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When bash is executing a script, ls -la /proc/<pid of bash>/fd shows "255 -> /path/to/script". I think that bash reads the script via file discriptor 255, and executes the commands line by line.

But when bash is sourcing a script, I can't find such an entry in ls -la /proc/<pid of bash>/fd. I can't find in cat /proc/<pid of bash>/maps, either.

I have a question: When bash sources a script, how does bash read the script? Is there any special way to read the script file?

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  • When bash sources a script, how does bash read the script I would think the commands are read in the same way as when you type commands in your shell. when you run a script, another shell process is created but when you source a script you are using the same shell process and for that reason you don't see the file descriptor. I think that but if I'm wrong it should be useful someone provide the correct answer. It's interesting.
    – Edgar Magallon
    Oct 20, 2022 at 8:36

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I hit upon an idea to analyze the situation by strace.

When just executing, the output of strace -f -p <pid> is like this:

openat(AT_FDCWD, "./test", O_RDONLY) = 3

...

[pid 3408] dup2(3, 255) = 255

[pid 3408] close(3) = 0

"test" is the shell script to be executed. "3408" is the pid of the child shell. "test" is opened at file discriptor 3, and dupulicated to file discriptor 255. So ls -la /proc/<pid of bash>/fd shows "255 -> /some/directory/test".

But when sourcing the script, "test" is opened, read into the buffer, and immediately closed. So ls -la /proc/<pid of bash>/fd show no such entry as above.

I don't know why bash is implemented like this.

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