checkbashisms-compliant way to determine the current shell

Question

In my .profile, I use the following code to ensure that Bash-related aliases and functions are only sourced if the login shell actually is Bash:

# If the current (login) shell is Bash, then
if [ "${BASH_VERSION:-}" ]; then
  # source ~/.bashrc if it exists.
  if [ -f "$HOME/.bashrc" ]; then
    . "$HOME/.bashrc"
  fi
fi

I’m currently in the process of putting my shell configuration files, scripts and functions under version control. I’ve also recently started the process of removing casual Bashisms from shell scripts that don’t benefit from Bash-specific features, e.g., replacing function funcname() with funcname().

For my shell files repository, I’ve configured a pre-commit hook that runs the checkbashisms utility from Debian’s devscripts package on each sh file in the repository to ensure that I don’t inadvertently introduce Bash-specific syntax. However, this raises an error for my .profile:

possible bashism in .profile line 51 ($BASH_SOMETHING):
if [ "${BASH_VERSION:-}" ]; then

I was wondering if there was a way to check which shell is running that wouldn’t trigger a warning in checkbashisms.

I checked the list of shell-related variables listed by POSIX in the hope that one of them could used to show the current shell. I’ve also looked at the variables set in an interactive Dash shell but, again, failed to find a suitable candidate.

At the moment, I’ve excluded .profile from being processed by checkbashisms; it’s a small file so it’s not hard to check it manually. However, having researched the issue, I’d still like to know if there is a POSIX compliant method to determine which shell is running (or at least a way that doesn’t cause checkbashisms to fail).

---

Further background/clarification

One of the reasons I’m putting my shell configuration files under version control is to configure my environment on all the systems I currently log in to on a regular basis: Cygwin, Ubuntu and CentOS (both 5 and 7, using Active Directory for user authentication). I most often log on via X Windows / desktop environments and SSH for remote hosts. However, I’d like this to be future proof and have the least reliance on system dependencies and other tools as possible.

I’ve been using checkbashisms as a simple, automated sanity check for the syntax of my shell-related files. It’s not a perfect tool, e.g., I’ve already applied a patch to it so that it doesn’t complain about the use of command -v in my scripts. While researching, I’ve learned that the program’s actual purpose is to ensure compliance with Debian policy which, as I understand it, is based on POSIX 2004 rather than 2008 (or its 2013 revision).

Answer 1

Usually one uses $0 for this purpose. On the site which you linked it stands:

0
   (Zero.) Expands to the name of the shell or shell script.

Answer 2

Your

# If the current (login) shell is Bash, then
if [ "${BASH_VERSION:-}" ]; then
  # source ~/.bashrc if it exists.
  if [ -f "$HOME/.bashrc" ]; then
    . "$HOME/.bashrc"
  fi
fi

code is completely POSIX conformant and the best way to check that you're currently running bash. Of course the $BASH_VERSION variable is bash-specific, but that's specifically why you're using it! To check that you're running bash!

Note that $BASH_VERSION will be set whether bash is invoked as bash or sh. Once you've asserted that you're running bash, you can use [ -o posix ] as an indicator that the shell was invoked as sh (though that option is also set when POSIXLY_CORRECT is in the environment or bash is called with -o posix, or with SHELLOPTS=posix in the environment. But in all those cases, bash will behave as if called as sh).

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Another variable you could use instead of $BASH_VERSION and that checkbashism doesn't seem to complain about unless passed the -x option is $BASH. That is also specific to bash so one you should also be able to use to determine whether you're running bash or not.

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I'd also argue it's not really a proper use of checkbashisms. checkbashisms is a tool to help you write portable sh scripts (as per the sh specification in the Debian policy, a superset of POSIX), it helps identify non-standard syntax introduced by people writing scripts on systems where sh is a symlink to bash.

A .profile is interpreted by different shells, many of which are not POSIX compliant. Generally, you don't use sh as your login shell, but shells like zsh, fish or bash with more advanced interactive features.

bash and zsh, when not called as sh and when their respective profile session file (.bash_profile, .zprofile) are not POSIX conformant (especially zsh) but still read .profile.

So it's not POSIX syntax you want for .profile but a syntax that is compatible with POSIX (for sh), bash and zsh if you're ever to use those shells (possibly even Bourne as the Bourne shell also reads .profile but is not commonly found on Linux-based systems).

checkbashisms would definitely help you find out bashisms but may not point out POSIX syntax that is not compatible with zsh or bash.

Here, if you want to use bash-specific code (like the work around of that bash bug whereby it doesn't read ~/.bashrc in interactive login shells), a better approach would be to have ~/.bash_profile do that (before or after sourcing ~/.profile where you put your common session initialisations).

Answer 3

Usually the SHELL environment variable tells you the default shell. You should not need to manually source the .bashrc file (not true see update bellow), bash should do this automatically just make sure it is in the $HOME directory.

The other option is to do something like:

 ps -o cmd= $$

that will tell you the command (without arguments, the = without a value will not display column headers) of the current process. Example output:

 $ps -o cmd= $$
 bash
 $sh
 $ps -o cmd= $$
 sh

UPDATE:

I stand corrected! :)

The .bashrc is not always sourced as was mentioned in the comments bellow and https://stackoverflow.com/questions/415403/whats-the-difference-between-bashrc-bash-profile-and-environment

So you could move your .bashrc to .bash_profile and see if that works without having to do the test. If not you have the test above.

Answer 4

The question asks for the user's login shell as well as the current shell in a manner that appeases checkbashisms. If that is meant to be the shell on which the user logged in, I would use the one from /etc/passwd, e.g.,

MY_UID=$(id -u)
MYSHELL=$(awk -F: '$3 == '$MY_UID'{ print $7; }' </etc/passwd )

Users can of course start a new shell after logging in. Of course, if one is bash and the other is not, tests of bash's environment variables may not help.

Some may want to use getent rather than just the passwd file (but that wouldn't be in the scope of the question).

Based on the comment about LDAP, and the suggestion for logname, this alternative form could be used:

MY_NAME=$(logname)
MYSHELL=$(getent passwd | awk -F: '$1 ~ /^'$MY_NAME'$/ {print $7;}' )

While testing it, I noticed that logname does not like its input redirected (so I left the expression split up). A quick check shows getent should work on the platforms mentioned (though that should have been provided in the original question):

• New package: getent-2.18.90-1
• Authenticate RHEL 5 and 6 SSSD Using Kerberos and LDAP Against Active Directory on Windows Server 2008 R2
• getent - get entries from administrative database (Ubuntu)