How can I get the pid of a subshell? [duplicate]

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This question already has an answer here:

$BASHPID And $$ differ in some cases

2 answers

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.

edited Nov 27 at 14:59

Kusalananda

118k16223360

asked Nov 27 at 13:23

Tim

25.1k72243442

marked as duplicate by elbarna, RalfFriedl, roaima, Isaac, thrig Nov 28 at 0:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment |

up vote
4
down vote

favorite

This question already has an answer here:

$BASHPID And $$ differ in some cases

2 answers

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.

edited Nov 27 at 14:59

Kusalananda

118k16223360

asked Nov 27 at 13:23

Tim

25.1k72243442

marked as duplicate by elbarna, RalfFriedl, roaima, Isaac, thrig Nov 28 at 0:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment |

up vote
4
down vote

favorite

This question already has an answer here:

$BASHPID And $$ differ in some cases

2 answers

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.

edited Nov 27 at 14:59

Kusalananda

118k16223360

asked Nov 27 at 13:23

Tim

25.1k72243442

This question already has an answer here:

$BASHPID And $$ differ in some cases

2 answers

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.

This question already has an answer here:

$BASHPID And $$ differ in some cases

2 answers

bash process subshell

edited Nov 27 at 14:59

Kusalananda

118k16223360

asked Nov 27 at 13:23

Tim

25.1k72243442

edited Nov 27 at 14:59

Kusalananda

118k16223360

asked Nov 27 at 13:23

Tim

25.1k72243442

edited Nov 27 at 14:59

Kusalananda

118k16223360

edited Nov 27 at 14:59

Kusalananda

118k16223360

edited Nov 27 at 14:59

Kusalananda

118k16223360

asked Nov 27 at 13:23

Tim

25.1k72243442

asked Nov 27 at 13:23

Tim

25.1k72243442

asked Nov 27 at 13:23

Tim

25.1k72243442

marked as duplicate by elbarna, RalfFriedl, roaima, Isaac, thrig Nov 28 at 0:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

marked as duplicate by elbarna, RalfFriedl, roaima, Isaac, thrig Nov 28 at 0:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment |

2 Answers
2

active

oldest

votes

up vote
13
down vote

accepted

$ 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.

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

edited Nov 27 at 13:42

answered Nov 27 at 13:34

Kusalananda

118k16223360

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

@Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
– Kusalananda
Nov 27 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 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 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 at 14:57

|
show 1 more comment

up vote
3
down vote

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")

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

edited Nov 27 at 16:49

answered Nov 27 at 15:00

mosvy

4,996323

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

Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
– mosvy
Nov 27 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 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 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 at 16:01

|
show 4 more comments

2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
13
down vote

accepted

$ 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.

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

edited Nov 27 at 13:42

answered Nov 27 at 13:34

Kusalananda

118k16223360

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

@Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
– Kusalananda
Nov 27 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 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 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 at 14:57

|
show 1 more comment

up vote
13
down vote

accepted

$ 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.

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

edited Nov 27 at 13:42

answered Nov 27 at 13:34

Kusalananda

118k16223360

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

@Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
– Kusalananda
Nov 27 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 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 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 at 14:57

|
show 1 more comment

up vote
13
down vote

accepted

$ 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.

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

edited Nov 27 at 13:42

answered Nov 27 at 13:34

Kusalananda

118k16223360

$ 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.

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

edited Nov 27 at 13:42

answered Nov 27 at 13:34

Kusalananda

118k16223360

edited Nov 27 at 13:42

answered Nov 27 at 13:34

Kusalananda

118k16223360

answered Nov 27 at 13:34

Kusalananda

118k16223360

answered Nov 27 at 13:34

Kusalananda

118k16223360

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

@Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
– Kusalananda
Nov 27 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 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 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 at 14:57

|
show 1 more comment

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

@Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
– Kusalananda
Nov 27 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 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 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 at 14:57

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

@Tim I believe this is answered by Gilles here. Bash simply does not update $$ in subshells.
– Kusalananda
Nov 27 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 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 at 14:20

@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 at 14:57

|
show 1 more comment

up vote
3
down vote

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")

edited Nov 27 at 16:49

answered Nov 27 at 15:00

mosvy

4,996323

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

Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
– mosvy
Nov 27 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 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 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 at 16:01

|
show 4 more comments

up vote
3
down vote

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")

edited Nov 27 at 16:49

answered Nov 27 at 15:00

mosvy

4,996323

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

Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
– mosvy
Nov 27 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 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 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 at 16:01

|
show 4 more comments

up vote
3
down vote

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")

edited Nov 27 at 16:49

answered Nov 27 at 15:00

mosvy

4,996323

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")

edited Nov 27 at 16:49

answered Nov 27 at 15:00

mosvy

4,996323

edited Nov 27 at 16:49

answered Nov 27 at 15:00

mosvy

4,996323

answered Nov 27 at 15:00

mosvy

4,996323

answered Nov 27 at 15:00

mosvy

4,996323

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

Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
– mosvy
Nov 27 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 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 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 at 16:01

|
show 4 more comments

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

Should work in any POSIX shell -- eg in debian's /bin/sh (dash) and in busybox (ash).
– mosvy
Nov 27 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 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 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 at 16:01

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

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

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 at 15:18

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 at 15:21

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 at 16:01

|
show 4 more comments

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