Get GNU tar PID inside of --to-command

Question

We have this GNU tar command as shown:

$ tar -xjf /dev/st0 --to-command='sh -c '\''printf "%s\n" "$TAR_SIZE $TAR_REALNAME" >> "$TAR_ARCHIVE.sizes.txt"'\'

Is there any way we can get tar's PID inside of --to-command? Something like this:

$ [...] --to-command='sh -c '\''printf "%s\n" "$TAR_SIZE $TAR_REALNAME" >> "$TAR_ARCHIVE.sizes.$TAR_PID.txt"'\'

(note the addition of fictional $TAR_PID near the end)

I read the manpages of GNU tar 1.34 but see no relevant variables.

Answer 1

All POSIX sh implementations set the $PPID variable to the pid of the parent of the process that executed it, so:

tar -xvf foo.tgz --to-command='
  printf "%s\n" "$TAR_SIZE $TAR_REALNAME" >> "$TAR_ARCHIVE.sizes.$PPID"'

Note that that variable doesn't show up in the output of printenv as it's not exported to the environment (understandably as its value is only valid for the current shell), use the set command instead of the printenv command to list all shell variables (which includes a subset of those reported by printenv excluding the environment variables that cannot be mapped to shell variables).

$ tar cf - /etc/issue | env -i tar -xf - --to-command='set -x; printenv | grep -i pid; set | grep -i pid'
tar: Removing leading `/' from member names
+ printenv
+ grep -i pid
+ set
+ grep -i pid
PPID='27812'

If for some reason, your sh didn't set the $PPID variable, you could always do:

sh -c 'TAR_PID=$$ exec tar -xvf "$1" --to-command="$2"' sh foo.tgz '
  printf "%s\n" "$TAR_SIZE $TAR_REALNAME" >> "$TAR_ARCHIVE.sizes.$TAR_PID"'

Where, thanks to exec, it's the same process that runs sh and then tar, so we can make its pid ($$) available in the TAR_PID environment variable.

Now, since $TAR_ARCHIVE and $TAR_PID don't change, there's no need to reopen the file for every file and you can redirect all that output at once and use > instead of >>:

sh -c 'exec tar -xvf "$1" --to-command="$2" > "$1.sizes.$$' sh foo.tgz '
  printf "%s\n" "$TAR_SIZE $TAR_REALNAME"'

You can also get size and name information using the standard pax command.

gunzip < foo.tgz | pax -o listopt='%(size)u %(name)s\n'

Though the implementations commonly found on GNU/Linux systems in my experience don't implement that listopt option.

Answer 2

GNU Tar 1.35 on my Arch Linux system runs the command in a shell, so this outputs tar's PID for me:

tar ... --to-command='ps -o ppid= $$'

Example:

% tar xvf foo.tgz --to-command='ps -o ppid= $$' foo.c &
[1] 26000
foo.c
26000
[1]  + 26000 done       tar xvf foo.tgz --to-command='ps -o ppid= $$' foo.c

So maybe this should work:

tar --to-command='printf "%s\n" "$TAR_SIZE $TAR_REALNAME" >> "$TAR_ARCHIVE.sizes.$(ps -o ppid= $$).txt"'

Testing it out:

% tar xvf foo.tgz --to-command='printf "%s\n" "$TAR_SIZE $TAR_REALNAME" >> "$TAR_ARCHIVE.sizes.$(ps -o ppid= $$).txt"' &
[1] 26100
foo
foo.c
foo.compressed
[1]  + 26100 done       tar xvf foo.tgz
% cat foo.tgz.sizes.26100.txt
7568 foo
153 foo.c
148 foo.compressed