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How can I get the pid of a subshell?

Do parentheses really put the command in a subshell?How can I detect if I'm in a subshell?$BASHPID And $$ differ in some casesBash subshell creation with curly bracesHow to get the pid to the last background appWhy is subshell created by background control operator (&) not displayed under pstreeHow can we prevent parameter expansion?Subshell does not create a separate processGet PID and return code from 1 line bash callSupress errors in a subshell?Why do `jobs` and `dirs` run in command subsitution, process substitution, pipeline, and background jobs output the same as in original shell?Why does bash not remove backslash in the quote removal step in this example?How can I stop a child process of a subshell (as per SIGSTOP) before the subshell exits?

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ margin-bottom:0;
}

7

1

How can I get the pid of a subshell?

For example:

$ echo $$

16808

This doesn't work, because the original shell expands $$:

$ ( echo $$ )

16808

Why does single quoting not work? After the original shell removes the single quote, does the subshell not expand $$ in itself?

$ ( echo '$$' )

$$

Why does eval not work either? Is eval run by the subshell? Why does it give me the original shell's PID?

$ ( eval echo '$$' )

16808

Thanks.

bash process subshell

share|improve this question

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I suggest a reopen, because the questions are essentially different in my opinion ("how to avoid $$ expansion" vs. "different pid in subshell").
  
  – peterh
  2 hours ago
  
  
  

  
  
  
  

add a comment | 

7

1

How can I get the pid of a subshell?

For example:

$ echo $$

16808

This doesn't work, because the original shell expands $$:

$ ( echo $$ )

16808

Why does single quoting not work? After the original shell removes the single quote, does the subshell not expand $$ in itself?

$ ( echo '$$' )

$$

Why does eval not work either? Is eval run by the subshell? Why does it give me the original shell's PID?

$ ( eval echo '$$' )

16808

Thanks.

bash process subshell

share|improve this question

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I suggest a reopen, because the questions are essentially different in my opinion ("how to avoid $$ expansion" vs. "different pid in subshell").
  
  – peterh
  2 hours ago
  
  
  

  
  
  
  

add a comment | 

How can I get the pid of a subshell?

For example:

$ echo $$

16808

This doesn't work, because the original shell expands $$:

$ ( echo $$ )

16808

Why does single quoting not work? After the original shell removes the single quote, does the subshell not expand $$ in itself?

$ ( echo '$$' )

$$

Why does eval not work either? Is eval run by the subshell? Why does it give me the original shell's PID?

$ ( eval echo '$$' )

16808

Thanks.

bash process subshell

share|improve this question

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

$ echo $$

16808

$ ( echo $$ )

16808

$ ( echo '$$' )

$$

$ ( eval echo '$$' )

16808

share|improve this question

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

share|improve this question

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

edited Nov 27 '18 at 14:59

Kusalananda♦

151k18298478

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

asked Nov 27 '18 at 13:23

TimTim

29.6k85279515

2 Answers
2

active

oldest

votes

7

In addition to bash's $BASHPID, you can do it portably with:

pid=$(exec sh -c 'echo "$PPID"')

Example:

(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")

You can make it into a function:

# usage getpid [varname]

getpid(){

    pid=$(exec sh -c 'echo "$PPID"')

    test "$1" && eval "$1=$pid"

}

Notice that some shells (eg. zsh or ksh93) do NOT start a subprocess for each subshell created with (...); in that case, $pid may be end up being the same as $$, which is just right, because that's the PID of the process getpid was called from.

share|improve this answer

edited Dec 2 '18 at 0:29

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. By portably, you mean ...?
  
  – Tim
  Nov 27 '18 at 15:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
  
  – mosvy
  Nov 27 '18 at 15:15
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No. But please do not assume that a subshell is necessarily run in a subprocess -- that is not the case in ksh93, for instance.
  
  – mosvy
  Nov 27 '18 at 15:18
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It will work fine in ksh93 -- it will always return the pid of the process it was called from. It's the (...) from the example which may not spawn a separate process, as it does in bash.
  
  – mosvy
  Nov 27 '18 at 15:21
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, some shells like zsh or yash optimise out a fork() for the last command in a subshell. They may even optimise out the fork for the subshell if it's the last command in a script so your getpid could even report the parent of $$. You could define getpid as: getpid(){ sh -c 'echo "$PPID"'; return; } to disable avoid the problem.
  
  – Stéphane Chazelas
  Nov 27 '18 at 16:01
  

  
  
  
  

 | 
show 5 more comments

14

$ echo $BASHPID

37152

$ ( echo $BASHPID )

18633

From the manual:

BASHPID

Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.

$

Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the
subshell.

Related:

• 
  Do parentheses really put the command in a subshell?, especially parts of Gilles' answer.

share|improve this answer

edited Nov 27 '18 at 13:42

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. (1) What does "re-initialized" mean? (2) Could you also consider why those ways I have tried do not work?
  
  – Tim
  Nov 27 '18 at 13:36
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
  
  – Kusalananda♦
  Nov 27 '18 at 13:44
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you mean I should always use $BASHPID in place of $$ in any case in bash? When shall I use which?
  
  – Tim
  Nov 27 '18 at 14:09
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tim It depends on whether you, in a subshell, wants to get the process ID of the script or of the subshell. Both possibilities are provided and which is the correct one is dependent on the application. No more specific answer can be given to that.
  
  – Kusalananda♦
  Nov 27 '18 at 14:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Tim The PID of a parent shell of a subshell can't reliably be found unless you arrange to save $BASHPID in a variable and use that in the subshell. There is $PPID, but that's the parent PID of the shell in the same sense that $$ is the PID of the shell (it's not reset in a subshell). There is no $BASHPPID variable.
  
  – Kusalananda♦
  Nov 27 '18 at 14:57
  
  
  

  
  
  
  

 | 
show 1 more comment

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7

In addition to bash's $BASHPID, you can do it portably with:

pid=$(exec sh -c 'echo "$PPID"')

Example:

(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")

You can make it into a function:

# usage getpid [varname]

getpid(){

    pid=$(exec sh -c 'echo "$PPID"')

    test "$1" && eval "$1=$pid"

}

Notice that some shells (eg. zsh or ksh93) do NOT start a subprocess for each subshell created with (...); in that case, $pid may be end up being the same as $$, which is just right, because that's the PID of the process getpid was called from.

share|improve this answer

edited Dec 2 '18 at 0:29

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. By portably, you mean ...?
  
  – Tim
  Nov 27 '18 at 15:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
  
  – mosvy
  Nov 27 '18 at 15:15
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No. But please do not assume that a subshell is necessarily run in a subprocess -- that is not the case in ksh93, for instance.
  
  – mosvy
  Nov 27 '18 at 15:18
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It will work fine in ksh93 -- it will always return the pid of the process it was called from. It's the (...) from the example which may not spawn a separate process, as it does in bash.
  
  – mosvy
  Nov 27 '18 at 15:21
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, some shells like zsh or yash optimise out a fork() for the last command in a subshell. They may even optimise out the fork for the subshell if it's the last command in a script so your getpid could even report the parent of $$. You could define getpid as: getpid(){ sh -c 'echo "$PPID"'; return; } to disable avoid the problem.
  
  – Stéphane Chazelas
  Nov 27 '18 at 16:01
  

  
  
  
  

 | 
show 5 more comments

7

In addition to bash's $BASHPID, you can do it portably with:

pid=$(exec sh -c 'echo "$PPID"')

Example:

(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")

You can make it into a function:

# usage getpid [varname]

getpid(){

    pid=$(exec sh -c 'echo "$PPID"')

    test "$1" && eval "$1=$pid"

}

Notice that some shells (eg. zsh or ksh93) do NOT start a subprocess for each subshell created with (...); in that case, $pid may be end up being the same as $$, which is just right, because that's the PID of the process getpid was called from.

share|improve this answer

edited Dec 2 '18 at 0:29

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. By portably, you mean ...?
  
  – Tim
  Nov 27 '18 at 15:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
  
  – mosvy
  Nov 27 '18 at 15:15
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No. But please do not assume that a subshell is necessarily run in a subprocess -- that is not the case in ksh93, for instance.
  
  – mosvy
  Nov 27 '18 at 15:18
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It will work fine in ksh93 -- it will always return the pid of the process it was called from. It's the (...) from the example which may not spawn a separate process, as it does in bash.
  
  – mosvy
  Nov 27 '18 at 15:21
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, some shells like zsh or yash optimise out a fork() for the last command in a subshell. They may even optimise out the fork for the subshell if it's the last command in a script so your getpid could even report the parent of $$. You could define getpid as: getpid(){ sh -c 'echo "$PPID"'; return; } to disable avoid the problem.
  
  – Stéphane Chazelas
  Nov 27 '18 at 16:01
  

  
  
  
  

 | 
show 5 more comments

In addition to bash's $BASHPID, you can do it portably with:

pid=$(exec sh -c 'echo "$PPID"')

Example:

(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")

You can make it into a function:

# usage getpid [varname]

getpid(){

    pid=$(exec sh -c 'echo "$PPID"')

    test "$1" && eval "$1=$pid"

}

Notice that some shells (eg. zsh or ksh93) do NOT start a subprocess for each subshell created with (...); in that case, $pid may be end up being the same as $$, which is just right, because that's the PID of the process getpid was called from.

share|improve this answer

edited Dec 2 '18 at 0:29

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

pid=$(exec sh -c 'echo "$PPID"')

(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")

# usage getpid [varname]

getpid(){

    pid=$(exec sh -c 'echo "$PPID"')

    test "$1" && eval "$1=$pid"

}

share|improve this answer

edited Dec 2 '18 at 0:29

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

answered Nov 27 '18 at 15:00

mosvymosvy

14.1k21547

14

$ echo $BASHPID

37152

$ ( echo $BASHPID )

18633

From the manual:

BASHPID

Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.

$

Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the
subshell.

Related:

• 
  Do parentheses really put the command in a subshell?, especially parts of Gilles' answer.

share|improve this answer

edited Nov 27 '18 at 13:42

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. (1) What does "re-initialized" mean? (2) Could you also consider why those ways I have tried do not work?
  
  – Tim
  Nov 27 '18 at 13:36
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
  
  – Kusalananda♦
  Nov 27 '18 at 13:44
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you mean I should always use $BASHPID in place of $$ in any case in bash? When shall I use which?
  
  – Tim
  Nov 27 '18 at 14:09
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tim It depends on whether you, in a subshell, wants to get the process ID of the script or of the subshell. Both possibilities are provided and which is the correct one is dependent on the application. No more specific answer can be given to that.
  
  – Kusalananda♦
  Nov 27 '18 at 14:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Tim The PID of a parent shell of a subshell can't reliably be found unless you arrange to save $BASHPID in a variable and use that in the subshell. There is $PPID, but that's the parent PID of the shell in the same sense that $$ is the PID of the shell (it's not reset in a subshell). There is no $BASHPPID variable.
  
  – Kusalananda♦
  Nov 27 '18 at 14:57
  
  
  

  
  
  
  

 | 
show 1 more comment

14

$ echo $BASHPID

37152

$ ( echo $BASHPID )

18633

From the manual:

BASHPID

Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.

$

Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the
subshell.

Related:

• 
  Do parentheses really put the command in a subshell?, especially parts of Gilles' answer.

share|improve this answer

edited Nov 27 '18 at 13:42

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. (1) What does "re-initialized" mean? (2) Could you also consider why those ways I have tried do not work?
  
  – Tim
  Nov 27 '18 at 13:36
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
  
  – Kusalananda♦
  Nov 27 '18 at 13:44
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you mean I should always use $BASHPID in place of $$ in any case in bash? When shall I use which?
  
  – Tim
  Nov 27 '18 at 14:09
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Tim It depends on whether you, in a subshell, wants to get the process ID of the script or of the subshell. Both possibilities are provided and which is the correct one is dependent on the application. No more specific answer can be given to that.
  
  – Kusalananda♦
  Nov 27 '18 at 14:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Tim The PID of a parent shell of a subshell can't reliably be found unless you arrange to save $BASHPID in a variable and use that in the subshell. There is $PPID, but that's the parent PID of the shell in the same sense that $$ is the PID of the shell (it's not reset in a subshell). There is no $BASHPPID variable.
  
  – Kusalananda♦
  Nov 27 '18 at 14:57
  
  
  

  
  
  
  

 | 
show 1 more comment

$ echo $BASHPID

37152

$ ( echo $BASHPID )

18633

From the manual:

BASHPID

Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.

$

Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the
subshell.

Related:

• 
  Do parentheses really put the command in a subshell?, especially parts of Gilles' answer.

share|improve this answer

edited Nov 27 '18 at 13:42

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478

$ echo $BASHPID

37152

$ ( echo $BASHPID )

18633

share|improve this answer

edited Nov 27 '18 at 13:42

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478

answered Nov 27 '18 at 13:34

Kusalananda♦Kusalananda

151k18298478