Linux – How Does Linux Handle Multiple Consecutive Path Separators?

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I'm working on a python script that passes file locations to an scp subprocess. That's all fine, but I'm in a situation where I may end up concatenating a path with a filename such that there's a double '/ in the path. I know that bash doesn't care if you have multiple file separators, but I'm wondering how exactly that is rectified. Is it bash that strips extra /s or does it really not matter ever?

I ask because it will save me several lines of code to check for extra /s while concatenating. I know it's not a big deal, but I'm curious as well. I have a bash script that has the line cd //usr (instead of cd /usr), which seems to imply there might be a significance to using multiple /s in a path

Best Answer

Multiple slashes are allowed and are equivalent to a single slash. From the Single Unix specification (version 4), base definitions §3.271 pathname: “Multiple successive slashes are considered to be the same as one slash.”

There is one exception: If a pathname begins with two successive slash characters, the first component following the leading slash characters may be interpreted in an implementation-defined manner. (ref: base definitions §4.13 pathname resolution). Linux itself doesn't do this, though some applications might, and other unix-ish system do (e.g. Cygwin).

A trailing / at the end of a pathname forces the pathname to refer to a directory. In (POSIX 1003.1-2001 (Single Unix v4) base definitions §4.11 pathname resolution, a trailing / is equivalent to a trailing /.. POSIX 1003.1-2008 (Single Unix v4) base definitions §4.13 removes the requirement to make it equivalent to /., in order to cope with non-existing directories (e.g. mkdir foo/ is required to work, whereas mkdir foo/. wouldn't — see the rationale for the change).

For programs that act on a directory entry, if foo is a symbolic link to a directory, then passing foo/ is a way to make the program act on the directory instead of the symbolic link.

¹ Note that this applies for pathname resolution only, i.e. when accessing files. Filename manipulations may work differently. For example basename and dirname ignore trailing slashes.

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