6.3.4 Reading Lines of a File using Standard Input

READ-LINE will read one line from a stream (which defaults to ‘*STANDARD INPUT*’) 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)))

Function: read-line &optional input-stream eof-error-p eof-value recursive-p => line ¶

Reads from input-stream a line of text that is terminated by a newline or end of file. Two values are returned: the primary value is the line that was read, while the secondary value is a boolean that is ‘false’ if the line was terminated by a newline, or ‘true’ if the line was terminated by the end of file for input-stream (or if the line is the eof-value).

INPUT-STREAM

an input stream designator. The default is ‘*STANDARD-INPUT*’.

EOF-ERROR-P

If an end of file occurs before any characters are read in the line, an error is signaled if ‘eof-error-p’ is ‘true’. If ‘eof-error-p’ is ‘false’ and the end of file for ‘input-stream’ is reached before any characters are read, ‘eof-value’ is returned as the line.

EOF-VALUE

an object. The default is ‘nil’.

= RECURSIVE-P :: A boolean. The default is ‘false’.

RETURN

‘LINE’ is the line that is read, represented as a string (without the trailing newline, if any).

 (setq a "line 1
 line2")
=>  "line 1
 line2"
 (read-line (setq input-stream (make-string-input-stream a)))
=>  "line 1", false
 (read-line input-stream)
=>  "line2", true
 (read-line input-stream nil nil)
=>  NIL, true