1   basename and $0

I encountered a small issue with someone’s code, basically it’s about

basename "$0"

In Bash, if run it in interactive shell or shell prompt, you will get the following error:

$ basename "$0"
basename: invalid option -- 'b'
Try 'basename --help' for more information.

Because $0 is -bash, -b makes basename thinks the user specifies an invalid option. The correct way is to terminate option list by the convention using --, that is:

$ basename -- "$0"
-bash

I’ve known Bash have unusual $0 when in shell prompt, but never really thought about it until this thing came up. So I decided to check on various shells, just to see if any other gives unusual $0.

2   Table of $0 in various shells

Shell Version Command $0 or equiv.
bash The standard GNU Bourne again shell
4.2.45 bash -bash
sh sh
ccsh UNIX Shell for people already familiar with the C language
0.0.4 ccsh /tmp/random text 
int main(int argc, char *argv[])
{
   printf("%s\n", argv[0]);
}
dash DASH is a direct descendant of the NetBSD version of ash (the Almquist SHell) and POSIX compliant
0.5.7.3 dash dash
fish fish is the Friendly Interactive SHell
2.0.0 fish see below
$0 is empty string and list in fish is 1-based index, so there is no such $argv[0]
ksh The Original Korn Shell, 1993 revision (ksh93)
93u+ ksh ksh
heirloom-sh Heirloom Bourne Shell, derived from OpenSolaris code SVR4/SVID3
050706 jsh jsh
posh Reimplementation of Bourne shell based on pdksh
0.12 posh posh
rc A reimplementation of the Plan 9 shell
1.7.2 rcsh rcsh
sash A small (static) UNIX Shell
3.7 sash sh
squirrelsh An advanced, cross-platform object oriented scripting shell based on the squirrel scripting language
1.2.7 squirrelsh squirrelsh 
printl(__argv[0])
tcsh Enhanced version of the Berkeley C shell (csh)
6.18.01 csh csh
tcsh tcsh
zsh UNIX Shell similar to the Korn shell
5.0.2 zsh zsh

3   Notes on the table

  • The descriptions are from Gentoo Portage.
  • Some shells may provides additional shells, for example, Bash also have sh mode.
  • ccsh doesn’t really have an interactive shell mode.
  • fish doesn’t seem to have $0-equivalent.

4   Conclusion

  • Only Bash gives unusual $0 in interactive shell.
  • Using -- is a good practice.