[SOLVED] env shebang enters infinite loop

doru · 12-11-2021, 09:35 AM

Code:

$ cat shebang.sh 
#!/usr/bin/env hisvar=8 /bin/bash 

echo hisvar=$hisvar
$ ./shebang.sh 
^C

This enters an infinite loop. Why?!

HappyTux · 12-11-2021, 09:59 AM

Quote:

Originally Posted by doru

Code:

$ cat shebang.sh 
#!/usr/bin/env hisvar=8 /bin/bash 

echo hisvar=$hisvar
$ ./shebang.sh 
^C

This enters an infinite loop. Why?!

According to shell check the shebang is wrong and you have undeclared variable. I modified it to actually use bash as intended and put the variable in a correct place and it works fine. Oh and it said at one point to double quote the $hisvar as well to have zero issues with it, though that disappeared when I put my version in it.

Code:

zeus@bullseye-raspi:~$ nano shebang.sh
zeus@bullseye-raspi:~$ chmod +x shebang.sh
zeus@bullseye-raspi:~$ ./shebang.sh 
^C
zeus@bullseye-raspi:~$ nano shebang.sh
zeus@bullseye-raspi:~$ ./shebang.sh 
hisvar=8
zeus@bullseye-raspi:~$ cat shebang.sh 
#!/bin/bash 
hisvar=8
echo hisvar=$hisvar

https://www.shellcheck.net/

shruggy · 12-11-2021, 10:05 AM

You forgot -S / --split-string. Traditional shebang allows only one parameter. If you want more, you have to use env -S.

And anyway, /bin/bash as an argument to env makes no sense whatsoever. You use an env shebang when you don't know the path to Bash.

doru · 12-11-2021, 11:00 AM

Quote:

Originally Posted by shruggy

You forgot -S / --split-string. Traditional shebang allows only one parameter. If you want more, you have to use env -S.

And anyway, /bin/bash as an argument to env makes no sense whatsoever. You use an env shebang when you don't know the path to Bash.

Thanks, excellent reference!
Do you have any idea why the infinite loop?

shruggy · 12-11-2021, 11:19 AM

Well, #!/usr/bin/env hisvar=8 /bin/bash is treated as

Code:

env hisvar='8 /bin/bash'

i.e. assigning a variable without running any command. When you run it on the command line it will just print the environment. But when you're calling it from a shebang it will try to run it like this

Code:

env hisvar='8 /bin/bash' ./shebang.sh

which will then translate to

Code:

env hisvar='8 /bin/bash' env hisvar='8 /bin/bash' ./shebang.sh

and so on.

Contrast that with the shebang line

Code:

#!/usr/bin/env -S hisvar=8 /bin/bash

which will be run as

Code:

env hisvar=8 /bin/bash ./shebang.sh

There are commands other than env that would enter the infinite loop for the same reason. The most obvious example is sudo:

Code:

#!/usr/bin/sudo hisvar=8 /bin/bash

doru · 12-15-2021, 07:41 AM

Quote:

Originally Posted by shruggy

Well, #!/usr/bin/env hisvar=8 /bin/bash is treated as

Code:

env hisvar='8 /bin/bash'

i.e. assigning a variable without running any command. When you run it on the command line it will just print the environment. But when you're calling it from a shebang it will try to run it like this

Code:

env hisvar='8 /bin/bash' ./shebang.sh

which will then translate to

Code:

env hisvar='8 /bin/bash' env hisvar='8 /bin/bash' ./shebang.sh

and so on.

shebang.sh, which was intended as an argument to bash, now becomes a command. This should happen for both

Code:

#!/usr/bin/bash

and

Code:

#!/usr/bin/env

but it happens only for the second. Why?

Do you know why

Code:

$ env -S 'OLDUSER=${USER} sh -c echo\ OLDUSER=\$OLDUSER'
env: invalid sequence '\ ' in -S

and they needed to do this

Code:

$ env -S 'OLDUSER=${USER} sh -c "echo\_OLDUSER=\$OLDUSER"'
OLDUSER=user

'\_' inside double quotes is replaced by a space

pan64 · 12-15-2021, 07:52 AM

Code:

env -S OLDUSER=${USER} sh -c 'echo OLDUSER=$OLDUSER'

doru · 12-15-2021, 07:59 AM

Quote:

Originally Posted by pan64

Code:

env -S OLDUSER=${USER} sh -c 'echo OLDUSER=$OLDUSER'

-S is redundant for that.

shruggy · 12-15-2021, 08:18 AM

Quote:

Originally Posted by doru

they needed to do this

Strictly speaking, they don't need it here:

Code:

env -S 'OLDUSER=${USER} sh -c "echo OLDUSER=\$OLDUSER"'

should work as well. You can always check how the string is going to be parsed with -v:

Code:

env -vS 'OLDUSER=${USER} sh -c "echo OLDUSER=\$OLDUSER"'

doru · 12-15-2021, 09:17 AM

Quote:

Originally Posted by shruggy

Strictly speaking, they don't need it here:

Code:

env -S 'OLDUSER=${USER} sh -c "echo OLDUSER=\$OLDUSER"'

should work as well. You can always check how the string is going to be parsed with -v:

Code:

env -vS 'OLDUSER=${USER} sh -c "echo OLDUSER=\$OLDUSER"'

I edited my reply here: https://www.linuxquestions.org/quest...8/#post6309691

These old pieces of code can be quite confusing.

shruggy · 12-15-2021, 09:40 AM

Quote:

Originally Posted by doru

shebang.sh, which was intended as an argument to bash, now becomes a command. This should happen for both

Code:

#!/usr/bin/bash

and

Code:

#!/usr/bin/env

but it happens only for the second. Why?

No. This shouldn't happen for both. Bash expects a script as an argument, passing a variable instead would end in an error, you wouldn't get an infinite loop.

doru · 12-15-2021, 12:12 PM

Quote:

Originally Posted by shruggy

No. This shouldn't happen for both. Bash expects a script as an argument, passing a variable instead would end in an error, you wouldn't get an infinite loop.

You are right again, bash expects a script. env works for binaries and scripts.
But this is a script. I was not explicit enough.

Code:

$ cat bash.sh 
#!/usr/bin/bash
$ cat env.sh 
#!/usr/bin/env
$ ./bash.sh # exits immediately 
$ ./env.sh # infinite loop 
^C

I am missing something here.

shruggy · 12-15-2021, 01:25 PM

What are you missing? How is this situation different from the one described in #5?

Shebang instructs the system to pass the script as an argument to the command specified in the shebang.

So, when the system encounters a script with the shebang #!/usr/bin/bash, it will be passed as an argument to /usr/bin/bash. Like this

Code:

/usr/bin/bash ./bash.sh

As the script is empty (for Bash, shebang is just a comment as any other), it does nothing.

And when the system encounters a script with the shebang #!/usr/bin/env, it will be passed as an argument to /usr/bin/env. Like this

Code:

/usr/bin/env ./env.sh

As the script is executable and the path to it is specified, env just tries to execute the script as if ./env.sh were specified at the command prompt. But this is the script with a shebang, so the system calls the program specified in the shebang (env) and passes it the script as argument for execution. And the whole process repeats itself. Over and over again.

MadeInGermany · 12-15-2021, 02:44 PM

./shebang.sh with
#!/usr/bin/bash

transforms to
/usr/bin/bash ./shebang.sh
And bash sees the shebang as a comment in the script.

./shebang.sh with
#!/usr/bin/env

transforms to
/usr/bin/env ./shebang.sh
And env does nothing (or sets an environment variable) then invokes
./shebang.sh

that again finds the shebang...
resulting in a loop.

doru · 12-16-2021, 03:01 AM

Quote:

Originally Posted by shruggy

What are you missing? How is this situation different from the one described in #5?

Shebang instructs the system to pass the script as an argument to the command specified in the shebang.

Quote:

Originally Posted by MadeInGermany

bash sees the shebang as a comment in the script.

This is the piece of information I was missing.

The fact that bash ignores the shebang is even more shocking to me than the fact that bash does not run binaries. This may be done precisely to stop the recursion.

Thank you all for your kind help.