Re: execve(2) optimisation for last command

[Harald van Dijk via austin-group-l at The Open Group]

On 15/04/2021 21:36, Martijn Dekker via austin-group-l at The Open Group 
wrote:
My question: why is this? I would have thought that a script is a script 
and that it should make no essential difference whether a script is 
taken from a -c argument or loaded from a file. What makes the 
optimisation appropriate for one but not the other?

The parsing of script files and -c arguments is different and is meant 
to be different. With -c, the whole command string is known and parsed 
in advance. With script files, in general, only as much is read as 
necessary to parse the next command, that command is executed, and so 
on. With -c, all shells already know at the time that a command is 
executed whether it is the last command the shell will execute. When a 
script file is executed, in the general case, the shell not only does 
not know, it cannot know when a command is the last command in the 
script file. The command will almost always be followed by a newline, 
and determining whether that newline is the last byte of the file 
requires trying to read at least one more byte. Trying to read at least 
one more byte is problematic if the file is non-seekable so that the 
shell cannot restore the state to what it was before that read (needed 
when the shell is reading from standard input and the fd is shared with 
other processes), and is problematic if the read may block.

In specific cases, the shell may be able to detect that these issues 
cannot be a problem. In those specific cases, the optimisation should be 
perfectly valid and any reasons why it should or should not be performed 
are not of a technical nature.

Cheers,
Harald van Dijk