adding variable containing current shell pid

[bstarynk]

Hello all,

I am missing the $$ special variable to denote the current shell's pid (see http://hyperpolyglot.org/unix-shells giving a nice table ...); has it been discussed and rejected? What files should I look in to consider adding it?

[zanchey]

You are looking for the %self process expansion - see http://fishshell.com/docs/current/index.html#expand-process .

[KamilaBorowska]

Perhaps it could be possible to mention %self when user types $$ followed by non-letter.

[bstarynk]

The point is that $$ is very common in most other shells. So I believe that it should be expanded like %self is. Using $$ lowers the fish learning effort.

[maxfl]

One of the problems with %self is that it is not expanded in a string. You can not create file file.%self
I dislike $$, but it has an advantage of expanding in this case.

[cljacobs]

You can create such a file using touch file.(echo %self). It's a lot more typing, and not as intuitive to people moving from other shells, but I disagree that $$ should be expandable to the shell's pid for the same reasons fish ignores convention for many of those other special-case variable expansions other shells have adopted.

I don't really use %self very much, and even less in scenarios where it would be difficult to implement without a construct like $$, but if it's something people do a lot and given that it's already been given a special recognized syntax in process expansion, perhaps a variable expansion a la $argv would suffice, e.g. $pid or $self? This would violate the law of orthogonality, but possibly satisfies the law of user focus much better than the current way of doing things.

[zanchey]

The lack of expansion in a string is #1074.

[krader1961]

I'm in agreement with @cljacobs. I don't think we should introduce $$ but introducing $PID or $SELF can be justified despite introducing a second way to do the same thing. Note the all uppercase names to mirror other vars like $PWD and minimize clashes with existing uses of their lowercase forms.

[ridiculousfish]

It's also very easy for any user to just write set SELF %self at the top of the script and then they have it. This wouldn't work in bash since bash forks for subprocesses, but it works in fish.

[krader1961]

That set SELF %self works actually surprises me. As does touch %self but not touch x.%self.

[kgadek]

I would add that using $$ is very common to detect the shell type itself. I use this to do that: ps -cp $$ | awk 'NR==2{sub(/^-?/, "", $NF); print $NF}' and it works quite nicely across macOS, Linux and FreeBSD and various shells… except fish.

[krader1961]

I'm removing the "RFC" label. This should be implemented. I have no objections to using %some-string to match process names containing that string. But given the quirks of %self (e.g., touch x.%self doesn't work) we should allow access to this info via normal variable expansion.

[faho]

But given the quirks of %self (e.g., touch x.%self doesn't work) we should allow access to this info via normal variable expansion.

Okay. What should the variable be called? Note that $ is currently not a valid variable name. It's also not in any way explanatory, so I'm leaning towards using a more descriptive name. Some suggestions:

• $SELF
• $PID
• $FISH_PID
• $SHELL_PID

I like the latter two the most since they best explain what the variable is.

[KamilaBorowska]

I think the name should be lowercase, considering shell PID is not an exported variable (for consistency).

[cben]

For the historic record, the new vars got called $fish_pid (for %self) and $last_pid (replacing %last).