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Is it possible to redefine the home directory? e.g. to /ext1/acheong instead of /home/acheong, i.e. expanding the ~ to another directory (as opposed to changing the actual home directory where users' home files are located).

(This question is mostly academic, as it seems like bad practice to do so. I also have no choice in the matter of using csh, despite having read the Top 10.)

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Do you mean change the actual home directory, or do you mean make the shell expand ~ to something other than the actual home directory? –  derobert Dec 4 '13 at 13:47
    
I meant expanding the ~ to another directory (not, if I understand your alternative correctly, changing where all users' home directories are to be located). I didn't realize that ~ was interpreted as a short form of $HOME, as @michas described in his answer. –  Andrew Cheong Dec 4 '13 at 14:42
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Not all users' home directories, you can change just one, as slm shows in his answer. –  derobert Dec 4 '13 at 14:44
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2 Answers

up vote 16 down vote accepted

The tilde ~ is interpreted by your shell. Your shell will interpret ~ as a short form of $HOME.

Try (echo ~; HOME=foo; echo ~). This should first print your real home directory and afterwards "foo", as you set $HOME to that.

The default value of $HOME comes from you system configuration. Use getent passwd to list all known users and their home directories. Depending on your system configuration those entries might come from /etc/passwd or any remote directory service.

If you only want to temporarily redefine your home directory, just set another $HOME.

If you permanently want to change it you have to change the passwd entry, e.g. by manually editing /etc/passwd.

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Thanks, @michas. Can't believe I missed that variable. (BTW, turns out in csh the relevant variable is $home (lowercase), and you can't setenv HOME /ext1/acheong—you can only set home=/ext1/acheong, and that worked for me.) –  Andrew Cheong Dec 4 '13 at 14:50
    
To change it permanently, you can also use usermod -d –  user606723 Dec 4 '13 at 15:02
    
Use vipw (or whatever GUI your OS provides) rather than editing /etc/passwd directly. Direct editing risks corrupting the file, which can be difficult to recover from. –  Keith Thompson Dec 4 '13 at 20:45
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The value that is used for ~ is determined from the value you get from the administrative database (getent passwd), typically in the /etc/passwd file, for each user's home directory that's defined there.

$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
sam:x:500:500:Sam Mingolelli:/home/sam:/bin/bash
tracy:x:501:501::/home/tracy:/bin/bash

The 6th column in this file is where the value used when someone types cd ~ comes from.

nsswitch

You can see what a system would use for the user's home directory using the command getent passwd:

$ getent passwd
root:x:0:0:root:/root:/bin/bash
sam:x:500:500:Sam Mingolelli:/home/sam:/bin/bash
tracy:x:501:501::/home/tracy:/bin/bash

The "database" that provides these is controlled through your systems resolver, defined in /etc/nsswitch.conf.

$ grep passwd /etc/nsswitch.conf 
#passwd:    db files nisplus nis
passwd:     files

Files above means /etc/passwd, but the "database" could come from LDAP, NIS, or other locations over the network, for example.

Moving/Redefining?

To perform this operation is a little tricky after the accounts have been created. If you're creating accounts from scratch then it's trivial to redefine a user's location of their home directory. When running the useradd command you can specify the location to be used for a user's home directory.

Example

$ useradd -d /ext1/acheong ...

excerpt from man page

-d, --home HOME_DIR
   The new user will be created using HOME_DIR as the value for the user’s 
   login directory. The default is to append the LOGIN name to BASE_DIR and 
   use that as the login directory name. The directory HOME_DIR does not 
   have to exist but will not be created if it is missing.

For existing accounts?

This becomes more of a surgical operation since often times the path of a user's home directory gets included statically in configuration files, making it trickier.

Example

$ grep home /home/sam/.*
/home/sam/.gtkrc-1.2-gnome2:include "/home/sam/.gtkrc.mine"

These will either need to be fixed, or you'll have to provide a link from /home/sam to the new location, /ext1/sam.

Moving when "database" isn't /etc/passwd

If the system is getting the home directories from LDAP, NIS, etc. then you'll need to peform the relocation in those systems, and coordinate with moving the files from /home/sam to /ext1/sam.

References

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Of course, changing that will require moving the user's files. And probably leaving a symlink in the old place (as otherwise a lot of things will break...) –  derobert Dec 4 '13 at 13:54
    
@derobert - yeah having done this a few times I try to avoid it like the plague. –  slm Dec 4 '13 at 14:01
    
@slm - Thanks for such a detailed answer (and the bit of history). I'm afraid I was looking to do something much simpler—and I was ignorant to the existence of an environment variable representing the session's home directory. I'm sure your answer will help another reader in the future, however. Thanks again. –  Andrew Cheong Dec 4 '13 at 14:48
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