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Input:

sudo bash -c '>> /media/$USER/rootfs/etc/profile'

Output:

bash: /media/root/rootfs/etc/profile: No such file or directory

Input:

TEST=$USER
sudo bash -c '>> /media/$TEST/rootfs/etc/profile'

Output:

bash: /media//rootfs/etc/profile: No such file or directory

Instead of:

/media/riccardo/rootfs/etc/profile

How can I solve the problem?

ctrl-alt-delor
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Riccardo La Marca
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2 Answers2

4

Shell variables aren't exported to commands launched from the shell, by default.

You need to explicitly do it with export, e.g.:

$ export test=foo
$ bash -c 'echo test=$test'
test=foo

Except that sudo may still clean up the environment before passing it to bash, so you may need to use the command line arguments instead:

$ test=foobar 
$ sudo bash -c 'echo "$1"' sudo_bash "$test"
foobar

No need to export here, and sudo_bash is just a string that's used as the "name" of that inner shell, you'd see it if the -c string had e.g. a syntax error.

See also:

ilkkachu
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  • Thank you ilkkachu, also if i have found an more elegant solution: – Riccardo La Marca Mar 14 '20 at 13:13
  • sudo bash -c ">> /media/$USER/rootfs/etc/profile" – Riccardo La Marca Mar 14 '20 at 13:13
  • double quotes instead of single quotes solve my problem. – Riccardo La Marca Mar 14 '20 at 13:14
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    @RiccardoLaMarca, ah, I should have mentioned that in the first place. In general, **_don't_** use double quotes like that. If you do, the contents of the variable get expanded as part of the inner shell's command line, _just like any other code there_. Shell metacharacters and expansions get processed, so they can break the structure of the command and even cause security issues. Try e.g. `test='foo;bar'; bash -c "echo $test"` – ilkkachu Mar 14 '20 at 13:26
  • Wow, it's true, this issue was not known to me. Thank you. – Riccardo La Marca Mar 16 '20 at 09:32
1

Quoting problems

sudo bash -c '>> /media/'"$USER"'/rootfs/etc/profile'
test=$USER
sudo bash -c '>> /media/'"$test"'/rootfs/etc/profile'

'' quotes are literal, "" quotes allow expansion of variables. I have use both, concatenating them together. It would also work if I only used "".

ctrl-alt-delor
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  • This has exactly the issue with unsafe expansion of `$USER` or `$test` in the command line interpreted by the shell started from `sudo`. – ilkkachu Mar 16 '20 at 11:28
  • What is the danger? These variables are expanded by the calling shell, so you have to be responsible for ensuring they are correct, and won't cause a problem. Can you explain what the danger is? – ctrl-alt-delor Mar 16 '20 at 13:02
  • The usual, shell metacharacters and whitespace being interpreted as part of the command, and not just as pure data. Compare `test='foo;bar'; sudo bash -c 'echo /media/'"$test"'/rootfs/etc/profile'` and `test='foo;bar'; sudo bash -c 'echo /media/"$1"/rootfs/etc/profile' sh "$test"`. Sure, `$USER` _probably_ contains just letters and digits, but it's not hard to use command line args or the environment to pass the value safely. – ilkkachu Mar 16 '20 at 13:09
  • @ilkkachu oh yes. Would I need to add more quotes (for sudo). You can edit if you like. – ctrl-alt-delor Mar 16 '20 at 13:12