Why does the following script delete itself?

shell-script files rm executable shebang

10,468

The kernel interprets the line starting with #! and uses it to run the script, passing in the script's name; so this ends up running

/bin/rm scriptname

which deletes the script. (As Stéphane Chazelas points out, scriptname here is sufficient to find the script — if you specified a relative or absolute path, that's passed in as-is, otherwise whatever path was found in PATH is prepended, including possibly the emptry string if your PATH contains that and the script is in the current directory. You can play around with an echo script — #!/bin/echo — to see how this works.)

As hobbs pointed out, this means your script is actually an rm script, not a bash script — the latter would start with #!/bin/bash.

See How programs get run for details of how this works in Linux; the comments on that article give details for other platforms. #! is called a shebang, you'll find lots of information by searching for that term (thanks to Aaron for the suggestion). As jlp pointed out, you'll also find it referred to as "pound bang" or "hash bang" (# is commonly known as "pound" — in countries that don't use £ — or "hash", and ! as "bang"). Wikipedia has more info.

10,468

user253751

Updated on September 18, 2022

Comments

user253751 over 1 year
If you create an executable file with the following contents, and run it, it will delete itself.
How does this work?
```
#!/bin/rm
```
- Admin about 8 years
  
  Related
- Admin almost 8 years
  
  @DigitalTrauma Heh, that was my first thought when I saw this.
- Admin almost 8 years
  
  it's not about rm, it is about the #!. The question could be rephrased to how does any executable script with a #! works.
- Admin almost 8 years
  
  How did you manage to stumble upon this?
- Admin almost 8 years
  
  @user1717828 Probably PPCG.
- Admin almost 6 years
  
  That's definitely an epic question. I regret that I can upvote it only once.
jlp about 8 years

Other names for "#!" you might hear are "pound bang" and "hash bang". See en.wikipedia.org/wiki/Shebang_(Unix) for details.
cat almost 8 years

@jlp Pound bang? Is that like "bang for your buck"? Heh...
Swiss Frank about 4 years

"passing in the script's name" -- not quite. That's how it worked in the 70s, but it invites a race condition hack. You can start a script with one heading, or set of file permissions, then change it before the interpreter is started. Instead, I believe such files are opened and read by the OS, then LEFT opened and passed to the interpreter as file handle 2 (or 0?). I'm not 1000% sure of this so I won't edit the actual answer
Stephen Kitt about 4 years

@SwissFrank yes, there is a race, and yes, there are loaders (in the Linux kernel) which support passing an open file descriptor, but not for scripts. In fact, if the race were fixed for scripts, arbitrary commands would no longer work as shebangs, and rm couldn’t be used in this way.