Suppose I have variables dir and file containing strings representing a directory and a filename, respectively . What is the proper way in emacs lisp to join them into a full path to the file?
For example, if dir is "/usr/bin" and file is "ls", then I want "/usr/bin/ls". But if instead dir is "/usr/bin/", I still want the same thing, with no repeated slash.
This question was asked in 2010, but at the time of writing it's the top hit for searches like "join file paths in elisp", so I thought I'd update the answer.
Since 2010, things have moved on a lot in the world of Emacs. This is somewhat of a duplicate answer since it was mentioned briefly in an answer below, but I'll flesh it out a little. There's now a dedicated library for file interactions, f.el:
Much inspired by @magnars's excellent s.el and dash.el, f.el is a modern API for working with files and directories in Emacs.
Don't try to reinvent the wheel. You should use this library for file path manipulations. The function you want is f-join:
(f-join "path") ;; => "path"
(f-join "path" "to") ;; => "path/to"
(f-join "/" "path" "to" "heaven") ;; => "/path/to/heaven"
You may need to install the package first. It should be available on MELPA.
Just to complete what was said before with a link to the Emacs manual:
As others have said before, the answer to the OP question is to use the expand-file-name. That is a built-in function, implemented in C and therefore does not require the use of any external library.
This is described in the Emacs Lisp Manual section titled Functions that Expand Filenames.
And according to Emacs on-line help this function was introduced in version ... 1.6 of Emacs! So... it should be available!
For those who come to the question after 2021. elisp builtin function file-name-concat would do the job. It's much simpler now.
Document can be found in emacs with following keystroke:
C-h f file-name-concat <enter>
Append COMPONENTS to DIRECTORY and return the resulting string.
Elements in COMPONENTS must be a string or nil. DIRECTORY or the non-final elements in COMPONENTS may or may not end with a slash -- if they don't end with a slash, a slash will be inserted before contatenating.
Other relevant functions are documented in the file-name group.
Probably introduced at or before Emacs version 28.1.
This function does not change global state, including the match data.
(file-name-concat "/usr/bin/" "ls")
;; ==> "/usr/bin/ls"
(file-name-concat "/usr" "bin" "ls")
;; ==> "/usr/bin/ls"