Testing whether a File Exists
edi@bird:/tmp> ln -s /etc/passwd foo edi@bird:/tmp> cmucl ; Loading #p"/home/edi/.cmucl-init". CMU Common Lisp 18d-pre, level-1 built 2002-01-15 on maftia1, running on bird Send questions to firstname.lastname@example.org. and bug reports to email@example.com. Loaded subsystems: Python native code compiler, target Intel x86 CLOS based on PCL version: September 16 92 PCL (f) Gray Streams Protocol Support CLX X Library MIT R5.02 * (probe-file "/etc/passwd") #p"/etc/passwd" * (probe-file "foo") #p"/etc/passwd" * (probe-file "bar") NIL
Opening a File
Common Lisp has
functions which resemble the functions of the same denominator from other
programming languages you’re probably familiar with. However, it is almost
always recommendable to use the macro
instead. Not only will this macro open the file for you and close it when you’re
done, it’ll also take care of it if your code leaves the body abnormally (such
as by a use of
typical use of WITH-OPEN-FILE looks like this:
(with-open-file (str <_file-spec_> :direction <_direction_> :if-exists <_if-exists_> :if-does-not-exist <_if-does-not-exist_>) <_your code here_>)
STRis a variable which’ll be bound to the stream which is created by opening the file.
- <_file-spec_> will be a truename or a pathname.
- <_direction_> is usually `:INPUT` (meaning you want to read from the file), `:OUTPUT` (meaning you want to write to the file) or `:IO` (which is for reading _and_ writing at the same time) - the default is `:INPUT`.
- <_if-exists_> specifies what to do if you want to open a file for writing and a file with that name already exists - this option is ignored if you just want to read from the file. The default is `:ERROR` which means that an error is signalled. Other useful options are `:SUPERSEDE` (meaning that the new file will replace the old one), `NIL` (the stream variable will be bound to `NIL`), and `:RENAME` (i.e. the old file is renamed).
- <_if-does-not-exist_> specifies what to do if the file you want to open does not exist. It is one of `:ERROR` for signalling an error, `:CREATE` for creating an empty file, or `NIL` for binding the stream variable to `NIL`. The default is, to be brief, to do the right thing depending on the other options you provided. See the CLHS for details.
Note that there are a lot more options to WITH-OPEN-FILE. See
the CLHS entry for
for all the details. You’ll find some examples on how to use WITH-OPEN-FILE
below. Also note that you usually don’t need to provide any keyword arguments if
you just want to open an existing file for reading.
Using Strings instead of Files
Reading a File one Line at a Time
will read one line from a stream (which defaults to
the end of which is determined by either a newline character or the end of the
file. It will return this line as a string without the trailing newline
character. (Note that READ-LINE has a second return value which is true if there
was no trailing newline, i.e. if the line was terminated by the end of the
file.) READ-LINE will by default signal an error if the end of the file is
reached. You can inhibit this by supplying NIL as the second argument. If you do
this, READ-LINE will return NIL if it reaches the end of the file.
(with-open-file (stream "/etc/passwd") (do ((line (read-line stream nil) (read-line stream nil))) ((null line)) (print line)))
You can also supply a third argument which will be used instead of NIL to signal the end of the file:
(with-open-file (stream "/etc/passwd") (loop for line = (read-line stream nil 'foo) until (eq line 'foo) do (print line)))
Reading a File one Character at a Time
is similar to READ-LINE, but it only reads one character as opposed to one
line. Of course, newline characters aren’t treated differently from other
characters by this function.
(with-open-file (stream "/etc/passwd") (do ((char (read-char stream nil) (read-char stream nil))) ((null char)) (print char)))
Reading a File into String
It’s quite common to need to access the contents of a file in string
form. While this can be achieved by using
that probably won’t be the best solution. File might not be divided into
multiple lines or reading one character at a time might bring significant
performance problems. To solve this problems, you can read files using buckets
of specific sizes.
(with-output-to-string (out) (with-open-file (in "/path/to/big/file") (loop with buffer = (make-array 8192 :element-type 'character) for n-characters = (read-sequence buffer in) while (< 0 n-characters) do (write-sequence buffer out :start 0 :end n-characters)))))
Furthermore, you’re free to change the format of the read/written data, instead
of using elements of type character everytime. For instance, you can set
:ELEMENT-TYPE type argument of
MAKE-ARRAY functions to
'(UNSIGNED-BYTE 8) to read data in octets.
Looking one Character ahead
You can ‘look at’ the next character of a stream without actually removing it
from there - this is what the function
is for. It can be used for three different purposes depending on its first
(optional) argument (the second one being the stream it reads from): If the
first argument is
NIL, PEEK-CHAR will just return the next character that’s
waiting on the stream:
CL-USER> (with-input-from-string (stream "I'm not amused") (print (read-char stream)) (print (peek-char nil stream)) (print (read-char stream)) (values)) #\I #\' #\'
If the first argument is
T, PEEK-CHAR will skip
characters, i.e. it will return the next non-whitespace character that’s waiting
on the stream. The whitespace characters will vanish from the stream as if they
had been read by READ-CHAR:
CL-USER> (with-input-from-string (stream "I'm not amused") (print (read-char stream)) (print (read-char stream)) (print (read-char stream)) (print (peek-char t stream)) (print (read-char stream)) (print (read-char stream)) (values)) #\I #\' #\m #\n #\n #\o
If the first argument to PEEK-CHAR is a character, the function will skip all characters until that particular character is found:
CL-USER> (with-input-from-string (stream "I'm not amused") (print (read-char stream)) (print (peek-char #\a stream)) (print (read-char stream)) (print (read-char stream)) (values)) #\I #\a #\a #\m
Note that PEEK-CHAR has further optional arguments to control its behaviour on end-of-file similar to those for READ-LINE and READ-CHAR (and it will signal an error by default):
CL-USER> (with-input-from-string (stream "I'm not amused") (print (read-char stream)) (print (peek-char #\d stream)) (print (read-char stream)) (print (peek-char nil stream nil 'the-end)) (values)) #\I #\d #\d THE-END
You can also put one character back onto the stream with the function
can use it as if, after you have read a character, you decide that you’d
better used PEEK-CHAR instead of READ-CHAR:
CL-USER> (with-input-from-string (stream "I'm not amused") (let ((c (read-char stream))) (print c) (unread-char c stream) (print (read-char stream)) (values))) #\I #\I
Note that the front of a stream doesn’t behave like a stack: You can only put back exactly one character onto the stream. Also, you must put back the same character that has been read previously, and you can’t unread a character if none has been read before.
Random Access to a File
Use the function
for random access to a file. If this function is used with one argument (a
stream), it will return the current position within the stream. If it’s used
with two arguments (see below), it will actually change the
in the stream.
CL-USER> (with-input-from-string (stream "I'm not amused") (print (file-position stream)) (print (read-char stream)) (print (file-position stream)) (file-position stream 4) (print (file-position stream)) (print (read-char stream)) (print (file-position stream)) (values)) 0 #\I 1 4 #\n 5
Getting file attributes (size, access time,…), with the Osicat library
Osicat (in Quicklisp) is a lightweight operating system interface for Common Lisp on POSIX-like systems, including Windows. With Osicat we can get and set environment variables, manipulate files and directories, pathnames and a bit more.
Once it is installed, Osicat also defines the
which permits us to get file attributes.
(ql:quickload :osicat) (let ((stat (osicat-posix:stat #P"./files.md"))) (osicat-posix:stat-size stat)) ;; => 10629
We can get the other attributes with the following methods:
osicat-posix:stat-dev osicat-posix:stat-gid osicat-posix:stat-ino osicat-posix:stat-uid osicat-posix:stat-mode osicat-posix:stat-rdev osicat-posix:stat-size osicat-posix:stat-atime osicat-posix:stat-ctime osicat-posix:stat-mtime osicat-posix:stat-nlink osicat-posix:stat-blocks osicat-posix:stat-blksize