I'm running a fresh Oneiric install (i.e. not an upgrade) on two different systems and running into the same set of seemingly related problems.

The most frustrating of the bunch is that, when I use the .profile and .bashrc that I have carried with me from Mac OS X, logging in to X via LightDM logs me out immediately. I believe this is caused by the fact that, when running "/bin/sh", it behaves as /bin/dash, but still has the $SHELL variable set to /bin/bash.


I have a huge .bashrc. You can see it here if you want, but its contents are probably not relevant, aside from the fact that it's full of bashisms, and the fact that it works with no errors inside xterm or on a virtual console.

My .profile looks like this (abbreviated):

case $SHELL in 
    if [ -f $HOME/.bashrc -a -r $HOME/.bashrc ]; then
        . $HOME/.bashrc

If I attempt to log in to X via LightDM, it will log me immediately back out. I get errors in .xsession-errors relating to my .bashrc that look like this (abbreviated):

/home/mrled/.bashrc: 103: [[: not found
[: 103: Linux: unexpected operator
[: 274: -P :: unexpected operator
/home/mrled/.bashrc: 520: complete: not found

As I said, when I run bash from a virtual console, I don't get these errors. Furthermore, if I remove my .profile, I can log in to X just fine. (I can also log in to a virtual console and use startx to initiate an X session that works, but this is of course not a long term solution.)

However, I discovered that if I run /bin/sh -l, I do get the errors. Here's an example session (note: the bash prompt I have simplified to bash>, and the sh prompt is just $):

bash> echo $SHELL
bash> echo $BASH_VERSION
bash> /bin/sh -l
/home/mrled/.bashrc: 103: [[: not found
[: 103: Linux: unexpected operator
[: 274: -P :: unexpected operator
/home/mrled/.bashrc: 520: complete: not found
$ echo $SHELL


Q1: Why is this happening?

I understand that /bin/sh now points to dash rather than bash, but if that's true, then why is $SHELL still returning /bin/bash?

Q2: What can I do to work around it?

Is there a way to work around this? I want to keep my profile loading .bashrc so that I get the same environment on both login and non-login shells, but obviously I only want it to load for bash itself, not /bin/sh masquerading as bash.

You may have noticed the difference in the contents of the $BASH_VERSION variables above. I have tried wrapping my .profile in something like this:

if [ -n $BASH_VERSION ]; then
    # the rest of my .profile as above

The -n test should return true only if the length of the string is non-zero, however, even though in the session above, when I'm running under /bin/sh -l it returns an empty string for $BASH_VERSION, when it's included in my .profile like this, it passes the test! It them proceeds to source my .bashrc and give me the same errors as before.

Now I'm really confused.

  • Note that dash -l also shows $SHELL having the value /bin/bash. – Scott Severance Jan 22 '12 at 8:16
  • 3
    $SHELL is whatever the last field in /etc/passwd (or getent passwd) says. – Dennis Williamson Jan 22 '12 at 15:53
  • @DennisWilliamson Thank you, that's what I needed to know to find the right answer. – Micah R Ledbetter Jan 23 '12 at 18:39
  • You're doing it wrong. You should put non-bash-specific environment in ~/.profile, bash-specific things in ~/.bashrc, and have ~/.bash_profile source both. – nyuszika7h Mar 22 '15 at 15:59

You can make the fact that $BASH_VERSION is blank in dash work for you:

if [ "$BASH_VERSION" = '' ]; then
    echo "This is dash."
    echo "This is bash."
  • 2
    The "x" technique is archaic and only needed in ancient shells. Use if [ "$BASH_VERSION" = '' ] – Dennis Williamson Jan 24 '12 at 19:23
  • @DennisWilliamson: Thanks. I had thought your technique was specific to Bash, but I just rested it in Dash and it worked. I've edited my answer. – Scott Severance Jan 25 '12 at 4:40
  • Or just use -n, or nothing. (+1, though. = '' works perfectly well.) – Eliah Kagan Aug 31 '17 at 18:26

You just have to use quotes on the variable BASH_VERSION to use -n

if [ -n "$BASH_VERSION" ];then
 echo "this is bash"; 
 echo "this is dash";
  • 1
    since [ "$EMPTY_STRING" ] evaluates false, you don't even need the -n. You just have to quote the variable. – jeberle Mar 11 '16 at 17:22

Use /proc/[PID]/cmdline to see what the script is being run with and test for what it contains. The $$ variable will give us the PID of the running shell. Thus we can make a script like this,

if grep -q 'bash' /proc/$$/cmdline ;
    echo "This is bash"
    echo "This is some other shell"

Here's a test of the same script:

$> bash test_script.sh                                                                                                
This is bash
$> dash test_script.sh                                                                                                
This is some other shell
  • This will not work on Mac. Check $BASH_VERSION. – Austin Burk Jun 27 '16 at 12:49
  • 2
    @AustinBurk it doesn't need to work on Mac. This is Ask Ubuntu. – TheWanderer Jun 27 '16 at 20:03
  • @Zacharee1 oh darn it, I wasn't paying attention :,) – Austin Burk Jun 27 '16 at 20:19
  • I don't want to edit this away from what you intend, but I suggest mentioning this method's limitations. Bash needn't have bash in its name; it's not uncommon for the bash executable to be run through a symlink with another name. Usually one would still want to consider that Bash. Also, the pattern is matched anywhere in /proc/$$/cmdline, which should be possible to fix, but keep in mind that arguments in cmdline are null-character delimited. grep -qE '(^|/)bash$' feels like it should work but gives a false positive when any argument is bash. – Eliah Kagan Aug 31 '17 at 18:24
  • @EliahKagan Feel free to edit my answers any time - you've sufficient expertise so I know your edits can offer only improvements. The answer was written when I was much greener with shells than I am now. – Sergiy Kolodyazhnyy Sep 17 '17 at 18:30

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