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How can I tell if two files are hard-linked from the command line? e.g. something link this:

$ ls
fileA fileB fileC

$ is-hardlinked fileA fileB
yes

$ is-hardlinked fileA fileC
no
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3 Answers 3

up vote 16 down vote accepted

On most filesystems¹, a file is uniquely determined by its inode number, so all you need to check is whether the two files have the same inode number and are on the same filesystem.

Ash, ksh, bash and zsh have a construct that does the check for you: the file equality operator -ef.

[ fileA -ef fileB ] && ! [ fileA -ef fileC ]

For more advanced cases, ls -i /path/to/file lists a file's inode number. df -P /path/to/file shows what filesystem the file is on (if two files are in the same directory, they're on the same filesystem). If your system has the stat command, it can probably show the inode and filesystem numbers (stat varies from system to system, check your documentation). If you want a quick glance of hard links inside a directory, try ls -i | sort (possibly piped to awk).

¹ All native unix filesystems, and a few others such as NTFS, but possibly not exotic cases like CramFS.

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And definitely not on anything FAT-based, where it would be detected as a "cross-linked" file. –  Ignacio Vazquez-Abrams Nov 8 '11 at 0:16
3  
Note that fileA -ef fileB also returns 0 (success) if fileA is a symlink to fileB, or vice versa, or they both link to the same file. –  janmoesen Nov 8 '11 at 16:50
    
how do you actually run the command you suggested? I tried [ .bashrc -ef .bash/.bashrc ] and variations of this but it didn't really work. –  Charlie Parker Jun 15 at 4:33
    
@CharlieParker [ .bashrc -ef .bash/.bashrc ] is correct. Without context, of course, I have no idea why it “didn't really work” — you could be comparing the wrong files, you could be not checking the outcome correctly, you could be using a shell without -ef, ... –  Gilles Jun 15 at 10:49
1  
@CharlieParker The command is actually [ and is a synonym of test. But man [ or man test will give you the man page of the external command, whereas just about every shell out there has a built-in command with slightly different options, so you need to look this one up in your shell's manual. –  Gilles Jun 15 at 18:33
function is-hardlinked() {
    r=yes
    [ "`stat -c '%i' $1`" != "`stat -c '%i' $2`" ] && r=no
    echo $r
}
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Note that this can be a false positive if the two files are on different filesystems but happen to have the same inode. You need to test the device number as well (stat -c %d). And if you're on Linux (given your stat command), your shell has the [ fileA -ef fileB ] to do all this directly. Also, your command gratuitously breaks with file names containing whitespace or \[?*, or begins with -: always put double quotes around command susbtitutions ("$(stat -c %i -- "$1")"). –  Gilles Jan 29 '13 at 15:41

As the first poster suggest, you can write a script based on something like this on Linux:

stat -c '%i' fileA fileB fileC
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1  
This isn't enough: you'll get the same number for the two files if are on different filesystems but happen to have the same inode. You need to test the device number as well (stat -c %d). And if you're on Linux (given your stat command), your shell has the [ fileA -ef fileB ] to do all this directly. –  Gilles Jan 29 '13 at 15:42

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