Some strange output on my command in linux shell

Question

I've written the following in the command:

$ cat /bin/ls > blah
$ cat blah blah blah > bbb
$ chmod u+x bbb
$ ./bbb

And it printed all the file names in the current working directory.

My question is why? and why not 3 times?

You essentially made a copy of the ls executable, with a bunch of extra garbage at the end (extra copies of the executable). Then you ran the copy... which listed files and exited. — Dmitri
– Dmitri, Commented Jun 19, 2014 at 16:20

Pavel · Accepted Answer · 2014-06-19 16:23:05Z

2

Because the Linux executable file format (ELF) is not a script that you can copy-paste three times in a row to get the same result. To be more precise, the header contains a single entry point (think of it as the address of where int main() has been stored), which is where the instructions are read from. Once you reach the final return 0; or whatever, the program stops, even if there is more (nicely structured) binary garbage following in the binary file.

TL;DR: Don't forget - /bin/ls is a compiled binary and not a shell script.

answered Jun 19, 2014 at 16:23

Pavel

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1 Comment

Mickey Over a year ago

Yes, I forgot that it's an already complied file. Now it's clear, thanks!

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