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I've read a lot about realpath command and how it has been deprecated with the readlink -f being now recommended. I have also seen in some places that the reason why realpath was introduced was for the lack of such functionality in readlink and that once it has been introduced, realpath was no longer needed and its support discontinued by most OS vendors.

The reason for my question is that I've also seen many people recommending readlink -f as a command "pretty much similar" to realpath, and that is what is bothering me, because no one elaborates on that "pretty much similar" part. What are the actual differences?

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There are several realpath commands around.

The realpath utility is a wrapper around the realpath library functions and has been reinvented many times.

Debian maintains a realpath package (separated from dwww since woody) which hasn't changed except regarding packaging and documentation since 2001. This utility is deprecated because there are now more standard alternatives (GNU readlink and soon GNU realpath), but at the time, GNU utilities didn't even have readlink at all. This implementation of realpath supports a few options to prevent symbolic link resolution or produce null-terminated output. BusyBox also includes its own realpath command (which takes no option).

GNU coreutils introduced a realpath command in version 8.15 in January 2012. This is a compatible replacement for BusyBox's and Debian's realpath, and also has many options in common with GNU readlink.

realpath has the same effect as readlink -f with GNU readlink. What distinguishes the two commands (or rather the various realpath commands from readlink -f) is the extra options that they support.

GNU realpath is not deprecated; it has the opposite problem: it's too new to be available everywhere. At this time, Debian still omits GNU realpath from its coreutils package and sticks with its own realpath. I don't know why, since GNU realpath should be a drop-in replacement. As of 14.04, Ubuntu doesn't gainsay Debian on this.

On Linux systems, at the moment, your best bet to canonicalize a path that may contain symbolic links is readlink -f.

BSD systems have a readlink command, with different capabilities from GNU readlink. In particular, BSD readlink does not have an option to canonicalize paths, it only traverses the symlink passed to it.

readlink, incidentally, had the same problem — it was also invented many times (not adding this utility when symbolic links were added to Unix was a regrettable omission). It has now stabilized in several implementations with many incompatible flags (in particular BSD vs. GNU).

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readlink -f was in OpenBSD long before GNU. All of NetBSD, FreeBSD and OpenBSD now have readlink -f (your link even mentions it). realpath has been in FreeBSD and IRIX for a long time (don't know if it predates the one in Debian). HPUX and IRIX also have readlink, though not -f. The realpath package in Debian experimental is now the one from coreutils (as an experiment to see if it breaks things). The dwww realpath acts more like readlink -e while the GNU one like readlink -f so it's not a complete dropin replace –  Stéphane Chazelas Jun 11 at 8:22
    
realpath has been in FreeBSD since 2002. Before that, pwd was doing it (since 2000, pwd some-file would call realpath() on file). Debian has had a realpath package since 1996. The one on IRIX probably predates it though I found no evidence other than it was in IRIX 6.5 in 1998. OpenBSD added a -f to readlink in 1997. GNU added readlink in 2003 and it had -f from the start. –  Stéphane Chazelas Jun 11 at 9:09
    
Excellent summary thanks. Note a better bug for the request for debian to switch to the GNU variant is bugs.debian.org/730779 Even the existing realpath maintainer wants the switch to happen –  Pádraig Brady Jun 11 at 12:46
    
Awesome answer. Missed only references to RHEL implementation of realpath. Does anyone know if it is somehow different from the readlink -f version? –  Felipe Leão Jun 11 at 12:50
    
@StéphaneChazelas Oh wow, lots of errors in my answers indeed. Thanks for pointing them out. Could you post a correct answer, and I'll delete mine? (Otherwise, I'll correct the errors, but it's more work to tack on to my long todo list…) –  Gilles Jun 11 at 18:05

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