, produced by texinfo-format-buffer -*-Text-*-
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This file documents the GNU Emacs editor.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
sections entitled "The GNU Manifesto", "Distribution" and "GNU Emacs
General Public License" are included exactly as in the original, and
provided that the entire resulting derived work is distributed under the
terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the sections entitled "The GNU Manifesto", "Distribution"
and "GNU Emacs General Public License" may be included in a translation
approved by the author instead of in the original English.

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File: emacs Node: Regexps, Prev: Regexp Search, Up: Search, Next: Search Case

Syntax of Regular Expressions
=============================

Regular expressions have a syntax in which a few characters are special
constructs and the rest are "ordinary". An ordinary character is a simple
regular expression which matches that character and nothing else. The
special characters are `$', `^', `.', `*', `+', `?', `[', `]' and `\'; no
new special characters will be defined. Any other character appearing in a
regular expression is ordinary, unless a `\' precedes it.

For example, `f' is not a special character, so it is ordinary, and
therefore `f' is a regular expression that matches the string `f' and no
other string. (It does not match the string `ff'.) Likewise, `o' is a
regular expression that matches only `o'.

Any two regular expressions A and B can be concatenated. The result is a
regular expression which matches a string if A matches some amount of the
beginning of that string and B matches the rest of the string.

As a simple example, we can concatenate the regular expressions `f'
and `o' to get the regular expression `fo', which matches only
the string `fo'. Still trivial. To do something nontrivial, you
need to use one of the special characters. Here is a list of them.

`. (Period)'
is a special character that matches any single character except a
newline. Using concatenation, we can make regular expressions like
`a.b' which matches any three-character string which begins with `a'
and ends with `b'.

`*'
is not a construct by itself; it is a suffix, which means the
preceding regular expression is to be repeated as many times as
possible. In `fo*', the `*' applies to the `o', so `fo*' matches one
`f' followed by any number of `o's. The case of zero `o's is allowed:
`fo*' does match `f'.

`*' always applies to the smallest possible preceding expression.
Thus, `fo*' has a repeating `o', not a repeating `fo'.

The matcher processes a `*' construct by matching, immediately, as
many repetitions as can be found. Then it continues with the rest of
the pattern. If that fails, backtracking occurs, discarding some of
the matches of the `*'-modified construct in case that makes it
possible to match the rest of the pattern. For example, matching
`ca*ar' against the string `caaar', the `a*' first tries to match all
three `a's; but the rest of the pattern is `ar' and there is only `r'
left to match, so this try fails. The next alternative is for `a*' to
match only two `a's. With this choice, the rest of the regexp matches
successfully.

`+'
Is a suffix character similar to `*' except that it requires that the
preceding expression be matched at least once. So, for example,
`ca+r' will match the strings `car' and `caaaar' but not the string
`cr', whereas `ca*r' would match all three strings.

`?'
Is a suffix character similar to `*' except that it can match the
preceding expression either once or not at all. For example,
`ca?r' will match `car' or `cr'; nothing else.

`[ ... ]'
`[' begins a "character set", which is terminated by a `]'. In the
simplest case, the characters between the two form the set. Thus,
`[ad]' matches either one `a' or one `d', and `[ad]*' matches any
string composed of just `a's and `d's (including the empty string),
from which it follows that `c[ad]*r' matches `cr', `car', `cdr',
`caddaar', etc.

Character ranges can also be included in a character set, by writing
two characters with a `-' between them. Thus, `[a-z]' matches any
lower-case letter. Ranges may be intermixed freely with individual
characters, as in `[a-z$%.]', which matches any lower case letter or
`$', `%' or period.

Note that the usual special characters are not special any more inside
a character set. A completely different set of special characters
exists inside character sets: `]', `-' and `^'.

To include a `]' in a character set, you must make it the first
character. For example, `[]a]' matches `]' or `a'. To include a `-',
write `---', which is a range containing only `-'. To include `^',
make it other than the first character in the set.

`[^ ... ]'
`[^' begins a "complement character set", which matches any character
except the ones specified. Thus, `[^a-z0-9A-Z]' matches all
characters except letters and digits.

`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (`-' and `]' are not special there).

Note that a complement character set can match a newline, unless
newline is mentioned as one of the characters not to match.

`^'
is a special character that matches the empty string, but only if at
the beginning of a line in the text being matched. Otherwise it fails
to match anything. Thus, `^foo' matches a `foo' which occurs
at the beginning of a line.

`$'
is similar to `^' but matches only at the end of a line. Thus,
`xx*$' matches a string of one `x' or more at the end of a line.

`\'
has two functions: it quotes the special characters (including
`\'), and it introduces additional special constructs.

Because `\' quotes special characters, `\$' is a regular expression
which matches only `$', and `\[' is a regular expression which matches
only `[', and so on.

Note: for historical compatibility, special characters are treated as
ordinary ones if they are in contexts where their special meanings make no
sense. For example, `*foo' treats `*' as ordinary since there is no
preceding expression on which the `*' can act. It is poor practice to
depend on this behavior; better to quote the special character anyway,
regardless of where is appears.

For the most part, `\' followed by any character matches only
that character. However, there are several exceptions: characters
which, when preceded by `\', are special constructs. Such
characters are always ordinary when encountered on their own. Here
is a table of `\' constructs.

`\|'
specifies an alternative. Two regular expressions A and B with `\|'
in between form an expression that matches anything that either A or B
will match.

Thus, `foo\|bar' matches either `foo' or `bar' but no other string.

`\|' applies to the largest possible surrounding expressions. Only a
surrounding `$ ... $' grouping can limit the grouping power of `\|'.

Full backtracking capability exists to handle multiple uses of `\|'.

`$ ... $'
is a grouping construct that serves three purposes:

1. To enclose a set of `\|' alternatives for other operations.
Thus, `$foo\|bar$x' matches either `foox' or `barx'.

2. To enclose a complicated expression for the postfix `*' to
operate on. Thus, `ba$na$*' matches `bananana', etc., with any
(zero or more) number of `na' strings.

3. To mark a matched substring for future reference.

This last application is not a consequence of the idea of a
parenthetical grouping; it is a separate feature which happens to be
assigned as a second meaning to the same `$ ... $' construct
because there is no conflict in practice between the two meanings.
Here is an explanation of this feature:

`\DIGIT'
after the end of a `$ ... $' construct, the matcher remembers the
beginning and end of the text matched by that construct. Then, later
on in the regular expression, you can use `\' followed by DIGIT to
mean "match the same text matched the DIGIT'th time by the `$ ... $'
construct."

The strings matching the first nine `$ ... $' constructs appearing
in a regular expression are assigned numbers 1 through 9 in order that the
open-parentheses appear in the regular expression. `\1' through
`\9' may be used to refer to the text matched by the corresponding
`$ ... $' construct.

For example, `$.*$\1' matches any newline-free string that is
composed of two identical halves. The `$.*$' matches the first
half, which may be anything, but the `\1' that follows must match
the same exact text.

`\`'
matches the empty string, provided it is at the beginning
of the buffer.

`\''
matches the empty string, provided it is at the end of
the buffer.

`\b'
matches the empty string, provided it is at the beginning or end of a
word. Thus, `\bfoo\b' matches any occurrence of `foo' as a separate
word. `\bballs?\b' matches `ball' or `balls' as a separate word.

`\B'
matches the empty string, provided it is not at the beginning or
end of a word.

`\<'
matches the empty string, provided it is at the beginning of a word.

`\>'
matches the empty string, provided it is at the end of a word.

`\w'
matches any word-constituent character. The editor syntax table
determines which characters these are.

`\W'
matches any character that is not a word-constituent.

`\sCODE'
matches any character whose syntax is CODE. CODE is a character which
represents a syntax code: thus, `w' for word constituent, `-' for
whitespace, `(' for open-parenthesis, etc. *Note Syntax::.

`\SCODE'
matches any character whose syntax is not CODE.

Here is a complicated regexp, used by Emacs to recognize the end of a
sentence together with any whitespace that follows. It is given in Lisp
syntax to enable you to distinguish the spaces from the tab characters. In
Lisp syntax, the string constant begins and ends with a double-quote.
`\"' stands for a double-quote as part of the regexp, `\\' for a
backslash as part of the regexp, `\t' for a tab and `\n' for a
newline.

"[.?!][]\"')]*\$$\\|\t\\| \$[ \t\n]*"

This contains four parts in succession: a character set matching period,
`?' or `!'; a character set matching close-brackets,
quotes or parentheses, repeated any number of times; an alternative in
backslash-parentheses that matches end-of-line, a tab or two spaces; and a
character set matching whitespace characters, repeated any number of times.

▶1f◀
File: emacs Node: Search Case, Prev: Regexps, Up: Search, Next: Replace

Searching and Case
==================

All sorts of searches in Emacs normally ignore the case of the text they
are searching through; if you specify searching for `FOO', then `Foo' and
`foo' are also considered a match. Regexps, and in particular character
sets, are included: `[aB]' would match `a' or `A' or `b' or `B'.

If you do not want this feature, set the variable `case-fold-search'
to `nil'. Then all letters must match exactly, including case. This
is a per-buffer variable; altering the variable affects only the current
buffer, but there is a default value which you can change as well.
*Note Locals::.

▶1f◀
File: emacs Node: Replace, Prev: Search Case, Up: Search, Next: Other Repeating Search

Replacement Commands
====================

Global search-and-replace operations are not needed as often in Emacs as
they are in other editors, but they are available. In addition to the
simple `replace-string' command which is like that found in most
editors, there is a `query-replace' command which asks you, for each
occurrence of the pattern, whether to replace it.

The replace commands all replace one string (or regexp) with one
replacement string. It is possible to perform several replacements in
parallel using the command `expand-region-abbrevs'. *Note Expanding Abbrevs::.

* Menu:

* Unconditional Replace:: Replacing all matches for a string.
* Regexp Replace:: Replacing all matches for a regexp.
* Replacement and Case:: How replacements preserve case of letters.
* Query Replace:: How to use querying.

▶1f◀
File: emacs Node: Unconditional Replace, Prev: Replace, Up: Replace, Next: Regexp Replace

Unconditional Replacement
-------------------------

`M-x replace-string RET STRING RET NEWSTRING RET'
Replace every occurrence of STRING with NEWSTRING.
`M-x replace-regexp RET REGEXP RET NEWSTRING RET'
Replace every match for REGEXP with NEWSTRING.

To replace every instance of `foo' after point with `bar', use
the command `M-x replace-string' with the two arguments `foo' and
`bar'. Replacement occurs only after point, so if you want to cover
the whole buffer you must go to the beginning first. All occurrences up to
the end of the buffer are replaced; to limit replacement to part of the
buffer, narrow to that part of the buffer before doing the replacement
(*Note Narrowing::).

When `replace-string' exits, point is left at the last occurrence
replaced. The value of point when the `replace-string' command was
issued is remembered on the mark ring; `C-u C-SPC' moves back
there.

A numeric argument restricts replacement to matches that are surrounded
by word boundaries.

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File: emacs Node: Regexp Replace, Prev: Unconditional Replace, Up: Replace, Next: Replacement and Case

Regexp Replacement
------------------

`replace-string' replaces exact matches for a single string. The
similar command `replace-regexp' replaces any match for a specified
pattern.

In `replace-regexp', the NEWSTRING need not be constant. It can refer to
all or part of what is matched by the REGEXP. `\&' in NEWSTRING stands for
the entire text being replaced. `\D' in NEWSTRING, where D is a digit,
stands for whatever matched the D'th parenthesized grouping in REGEXP. For
example,

M-x replace-regexp RET c[ad]+r RET \&-safe RET

would replace (for example) `cadr' with `cadr-safe' and `cddr'
with `cddr-safe'.

M-x replace-regexp RET $c[ad]+r$-safe RET \1 RET

would perform exactly the opposite replacements. To include a `\'
in the text to replace with, you must give `\\'.

▶1f◀
File: emacs Node: Replacement and Case, Prev: Regexp Replace, Up: Replace, Next: Query Replace

Replace Commands and Case
-------------------------

If the arguments to a replace command are in lower case, it preserves
case when it makes a replacement. Thus, the command

M-x replace-string RET foo RET bar RET

replaces a lower case `foo' with a lower case `bar', `FOO'
with `BAR', and `Foo' with `Bar'. If upper case letters are
used in the second argument, they remain upper case every time that
argument is inserted. If upper case letters are used in the first
argument, the second argument is always substituted exactly as given, with
no case conversion. Likewise, if the variable `case-replace' is set
to `nil', replacement is done without case conversion. If
`case-fold-search' is set to `nil', case is significant in
matching occurrences of `foo' to replace; also, case conversion of the
replacement string is not done.

▶1f◀
File: emacs Node: Query Replace, Prev: Replacement and Case, Up: Replace

Query Replace
-------------

`M-% STRING RET NEWSTRING RET'
`M-x query-replace RET STRING RET NEWSTRING RET'
Replace some occurrences of STRING with NEWSTRING.
`M-x query-replace-regexp RET REGEXP RET NEWSTRING RET'
Replace some matches for REGEXP with NEWSTRING.

If you want to change only some of the occurrences of `foo' to `bar', not
all of them, then you cannot use an ordinary `replace-string'. Instead,
use `M-%' (`query-replace'). This command finds occurrences of `foo' one
by one, displays each occurrence and asks you whether to replace it. A
numeric argument to `query-replace' tells it to consider only occurrences
that are bounded by word-delimiter characters.

Aside from querying, `query-replace' works just like `replace-string',
and `query-replace-regexp' works just like `replace-regexp'.

The things you can type when you are shown an occurrence of STRING
or a match for REGEXP are:

`SPC'
to replace the occurrence with NEWSTRING. This preserves case, just
like `replace-string', provided `case-replace' is non-`nil', as it
normally is.

`DEL'
to skip to the next occurrence without replacing this one.

`, (Comma)'
to replace this occurrence and display the result. You are then asked
for another input character, except that since the replacement has
already been made, DEL and SPC are equivalent. You could
type `C-r' at this point (see below) to alter the replaced text. You
could also type `C-x u' to undo the replacement; this exits the
`query-replace', so if you want to do further replacement you must use
`C-x ESC' to restart (*Note Repetition::).

`ESC'
to exit without doing any more replacements.

`. (Period)'
to replace this occurrence and then exit.

`!'
to replace all remaining occurrences without asking again.

`^'
to go back to the location of the previous occurrence (or what used to
be an occurrence), in case you changed it by mistake. This works by
popping the mark ring. Only one `^' in a row is allowed, because
only one previous replacement location is kept during `query-replace'.

`C-r'
to enter a recursive editing level, in case the occurrence needs to be
edited rather than just replaced with NEWSTRING. When you are
done, exit the recursive editing level with `C-M-c' and the next
occurrence will be displayed. *Note Recursive Edit::.

`C-w'
to delete the occurrence, and then enter a recursive editing level as
in `C-r'. Use the recursive edit to insert text to replace the
deleted occurrence of STRING. When done, exit the recursive
editing level with `C-M-c' and the next occurrence will be
displayed.

`C-l'
to redisplay the screen and then give another answer.

`C-h'
to display a message summarizing these options, then give another
answer.

If you type any other character, the `query-replace' is exited, and
the character executed as a command. To restart the `query-replace',
use `C-x ESC', which repeats the `query-replace' because it
used the minibuffer to read its arguments. *Note C-x ESC: Repetition.

▶1f◀
File: emacs Node: Other Repeating Search, Prev: Replace, Up: Search

Other Search-and-Loop Commands
==============================

Here are some other commands that find matches for a regular expression.
They all operate from point to the end of the buffer.

`M-x occur'
Print each line that follows point and contains a match for the
specified regexp. A numeric argument specifies the number of context
lines to print before and after each matching line; the default is
none.

The buffer `*Occur*' containing the output serves as a menu for
finding the occurrences in their original context. Find an occurrence
as listed in `*Occur*', position point there and type `C-c
C-c'; this switches to the buffer that was searched and moves point to
the original of the same occurrence.

`M-x list-matching-lines'
Synonym for `M-x occur'.

`M-x count-matches'
Print the number of matches following point for the specified regexp.

`M-x delete-non-matching-lines'
Delete each line that follows point and does not contain a match for
the specified regexp.

`M-x delete-matching-lines'
Delete each line that follows point and contains a match for the
specified regexp.

▶1f◀
File: emacs Node: Fixit, Prev: Search, Up: Top, Next: Files

Commands for Fixing Typos
*************************

In this chapter we describe the commands that are especially useful for
the times when you catch a mistake in your text just after you have made
it, or change your mind while composing text on line.

* Menu:

* Kill Errors:: Commands to kill a batch of recently entered text.
* Transpose:: Exchanging two characters, words, lines, lists...
* Fixing Case:: Correcting case of last word entered.
* Spelling:: Apply spelling checker to a word, or a whole file.

▶1f◀
File: emacs Node: Kill Errors, Prev: Fixit, Up: Fixit, Next: Transpose

Killing Your Mistakes
=====================

`DEL'
Delete last character (`delete-backward-char').
`M-DEL'
Kill last word (`backward-kill-word').
`C-x DEL'
Kill to beginning of sentence (`backward-kill-sentence').

The DEL character (`delete-backward-char') is the most
important correction command. When used among graphic (self-inserting)
characters, it can be thought of as canceling the last character typed.

When your mistake is longer than a couple of characters, it might be more
convenient to use `M-DEL' or `C-x DEL'. `M-DEL' kills back to the start of
the last word, and `C-x DEL' kills back to the start of the last sentence.
`C-x DEL' is particularly useful when you are thinking of what to write as
you type it, in case you change your mind about phrasing. `M-DEL' and `C-x
DEL' save the killed text for `C-y' and `M-y' to retrieve. *Note
Yanking::.

`M-DEL' is often useful even when you have typed only a few
characters wrong, if you know you are confused in your typing and aren't
sure exactly what you typed. At such a time, you cannot correct with
DEL except by looking at the screen to see what you did. It requires
less thought to kill the whole word and start over again.

▶1f◀
File: emacs Node: Transpose, Prev: Kill Errors, Up: Fixit, Next: Fixing Case

Transposing Text
================

`C-t'
Transpose two characters (`transpose-chars').
`M-t'
Transpose two words (`transpose-words').
`C-M-t'
Transpose two balanced expressions (`transpose-sexps').
`C-x C-t'
Transpose two lines (`transpose-lines').

The common error of transposing two characters can be fixed, when they
are adjacent, with the `C-t' command (`transpose-chars'). Normally,
`C-t' transposes the two characters on either side of point. When
given at the end of a line, rather than transposing the last character of
the line with the newline, which would be useless, `C-t' transposes the
last two characters on the line. So, if you catch your transposition error
right away, you can fix it with just a `C-t'. If you don't catch it so
fast, you must move the cursor back to between the two transposed
characters. If you transposed a space with the last character of the word
before it, the word motion commands are a good way of getting there.
Otherwise, a reverse search (`C-r') is often the best way.
*Note Search::.

`Meta-t' (`transpose-words') transposes the word before point
with the word after point. It moves point forward over a word, dragging
the word preceding or containing point forward as well. The punctuation
characters between the words do not move. For example, `FOO, BAR'
transposes into `BAR, FOO' rather than `BAR FOO,'.

`C-M-t' (`transpose-sexps') is a similar command for transposing
two expressions (*Note Lists::), and `{C-x C-t} (`transpose-lines')'exchanges lines. They work like `M-t' except in determining the
division of the text into syntactic units.

A numeric argument to a transpose command serves as a repeat count: it
tells the transpose command to move the character (word, sexp, line) before
or containing point across several other characters (words, sexps, lines).
For example, `C-u 3 C-t' moves the character before point forward across
three other characters. This is equivalent to repeating `C-t' three times.
`C-u - 4 M-t' moves the word before point backward across four words. `C-u
- C-M-t' would cancel the effect of plain `C-M-t'.

A numeric argument of zero is assigned a special meaning (because
otherwise a command with a repeat count of zero would do nothing): to
transpose the character (word, sexp, line) ending after point with the
one ending after the mark.

▶1f◀
File: emacs Node: Fixing Case, Prev: Transpose, Up: Fixit, Next: Spelling

Case Conversion
===============

`M-- M-l'
Convert last word to lower case. Note `Meta--' is Meta-minus.
`M-- M-u'
Convert last word to all upper case.
`M-- M-c'
Convert last word to lower case with capital initial.

A very common error is to type words in the wrong case. Because of this,
the word case-conversion commands `M-l', `M-u' and `M-c' have a special
feature when used with a negative argument: they do not move the cursor.
As soon as you see you have mistyped the last word, you can simply
case-convert it and go on typing. *Note Case::.

▶1f◀
File: emacs Node: Spelling, Prev: Fixing Case, Up: Fixit

Checking and Correcting Spelling
================================

`M-$'
Check and correct spelling of word (`spell-word').
`M-x spell-buffer'
Check and correct spelling of each word in the buffer.
`M-x spell-region'
Check and correct spelling of each word in the region.
`M-x spell-string'
Check spelling of specified word.

To check the spelling of the word before point, and optionally correct it
as well, use the command `M-$' (`spell-word'). This command runs
an inferior process containing the `spell' program to see whether the
word is correct English. If it is not, it asks you to edit the word (in
the minibuffer) into a corrected spelling, and then does a `query-replace'
to substitute the corrected spelling for the old one throughout the buffer.

If you exit the minibuffer without altering the original spelling, it
means you do not want to do anything to that word. Then the `query-replace'
is not done.

`M-x spell-buffer' checks each word in the buffer the same way that
`spell-word' does, doing a `query-replace' if appropriate for every
incorrect word.

`M-x spell-region' is similar but operates only on the region, not
the entire buffer.

`M-x spell-string' reads a string as an argument and checks whether
that is a correctly spelled English word. It prints in the echo area a
message giving the answer.

▶1f◀
File: emacs Node: Files, Prev: Fixit, Up: Top, Next: Buffers

File Handling
*************

The basic unit of stored data in Unix is the "file". To edit a file,
you must tell Emacs to examine the file and prepare a buffer containing a
copy of the file's text. This is called "visiting" the file. Editing
commands apply directly to text in the buffer; that is, to the copy inside
Emacs. Your changes appear in the file itself only when you "save" the
buffer back into the file.

In addition to visiting and saving files, Emacs can delete, copy, rename,
and append to files, and operate on file directories.

* Menu:

* File Names:: How to type and edit file name arguments.
* Visiting:: Visiting a file prepares Emacs to edit the file.
* Saving:: Saving makes your changes permanent.
* Reverting:: Reverting cancels all the changes not saved.
* Auto Save:: Auto Save periodically protects against loss of data.
* ListDir:: Listing the contents of a file directory.
* Dired:: "Editing" a directory to delete, rename, etc.
the files in it.
* Misc File Ops:: Other things you can do on files.

▶1f◀
File: emacs Node: File Names, Prev: Files, Up: Files

File Names
==========

Most Emacs commands that operate on a file require you to specify the
file name. (Saving and reverting are exceptions; the buffer knows which
file name to use for them.) File names are specified using the minibuffer
(*Note Minibuffer::). "Completion" is available, to make it easier to
specify long file names. *Note Completion::.

There is always a "default file name" which will be used if you type
just RET, entering an empty argument. Normally the default file name
is the name of the file visited in the current buffer; this makes it easy
to operate on that file with any of the Emacs file commands.

Each buffer has a default directory, normally the same as the directory
of the file visited in that buffer. When Emacs reads a file name, if you
do not specify a directory, the default directory is used. If you specify
a directory in a relative fashion, with a name that does not start with a
slash, it is interpreted with respect to the default directory. The
default directory is kept in the variable `default-directory', which
has a separate value in every buffer.

For example, if the default file name is `/u/rms/gnu/gnu.tasks' then
the default directory is `/u/rms/gnu/'. If you type just `foo',
which does not specify a directory, it is short for `/u/rms/gnu/foo'.
`../.login' would stand for `/u/rms/.login'. `new/foo'
would stand for the filename `/u/rms/gnu/new/foo'.

The command `M-x pwd' prints the current buffer's default directory,
and the command `M-x cd' sets it (to a value read using the
minibuffer). A buffer's default directory changes only when the `cd'
command is used. A file-visiting buffer's default directory is initialized
to the directory of the file that is visited there. If a buffer is made
randomly with `C-x b', its default directory is copied from that of the
buffer that was current at the time.

The default directory actually appears in the minibuffer when the
minibuffer becomes active to read a file name. This serves two purposes:
it shows you what the default is, so that you can type a relative file name
and know with certainty what it will mean, and it allows you to edit the
default to specify a different directory. This insertion of the default
directory is inhibited if the variable `insert-default-directory' is
set to `nil'.

Note that it is legitimate to type an absolute file name after you enter
the minibuffer, ignoring the presence of the default directory name as part
of the text. The final minibuffer contents may look invalid, but that is
not so. *Note Minibuffer File::.

▶1f◀
File: emacs Node: Visiting, Prev: File Names, Up: Files, Next: Saving

Visiting Files
==============

`C-x C-f'
Visit a file (`find-file').
`C-x C-v'
Visit a different file instead of the one visited last
(`find-alternate-file').
`C-x 4 C-f'
Visit a file, in another window (`find-file-other-window'). Don't
change this window.

"Visiting" a file means copying its contents into Emacs where you can
edit them. Emacs makes a new buffer for each file that you visit. We say
that the buffer is visiting the file that it was created to hold. Emacs
constructs the buffer name from the file name by throwing away the
directory, keeping just the name proper. For example, a file named
`/usr/rms/emacs.tex' would get a buffer named `emacs.tex'. If
there is already a buffer with that name, a unique name is constructed by
appending `<2>', `<3>', or so on, using the lowest number that
makes a name that is not already in use.

Each window's mode line shows the name of the buffer that is being displayed
in that window, so you can always tell what buffer you are editing.

The changes you make with Emacs are made in the Emacs buffer. They do
not take effect in the file that you visited, or any place permanent, until
you "save" the buffer. Saving the buffer means that Emacs writes the
current contents of the buffer into its visited file. *Note Saving::.

If a buffer contains changes that have not been saved, the buffer is said
to be "modified". This is important because it implies that some
changes will be lost if the buffer is not saved. The mode line displays
two stars near the left margin if the buffer is modified.

To visit a file, use the command `C-x C-f' (`find-file'). Follow
the command with the name of the file you wish to visit, terminated by a
RET.

The file name is read using the minibuffer (*Note Minibuffer::), with
defaulting and completion in the standard manner (*Note File Names::).
While in the minibuffer, you can abort `C-x C-f' by typing `C-g'.

Your confirmation that `C-x C-f' has completed successfully is the
appearance of new text on the screen and a new buffer name in the mode
line. If the specified file does not exist and could not be created, or
cannot be read, then an error results. The error message is printed in the
echo area, and includes the file name which Emacs was trying to visit.

If you visit a file that is already in Emacs, `C-x C-f' does not make
another copy. It selects the existing buffer containing that file.
However, before doing so, it checks that the file itself has not changed
since you visited or saved it last. If the file has changed, a warning
message is printed. *Note Simultaneous Editing: Interlocking.

What if you want to create a file? Just visit it. Emacs prints
`(New File)' in the echo area, but in other respects behaves as if you
had visited an existing empty file. If you make any changes and save them,
the file is created.

If you visit a nonexistent file unintentionally (because you typed the
wrong file name), use the `C-x C-v' (`find-alternate-file')
command to visit the file you wanted. `C-x C-v' is similar to `C-x
C-f', but it kills the current buffer (after first offering to save it if
it is modified). `C-x C-v' is allowed even if the current buffer
is not visiting a file.

If the file you specify is actually a directory, Dired is called on that
directory (*Note Dired::). This can be inhibited by setting the variable
`find-file-run-dired' to `nil'; then it is an error to try to
visit a directory.

`C-x 4 f' (`find-file-other-window') is like `C-x C-f'
except that the buffer containing the specified file is selected in another
window. The window that was selected before `C-x 4 f' continues to
show the same buffer it was already showing. If this command is used when
only one window is being displayed, that window is split in two, with one
window showing the same before as before, and the other one showing the
newly requested file. *Note Windows::.

There are two hook variables that allow extensions to modify the
operation of visiting files. Visiting a file that does not exist runs the
functions in the list `find-file-not-found-hooks'; the value of this
variable is expected to be a list of functions, and the functions are
called one by one until one of them returns non-`nil'. Any visiting
of a file, whether extant or not, expects `find-file-hooks' to
contain list of functions and calls them all, one by one. In both cases
the functions receive no arguments. Visiting a nonexistent file
runs the `find-file-not-found-hooks' first.

▶1f◀
File: emacs Node: Saving, Prev: Visiting, Up: Files, Next: Reverting

Saving Files
============

"Saving" a buffer in Emacs means writing its contents back into the file
that was visited in the buffer.

`C-x C-s'
Save the current buffer in its visited file (`save-buffer').
`C-x s'
Save any or all buffers in their visited files (`save-some-buffers').
`M-~'
Forget that the current buffer has been changed (`not-modified').
`C-x C-w'
Save the current buffer in a specified file, and record that file as
the one visited in the buffer (`write-file').
`M-x set-visited-file-name'
Change file the name under which the current buffer will be saved.

When you wish to save the file and make your changes permanent, type
`C-x C-s' (`save-buffer'). After saving is finished, `C-x C-s'
prints a message such as

Wrote /u/rms/gnu/gnu.tasks

If the selected buffer is not modified (no changes have been made in it
since the buffer was created or last saved), saving is not really done,
because it would have no effect. Instead, `C-x C-s' prints a message
in the echo area saying

(No changes need to be written)

The command `C-x s' (`save-some-buffers') can save any or all modified
buffers. First it asks, for each modified buffer, whether to save it.
These questions should be answered with `y' or `n'. `C-x C-c',
the key that kills Emacs, invokes `save-some-buffers' and therefore
asks the same questions.

If you have changed a buffer and do not want the changes to be saved, you
should take some action to prevent it. Otherwise, each time you use
`save-some-buffers' you are liable to save it by mistake. One thing
you can do is type `M-~' (`not-modified'), which clears out the
indication that the buffer is modified. If you do this, none of the save
commands will believe that the buffer needs to be saved. (`~' is often
used as a mathematical symbol for `not'; thus `Meta-~' is `not', metafied.)
You could also use `set-visited-file-name' (see below) to mark the
buffer as visiting a different file name, one which is not in use for
anything important. Alternatively, you can undo all the changes made since
the file was visited or saved, by reading the text from the file again.
This is called "reverting". *Note Reverting::. You could also undo all
the changes by repeating the undo command `C-x u' until you have undone
all the changes; but this only works if you have not made more changes than
the undo mechanism can remember.

`M-x set-visited-file-name' alters the name of the file that the
current buffer is visiting. It reads the new file name using the
minibuffer. It can be used on a buffer that is not visiting a file, too.
The buffer's name is changed to correspond to the file it is now visiting
in the usual fashion (unless the new name is in use already for some other
buffer; in that case, the buffer name is not changed).
`set-visited-file-name' does not save the buffer in the newly visited
file; it just alters the records inside Emacs so that, if you save the
buffer, it will be saved in that file. It also marks the buffer as
"modified" so that `C-x C-s' will save.

If you wish to mark the buffer as visiting a different file and save it
right away, use `C-x C-w' (`write-file'). It is precisely
equivalent to `set-visited-file-name' followed by `C-x C-s'.
`C-x C-s' used on a buffer that is not visiting with a file has the
same effect as `C-x C-w'; that is, it reads a file name, marks the
buffer as visiting that file, and saves it there. The default file name in
a buffer that is not visiting a file is made by combining the buffer name
with the buffer's default directory.

If Emacs is about to save a file and sees that the date of the latest
version on disk does not match what Emacs last read or wrote, Emacs
notifies you of this fact, because it probably indicates a problem caused
by simultaneous editing and requires your immediate attention.
*Note Simultaneous Editing: Interlocking.

If the variable `require-final-newline' is non-`nil', Emacs
puts a newline at the end of any file that doesn't already end in one,
every time a file is saved or written.

You can implement other ways to write files, and other things to be done
before writing them, using the hook variable `write-file-hooks'. The
value of this variable should be a list of Lisp functions. When a file is
to be written, the functions in the list are called, one by one, with no
arguments. If one of them returns a non-`nil' value, Emacs takes this
to mean that the file has been written in some suitable fashion; the rest
of the functions are not called, and normal writing is not done.

* Menu:

* Backup:: How Emacs saves the old version of your file.
* Interlocking:: How Emacs protects against simultaneous editing
of one file by two users.

▶1f◀
File: emacs Node: Backup, Prev: Saving, Up: Saving, Next: Interlocking

Backup Files
------------

Because Unix does not provide version numbers in file names, rewriting a
file in Unix automatically destroys all record of what the file used to
contain. Thus, saving a file from Emacs throws away the old contents of
the file---or it would, except that Emacs carefully copies the old contents
to another file, called the "backup" file, before actually saving.
(Provided the variable `make-backup-files' is non-`nil'.
Backup files are not written if this variable is `nil').

At your option, Emacs can keep either a single backup file or a series of
numbered backup files for each file that you edit.

Emacs makes a backup for a file only the first time the file is saved
from one buffer. No matter how many times you save a file, its backup file
continues to contain the contents from before the file was visited.
Normally this means that the backup file contains the contents from before
the current editing session; however, if you kill the buffer and then visit
the file again, a new backup file will be made by the next save.

* Menu:

* Names: Backup Names. How backup files are named;
Choosing single or numbered backup files.
* Deletion: Backup Deletion. Emacs deletes excess numbered backups.
* Copying: Backup Copying. Backups can be made by copying or renaming.

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File: emacs Node: Backup Names, Prev: Backup, Up: Backup, Next: Backup Deletion

Single or Numbered Backups
..........................

If you choose to have a single backup file (this is the default),
the backup file's name is constructed by appending `~' to the
file name being edited; thus, the backup file for `eval.c' would
be `eval.c~'.

If you choose to have a series of numbered backup files, backup file
names are made by appending `.~', the number, and another `~' to
the original file name. Thus, the backup files of `eval.c' would be
called `eval.c.~1~', `eval.c.~2~', and so on, through names
like `eval.c.~259~' and beyond.

If protection stops you from writing backup files under the usual names,
the backup file is written as `%backup%~' in your home directory.
Only one such file can exist, so only the most recently made such backup is
available.

The choice of single backup or numbered backups is controlled by the
variable `version-control'. Its possible values are

`t'
Make numbered backups.
`nil'
Make numbered backups for files that have numbered backups already.
Otherwise, make single backups.
`never'
Do not in any case make numbered backups; always make single backups.

`version-control' may be set locally in an individual buffer to
control the making of backups for that buffer's file. For example,
Rmail mode locally sets `version-control' to `never' to make sure
that there is only one backup for an Rmail file. *Note Locals::.

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File: emacs Node: Backup Deletion, Prev: Backup Names, Up: Backup, Next: Backup Copying

Automatic Deletion of Backups
.............................

To prevent unlimited consumption of disk space, Emacs can delete numbered
backup versions automatically. Generally Emacs keeps the first few backups
and the latest few backups, deleting any in between. This happens every
time a new backup is made. The two variables that control the deletion are
`kept-old-versions' and `kept-new-versions'. Their values are, respectively
the number of oldest (lowest-numbered) backups to keep and the number of
newest (highest-numbered) ones to keep, each time a new backup is made.
Recall that these values are used just after a new backup version is made;
that newly made backup is included in the count in `kept-new-versions'.
By default, both variables are 2.

If `trim-versions-without-asking' is non-`nil', the excess
middle versions are deleted without a murmur. If it is `nil', the
default, then you are asked whether the excess middle versions should
really be deleted.

Dired's `.' (Period) command can also be used to delete old versions.
*Note Dired::.

▶1f◀
File: emacs Node: Backup Copying, Prev: Backup Deletion, Up: Backup

Copying vs. Renaming
....................

Backup files can be made by copying the old file or by renaming it. This
makes a difference when the old file has multiple names. If the old file
is renamed into the backup file, then the alternate names become names for
the backup file. If the old file is copied instead, then the alternate
names remain names for the file that you are editing, and the contents
accessed by those names will be the new contents.

The method of making a backup file may also affect the file's owner
and group. If copying is used, these do not change. If renaming is used,
you become the file's owner, and the file's group becomes the default
(different operating systems have different defaults for the group).

Having the owner change is usually a good idea, because then the owner
always shows who last edited the file. Also, the owners of the backups
show who produced those versions. Occasionally there is a file whose
owner should not change; it is a good idea for such files to contain
local variable lists to set `backup-by-copying-when-mismatch' for
them alone (*Note File Variables::).

The choice of renaming or copying is controlled by three variables.
Normally, renaming is done. If the variable `backup-by-copying' is
non-`nil', copying is used. Otherwise, if the variable
`backup-by-copying-when-linked' is non-`nil', then copying is done for
files that have multiple names, but renaming may still done when the file
being edited has only one name. If the variable
`backup-by-copying-when-mismatch' is non-`nil', then copying is done if
renaming would cause the file's owner or group to change.

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File: emacs Node: Interlocking, Prev: Backup, Up: Saving

Protection against Simultaneous Editing
---------------------------------------

Simultaneous editing occurs when two users visit the same file, both make
changes, and then both save them. If nobody were informed that this was
happening, whichever user saved first would later find that his changes
were lost. On some systems, Emacs notices immediately when the second user
starts to change the file, and issues an immediate warning. When this is
not possible, or if the second user has gone on to change the file despite
the warning, Emacs checks later when the file is saved, and issues a second
warning when a user is about to overwrite a file containing another user's
changes. If the editing user takes the proper corrective action at this
point, he can prevent actual loss of work.

When you make the first modification in an Emacs buffer that is visiting
a file, Emacs records that you have locked the file. (It does this by
writing another file in a directory reserved for this purpose.) The lock
is removed when you save the changes. The idea is that the file is locked
whenever the buffer is modified. If you begin to modify the buffer while
the visited file is locked by someone else, this constitutes a collision,
and Emacs asks you what to do. It does this by calling the Lisp function
`ask-user-about-lock', which you can redefine for the sake of
customization. The standard definition of this function asks you a
question and accepts three possible answers:

`s'
Steal the lock. Whoever was already changing the file loses the lock,
and you gain the lock.
`p'
Proceed. Go ahead and edit the file despite its being locked by someone else.
`q'
Quit. This causes an error (`file-locked') and the modification you
were trying to make in the buffer does not actually take place.

Note that locking works on the basis of a file name; if a file has
multiple names, Emacs does not realize that the two names are the same file
and cannot prevent two user from editing it simultaneously under different
names. However, basing locking on names means that Emacs can interlock the
editing of new files that will not really exist until they are saved.

Some systems are not configured to allow Emacs to make locks. On
these systems, Emacs cannot detect trouble in advance, but it still can
detect it in time to prevent you from overwriting someone else's changes.

Every time Emacs saves a buffer, it first checks the last-modification
date of the existing file on disk to see that it has not changed since the
file was last visited or saved. If the date does not match, it implies
that changes were made in the file in some other way, and these changes are
about to be lost if Emacs actually does save. To prevent this, Emacs
prints a warning message and asks for confirmation before saving.
Occasionally you will know why the file was changed and know that it does
not matter; then you can answer `yes' and proceed. Otherwise, you should
cancel the save with `C-g' and investigate the situation.

The first thing you should do when notified that simultaneous editing has
already taken place is to list the directory with `C-u C-x C-d' (*Note
Directory Listing: ListDir.). This will show the file's current author.
You should attempt to contact him to warn him not to continue editing.
Often the next step is to save the contents of your Emacs buffer under a
different name, and use `diff' to compare the two files.

Simultaneous editing checks are also made when you visit with `C-x
C-f' a file that is already visited and when you start to modify a file.
This is not strictly necessary, but it can cause you to find out about the
problem earlier, when perhaps correction takes less work.

▶1f◀
File: emacs Node: Reverting, Prev: Saving, Up: Files, Next: Auto Save

Reverting a Buffer
==================

If you have made extensive changes to a file and then change your mind
about them, you can get rid of them by reading in the previous version of
the file. To do this, use `M-x revert-buffer', which operates on the
current buffer. Since this is a very dangerous thing to do, you must
confirm it with `yes'.

If the current buffer has been auto-saved more recently than it has been
saved for real, `revert-buffer' offers to read the auto save file
instead of the visited file (*Note Auto Save::). This question comes
before the usual request for confirmation, and demands `y' or `n'
as an answer. If you have started to type `yes' for confirmation
without realizing that the other question was going to be asked, the
`y' will answer that question, but the `es' will not be valid
confirmation. So you will have a chance to cancel the operation with
`C-g' and try it again with the answers that you really intend.

`revert-buffer' keeps point at the same distance (measured in
characters) from the beginning of the file. If the file was edited only
slightly, you will be at approximately the same piece of text after
reverting as before. If you have made drastic changes, the same value of
point in the old file may address a totally different piece of text.

A buffer reverted from its visited file is marked "not modified" until
another change is made.

Some kinds of buffers whose contents reflect data bases other than files,
such as Dired buffers, can also be reverted. For them, reverting means
recalculating their contents from the appropriate data base. Buffers
created randomly with `C-x b' cannot be reverted; `revert-buffer'
reports an error when asked to do so.

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