5 ms·

As someone who uses the commandline a lot but isn't exactly a wizard, why does this work?

lsb 11y ago
globbing expression expand to all the files that match. if a directory has 1.txt, 2.txt, and 3.txt then << rm * >> expands to "rm 1.txt 2.txt 3.txt" and is then executed. if you have -@, 1.txt, 2.txt, and 3.txt, that expands to << rm -@ 1.txt 2.txt 3.txt >>, and that can't execute. (if you really wanted to remove your -@ you'd do << rm -- * >> because a double-dash signals the end of command-line options.)
ulrikrasmussen 11y ago
The * is expanded by the shell to a space-delimited list of filenames, but the shell does not adequately escape filenames that can be misinterpreted as arguments to 'rm'.
- gmac 11y ago
  That's kind of scary — in that case I guess I should avoid creating a file named -rf.
  - wodenokoto 11y ago
    That is really interesting. Can someone knowledgeable about the shell expand on this? I don't dare test it on my machine.
    - vbezhenar 11y ago
      Shell session to demonstrate (DO NOT DO IT IN THE DIRECTORY WITH IMPORTANT FILES): $ touch important $ chmod 400 important $ rm * override r-------- vbezhenar/staff for important? n $ touch -- -rf $ ls -l total 0 -rw-r--r-- 1 vbezhenar staff 0 Mar 24 14:35 -rf -r-------- 1 vbezhenar staff 0 Mar 24 14:35 important $ rm * $ ls -l total 0 -rw-r--r-- 1 vbezhenar staff 0 Mar 24 14:35 -rf $ rm -- -rf
    - theothertom 11y ago
      I've not tested it, but it should expand just like anything else. The effect would broadly be that running "rm *" in the directory would recurse into subfolders without warning.
    - pbhjpbhj 11y ago
      Re testing. If you follow this link - https://www.digitalocean.com/?refcode=3fc9a5a35c52 https://www.digitalocean.com/?refcode=3fc9a5a35c52 - you get $10 free credit (affiliate link I get $25 if you spend that much in future) on DigitalOcean. You can spin up a droplet and use the online shell tool or ssh in (very easy when you've set up a cert as the droplet can have the cert setup automatically). Then you can mess about with a droplet as much as you like, virtually speaking. Once you're done then use the control panel to destroy the droplet - it costs a few ¢ a day and if you don't have a droplet in use (which means active or paused; preserving images is cheaper but non-zero) then you don't pay anything. Basically sign up and have a year of uptime to mess with a full install of various OS with no charge. Make sure you don't write "rm -rf /*" in the wrong terminal!
  - coldpie 11y ago
    Yes. There was an article linked from HN ages ago (at least a year) that went into mitigation techniques for these issues. As you expect, it basically became fractal, and even then still had bugs. I wish I still had the URL.
  - un1xl0ser 11y ago
    I think it may be more scary for code that allows arbitrary execution using command-line arguments. Commands like find or xargs using without defense against this would be a problem. For example, site that does something precious with your uploaded pet pictures. Defending against this being the use of -- to signal an end of command line arguments.
- bentcorner 11y ago
  > shell does not adequately escape filenames that can be misinterpreted as arguments to 'rm' That sounds like a bug to me, or at least depending on suboptimal behavior.
raverbashing 11y ago
More specifically, it is expanded on the shell Example: $ echo /* /bin /boot /dev /etc /home (...)

lsb 11y ago

globbing expression expand to all the files that match. if a directory has 1.txt, 2.txt, and 3.txt then << rm * >> expands to "rm 1.txt 2.txt 3.txt" and is then executed. if you have -@, 1.txt, 2.txt, and 3.txt, that expands to << rm -@ 1.txt 2.txt 3.txt >>, and that can't execute. (if you really wanted to remove your -@ you'd do << rm -- * >> because a double-dash signals the end of command-line options.)

ulrikrasmussen 11y ago

The * is expanded by the shell to a space-delimited list of filenames, but the shell does not adequately escape filenames that can be misinterpreted as arguments to 'rm'.

gmac 11y ago
That's kind of scary — in that case I guess I should avoid creating a file named -rf.
- wodenokoto 11y ago
  That is really interesting. Can someone knowledgeable about the shell expand on this? I don't dare test it on my machine.
  - vbezhenar 11y ago
    Shell session to demonstrate (DO NOT DO IT IN THE DIRECTORY WITH IMPORTANT FILES): $ touch important $ chmod 400 important $ rm * override r-------- vbezhenar/staff for important? n $ touch -- -rf $ ls -l total 0 -rw-r--r-- 1 vbezhenar staff 0 Mar 24 14:35 -rf -r-------- 1 vbezhenar staff 0 Mar 24 14:35 important $ rm * $ ls -l total 0 -rw-r--r-- 1 vbezhenar staff 0 Mar 24 14:35 -rf $ rm -- -rf
  - theothertom 11y ago
    I've not tested it, but it should expand just like anything else. The effect would broadly be that running "rm *" in the directory would recurse into subfolders without warning.
  - pbhjpbhj 11y ago
    Re testing. If you follow this link - https://www.digitalocean.com/?refcode=3fc9a5a35c52 https://www.digitalocean.com/?refcode=3fc9a5a35c52 - you get $10 free credit (affiliate link I get $25 if you spend that much in future) on DigitalOcean. You can spin up a droplet and use the online shell tool or ssh in (very easy when you've set up a cert as the droplet can have the cert setup automatically). Then you can mess about with a droplet as much as you like, virtually speaking. Once you're done then use the control panel to destroy the droplet - it costs a few ¢ a day and if you don't have a droplet in use (which means active or paused; preserving images is cheaper but non-zero) then you don't pay anything. Basically sign up and have a year of uptime to mess with a full install of various OS with no charge. Make sure you don't write "rm -rf /*" in the wrong terminal!
- coldpie 11y ago
  Yes. There was an article linked from HN ages ago (at least a year) that went into mitigation techniques for these issues. As you expect, it basically became fractal, and even then still had bugs. I wish I still had the URL.
- un1xl0ser 11y ago
  I think it may be more scary for code that allows arbitrary execution using command-line arguments. Commands like find or xargs using without defense against this would be a problem. For example, site that does something precious with your uploaded pet pictures. Defending against this being the use of -- to signal an end of command line arguments.
bentcorner 11y ago
> shell does not adequately escape filenames that can be misinterpreted as arguments to 'rm' That sounds like a bug to me, or at least depending on suboptimal behavior.

raverbashing 11y ago

More specifically, it is expanded on the shell Example: $ echo /* /bin /boot /dev /etc /home (...)