probe-file

(probe-file pathname)

Answers whether a file exists: the pathname when it does, nil when it does not. This is the only file operation that asks the question without opening anything, and the only one that does not fail on a missing path -- open (and therefore with-open-file) signals an error on every backend. Catching that error is a workable substitute, but a heavier one: handler-case puts the WASM backends into exception mode, and --no-gc rejects catching entirely, so a plain probe is the portable spelling. Works on all four backends. truename is the signalling twin of this function.

The "truename" answered on success is a pathname carrying the argument namestring: no backend resolves symbolic links or makes the path absolute. The argument takes either spelling (a pathname or a namestring). The path is interpreted exactly as open interprets it -- relative to the process working directory on the interpreter and the JVM, and to the preopened directories on WASM (a relative path against the first one, an absolute path against the preopened directory whose name is its longest prefix; run with --dir). A directory counts as existing. uiop:file-exists-p is the same operation under its ASDF/UIOP name.

(if (probe-file "config.lisp")
    (load "config.lisp")
    (print "no config"))

Here the file is loaded only when it is there; without probe-file the missing-file case would abort the program rather than take the else branch.