Hi,
You are correct, export COLUMNS does truncate, I had made the mistake of
replaying the previous execution of ps which was with “axww”.
Something does go wrong once COLUMNS is in the environment, it seems that in
that case, it can get modified when executing a pipe’d command.
As the following examples show, we have narrowed this down to two things
1. set -a causes, in certain situation, to be clarified, but the pipe
below gives us a big hint, the COLUMNS variable to be exported when it should
NOT be
2. COLUMNS gets modified, by the shell, when a command with a ‘|’ is
processed
3. Those two conditions trigger what I saw as a bug in the output of “ps
ax”. Now it seems that “ps” is the revealer and not buggy as I suspected at
some point.
Starting a new bash :
host:/$ export COLUMNS=60
host:/$ ps ax | grep java | head -1
2051669 ? Sl 0:17 /usr/lib/jvm/java-17-openjdk-am
host:/$ export COLUMNS=80
host:/$ ps ax | grep java | head -1
2051669 ? Sl 0:17 /usr/lib/jvm/java-17-openjdk-amd64/bin/java -Xms2g
host:/$ export COLUMNS=120
host:/$ ps ax | grep java | head -1
2051669 ? Sl 0:17 /usr/lib/jvm/java-17-openjdk-amd64/bin/java -Xms2g
-Xmx2g -server -XX:+UseG1GC -XX:MaxGCPau
host:/$ ps ax | grep java | head -1 ### next output should be the
same as previous, but it is truncated
2051669 ? Sl 0:17 /usr/lib/jvm/java-17-openjdk-amd64/bin/java -Xms2g
-Xmx2g -server -XX:+Us
host:/$ echo $COLUMNS # COLUMNS was reset by bash
102
My conclusion is that the variable COLUMNS gets “reset” when there is a pipe
in a command and further gets EXPORTED if “set -a” was set before hand :
host:/$ COLUMNS=80
host:/$ echo $COLUMNS
80
host:/$ echo $COLUMNS
80
host:/$ echo $COLUMNS | cat
80
host:/$ echo $COLUMNS
102
PS1 is :
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$
I would challenge the fact that “set -a” should move COLUMNS to the
environment when it gets modified by bash itself : not the expected behaviour.
@ Robert Elz, the POSIX definition does put an additional spin to this. I was
going by the very short explanation of set -a in the bash man page.
And I would challenge the fact that COLUMNS gets modified when a command
with a ‘|’ is executed. There is no way for a bash user or bash script writer
to expect such a behaviour in unforeseen circumstances. Also, it basically
makes “set -a” too dangerous to use, so why bother implementing it if it
shouldn’t be used ? That last bit is maybe a bit of a stretch, but you met my
meaning.
I do believe that this must be considered as a bug as it breaks scripts which
otherwise work perfectly. I added the following line in a file which is
getting source’d to define variables. I was not aware that the caller had set
“-a”. In fact, scripts should not be so vulnerable to a change to a bash
variable made by the caller. ( I did describe a work-around using “echo $-“
within a $( command ) in my bug report.)
echo $PATH | grep -q xx/scripts || PATH=${PATH}:/xx/scripts
I do not believe that adding this to a bash file should cause a failure
100’s of lines down in a totally different script bought from a third party
whose use of “set –a” I do not control. I do agree, now that I know about this
highly unexpected feature, that the third party should have used “ps axww”, but
then how could they expect the COLUMNS variable to be exported ? There is no
testing in the world which would catch a case like this. Hence, I do insist
that this is a bug.
The only way to prevent the ‘|’ from modifying COLUMNS is to put the
command within $( ) from my empirical testing which is worth what it is worth.
Not straight forward…
Regards,
Alain Brossard
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Alain BROSSARD
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From: Oğuz <[email protected]>
Sent: Thursday, June 13, 2024 5:34 PM
To: Alain BROSSARD <[email protected]>
Cc: [email protected]
Subject: Re: it, soRE: set -a leads to truncated output from ps
On Thursday, June 13, 2024, Alain BROSSARD <abrossard@ reyl. com> wrote: export
COLUMNS=60 ps ax | grep java The output isn’t truncated. It is on my machine.
You must have something in your PS1/PROMPT_COMMAND interfering. It is critical
that
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On Thursday, June 13, 2024, Alain BROSSARD
<[email protected]<mailto:[email protected]>> wrote:
export COLUMNS=60
ps ax | grep java
The output isn’t truncated.
It is on my machine. You must have something in your PS1/PROMPT_COMMAND
interfering.
It is critical that application’s behaviour do not get impacted by the shell
used to call them.
Even more reason to use `ps axww' instead of `ps ax'.
What do you think ?
I think there's at least one Bash bug involved here.
--
Oğuz
