All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing)

with

(
cd somedir
do_something
)

The ostensible rationale is that it is shorter/easier to code, but the real rationale is that if the cd fails, putting it into a subshell "localizes"
the damage.

I find this analysis problematic for (at least) the following reasons:

1) First of all, I almost always run with either -e or (more likely)
"trap handler ERR", where handler is a shell function that prints
an error message and exits. So, if any "cd" fails, the script
aborts. Shellcheck fails to realize this and flags every "cd" in
the script with "Don't you mean: cd ... || exit". So, if it
recognized the trap better, both this and the SC2103 thing would
evaporate.

2) Subshells (still, as far as I know) require a fork() and run as
another process. Given that bash is a large program (it says so
right in the man page), this fork() is expensive. And note that it
isn't the cheap sort of fork() where it is immediately followed by
an exec(). It's the expensive kind which requires the COW
mechanism to kick in. So, it seems unwise for shellcheck to be
recommending using a subshell.

I certainly avoid it if possible, for this reason.

A further note: The most common case is to use (as shown above):

cd somewhere;...;cd ..

but the more general pattern would be:

cd somewhere;...;cd -

But note that "cd -" - even in a script - prints the name of the directory
it is cd'ing back to, which is annoying. I could not find any option to
turn this off, but: cd - > /dev/null
works. I'm guessing that people avoid using "cd -" in scripts for this
reason.

--
Modern Christian: Someone who can take time out from using Leviticus
to defend homophobia and Exodus to plaster the Ten Commandments on
every school and courthouse to claim that the Old Testament is merely
"ancient laws" that "only applies to Jews".

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[email protected] (Kenny McCormack):

All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing)

with

(
cd somedir
do_something
)

The ostensible rationale is that it is shorter/easier to code, but the real rationale is that if the cd fails, putting it into a subshell "localizes"
the damage.

Does


cd -- somedir &&
{
do_something
cd ..
}


make shellcheck happy while at the same time localizing the
damage and avoiding a subshell?

cd somewhere;...;cd -

But note that "cd -" - even in a script - prints the name of the directory
it is cd'ing back to, which is annoying. I could not find any option to
turn this off,

cd -- "$OLDPWD"

Will help.


But note, that


cd -- "$OLDPWD"


as well as


cd -


will not restore the working directory established at the start
of the


cd somewhere; do_something; cd -


commandline if that directory gets renamed or moved by a
(asynchronous) process while running.


Try the following command in an empty directory:  It creates a
subdirectory named “sandbox” and in it more subdirectories and
removes everything when it ends:


(
mkdir -- sandbox &&
{
n=1 &&
mkdir -p -- sandbox/"$n"/WD &&
{
while
sleep 1 &&
mv -- "$n"/ "$((n+1))"/ &&
n="$((n+1))"
do
:
done &
} &&
pid="$!" &&
{
(
CDPATH= cd -- sandbox/"$n"/WD &&
exec "$SHELL"
)
kill -s INT -- "$pid"
}
sleep 1
rm -R -- sandbox
}
)


The command creates the subdirectories “sandbox/1” and
“sandbox/1/WD” and launches a process that, running in the
background, will rename the directory “sandbox/1/” after 1 second
to “sandbox/2/”, then, after an additional second to “sandbox/3/”
and so on, continuing incrementing the number, until terminated.


At the same time the command launches in the foreground an
interactive (sub‐)shell (“"$SHELL"”), using the directory
“sandbox/1/WD” as its working directory, allowing the user to
type commands.  If they type “exit”, the “"$SHELL"” will exit.


The command then will signal the background process an INT signal
and removes the directory “sandbox/” including its subhierarchy. 
Finally it exits.


Each second the launched “"$SHELL"” will have its working
directory renamed to a new path in the file system without being
notificated about that fact.


In the launched interactive “"$SHELL"” one can repeatedly type the
commands


pwd -P

pwd -L

cd .


or the like, observing, whether they continue to report the
original (obsolete) path of the working directory or follow the
changed path.

In this use case, the command

cd -- "$OLDPWD"


might fail to restore the former working directory.

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On 05.03.2025 19:40, Kaz Kylheku wrote:

On 2025-03-05, Kenny McCormack <[email protected]> wrote:

All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing) >>
with

(
cd somedir
do_something
)

That obviously won't work if do_something has to set a variable
that is then visible to the rest of the script.

Indeed. Only for strict hierarchical semantics it makes sense.

Forking a process just to preserve a current working directory
is wasteful; we wouldn't do that in a C program, where we might
open the current directory to be saved, and then fchdir back to it.

Shells may be different. While Bash regularly creates a subprocess,
Ksh in certain structures creates just a "subshell context" without forking/cloning an own process.

[...]

but the more general pattern would be:

cd somewhere;...;cd -

cd - will break if any of the steps in between happen to to cd;
it is hostile toward maintenance of the script.

Indeed.

Just note that spending a "subshell context"[Ksh] or a subprocess
[Bash] keeps the structure intact. (If you want to pay for that;
especially when it's costly [as in Bash].)

Janis

[...]

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On 2025-03-05, Kenny McCormack <[email protected]> wrote:

All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing)

with

(
cd somedir
do_something
)

That obviously won't work if do_something has to set a variable
that is then visible to the rest of the script.

Forking a process just to preserve a current working directory
is wasteful; we wouldn't do that in a C program, where we might
open the current directory to be saved, and then fchdir back to it.

However, most of the actions in a shell script fork and exec
something anyway.

The ostensible rationale is that it is shorter/easier to code, but the real rationale is that if the cd fails, putting it into a subshell "localizes"
the damage.

save_pwd=$(pwd) # local save_pwd=$(pwd) in shells that have local

if cd somedir ; then
...
cd "$save_pwd"
else
...
fi

but the more general pattern would be:

cd somewhere;...;cd -

cd - will break if any of the steps in between happen to to cd;
it is hostile toward maintenance of the script.

By the way, we should also try to exploit the capability of commands to
do their own chdir.

E.g:

save_pwd=$(pwd)
cd somewhere
tar czf "$save_pwd"/foo.tar.gz .
cd "$save_pwd"

becomes

tar -C somewhere -czf foo.tar.gz .

tar will change to somewhere for the sake of finding the
files, and will resolve the . argument relative to that location,
but the foo.tar.gz file is created in the original directory
where it was invoked.

Another utility with -C <dir> is make.


--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @[email protected]

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Kaz Kylheku <[email protected]>:

On 2025-03-05, Kenny McCormack <[email protected]> wrote:

All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing) >>
with

(
cd somedir
do_something
)

That obviously won't work if do_something has to set a variable
that is then visible to the rest of the script.

Forking a process just to preserve a current working directory
is wasteful; we wouldn't do that in a C program, where we might
open the current directory to be saved, and then fchdir back to it.

Unfortunately, there is no way to fchdir() by means of a shell
built‐in command, say “fchdir“.

cd somewhere;...;cd -

cd - will break if any of the steps in between happen to to cd;
it is hostile toward maintenance of the script.

Yes.  I really would appreciate using the imagined “fchdir” bash
built‐in command to be able to do


set_exit_status()
{
return ${1-}
}
unset -v es &&
exec {saved_wd}< . &&
{
cd -- somedir &&
{
do_something
es="$?"
fchdir -- "$saved_wd"
}
exec {saved_wd}<&-
set_exit_status ${es-}
}

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In article <[email protected]>,
Kaz Kylheku <[email protected]> wrote:

On 2025-03-05, Kenny McCormack <[email protected]> wrote:

All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing) >>
with

(
cd somedir
do_something
)

That obviously won't work if do_something has to set a variable
that is then visible to the rest of the script.

That's actually mentioned in the rationale on the web page.
That if you need to set a variable, you can't use subshells.

Forking a process just to preserve a current working directory
is wasteful; we wouldn't do that in a C program, where we might
open the current directory to be saved, and then fchdir back to it.

However, most of the actions in a shell script fork and exec
something anyway.

Yes, but those would be the cheap form of fork(), where if it is quickly followed by an exec(), you don't have to do COW.

Note that, in Linux, fork() is actually an alias for vfork().

--
The randomly chosen signature file that would have appeared here is more than 4 lines long. As such, it violates one or more Usenet RFCs. In order to remain in compliance with said RFCs, the actual sig can be found at the following URL:
http://user.xmission.com/~gazelle/Sigs/TedCruz

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In article <[email protected]>,
Helmut Waitzmann <[email protected]> wrote:
...

Does

cd -- somedir &&
{
do_something
cd ..
}

make shellcheck happy while at the same time localizing the
damage and avoiding a subshell?

Yes, it does - silence both warnings. But seems kind of end-around.
The point is that it should recognize that you are running in "trap ... ERR" mode and therefore there can't be an untrapped "cd".

Now, I get that this is probably just too hard for shellcheck to do
(although it is amazing it does as much as it does, and it is getting
better with each new version), and so we have to live with the warning, but
my overall point is that this could be problematic if you happen to work in
an environment where management insists that your script pass shellcheck.

cd somewhere;...;cd -

But note that "cd -" - even in a script - prints the name of the directory >> it is cd'ing back to, which is annoying. I could not find any option to
turn this off,

cd -- "$OLDPWD"

Will help.

Pick your poison.

I'm happy enough with just using "> /dev/null".
It is about the same # of characters to type.

--
Nov 4, 2008 - the day when everything went
from being Clinton's fault to being Obama's fault.

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On 2025-03-06, Kenny McCormack <[email protected]> wrote:

In article <[email protected]>,
Kaz Kylheku <[email protected]> wrote:

On 2025-03-05, Kenny McCormack <[email protected]> wrote:

All testing done with shellcheck version 0.10.0 and bash under Linux.

Shellcheck says that you should replace code like:

cd somedir
do_something
cd .. # (Or, cd -, which is almost, but not exactly the same thing) >>>
with

(
cd somedir
do_something
)

That obviously won't work if do_something has to set a variable
that is then visible to the rest of the script.

That's actually mentioned in the rationale on the web page.
That if you need to set a variable, you can't use subshells.

Forking a process just to preserve a current working directory
is wasteful; we wouldn't do that in a C program, where we might
open the current directory to be saved, and then fchdir back to it.

However, most of the actions in a shell script fork and exec
something anyway.

Yes, but those would be the cheap form of fork(), where if it is quickly followed by an exec(), you don't have to do COW.

Note that, in Linux, fork() is actually an alias for vfork().

Nope. In Linux, fork translates to a clone call with a
menu of options to bring about the fork behavior. Whereas
vfork is its own system call. You can easily see this with
strace.

Classic vfork shares address space between the child and parent,
including the stack frame where the the fork is happening.

Linux vfork mitigates the problems which could arise from that
sharing by suspending the parent until the child either
execs or terminates.

Either way, fork cannot be vfork; that is nuts. It would break amost all programs which do anything other than exec an image in the child.

Of course, fork *can* be vfork when it is vfork that is made
identical to fork.

I have an old vfork test program in my directory of such test
programs. It's showing that vfork does share the stack frame
with parent and child.

The output is "var == 43" showing that the parent sees the
value incremented by the child:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main(void)
{
volatile int var = 42;
int status;
pid_t child = vfork();

if (child > 0) {
waitpid(child, &status, 0);
printf("var == %d\n", var);
} else if (child == 0) {
var++;
_exit(0);
} else {
perror("vfork");
return EXIT_FAILURE;
}

return EXIT_SUCCESS;
}

If we reorder the declarations like this:

int main(void)
{
pid_t child = vfork();
volatile int var = 42;
int status;

The output is then "var == 42". Why? Because vfork() suspends the parent
until the child shits a new process, or gets off the can.
And only when vfork terminates does the parent execute the effect
of the "volatile int var = 42" declaration. So at that point, it has
clobbered the value to 42. While the parent is suspended, the
child also initializes the value to 42, and increments it to 43.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @[email protected]

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