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From: charles.fisher@arconic.com
To: dash@vger.kernel.org
Subject: EXIT trap does not fire
Date: 31 May 2022 16:39:47 -0400	[thread overview]
Message-ID: <NOATDC-APPGTW91Teip00015ab2@NOATDC-APPGTW91.arconic.com> (raw)

I think that I have found a bug in dash that impacts EXIT trap handling. The behavior is different (and appears correct) in bash, ksh93 and mksh. I am using the EPEL dash package, version 0.5.10.2 for CentOS 7 (full package info included).

I didn't see this already reported on the mailing list; hopefully I am not reporting a duplicate bug, sorry if so.

The ssh-agent used in the context immediately below seems to prevent an EXIT trap from firing upon abnormal termination:

  # cat dashbug_trivial
  #!/bin/dash

  eval $(ssh-agent)

  trap 'eval $(ssh-agent -k)' EXIT

  if ssh-add
  then echo testing
       read junk
  fi

The EXIT does not fire if the user aborts (it fires properly if the user does not abort):

  # ./dashbug_trivial 
  Agent pid 59714
  Enter passphrase for /root/.ssh/id_ed25519: 
  Identity added: /root/.ssh/id_ed25519 (root@bastion.mycompany.com)
  testing
  ^C
  # ps ax | grep agent | grep -v grep
  59714 ?        Ss     0:00 ssh-agent


I am actually iterating over a set of hosts inside the conditional:

  # cat dashbug
  #!/bin/dash

  eval $(ssh-agent)

  trap 'eval $(ssh-agent -k)' EXIT

  if ssh-add
  then while read -r host port user &lt;&9
       do printf '=%s=%d=%s=\n' "$host" "$port" "$user"
          ssh -p "$port" "$user@$host" "$@" 2>&1
       done 9&lt;&lt;-''EndOfHosts
	limsprd.amppcs.noa.alcoa.com 22 root
	darkstar.dpw.alcoa.com 24 root
  EndOfHosts
  fi

The iterative script executes perfectly in dash; the problem is abnormal termination:

  # ./dashbug uptime
  Agent pid 10809
  Enter passphrase for /root/.ssh/id_ed25519: 
  Identity added: /root/.ssh/id_ed25519 (root@bastion.mycompany.com)
  =host1.mycompany.com=22=root=
  13:12:52 up 233 days,  1:58,  0 users,  load average: 0.66, 0.63, 0.54
  =host2.mycompany.com=24=root=
  13:12:52 up 2 days,  1:59,  0 users,  load average: 0.00, 0.00, 0.00
  Agent pid 10809 killed

Oddly, when I left off the trailing "fi" the trap killed the agent:

  # ./dashbug uptime
  Agent pid 9953
  ./dashbug: 15: ./dashbug: Syntax error: end of file unexpected (expecting "fi")
  Agent pid 9953 killed

When I break the password prompt with a control-c, dash does not terminate the agent:

  # ./dashbug uptime
  Agent pid 10072
  Enter passphrase for /root/.ssh/id_ed25519: ^C

  #

I confirmed this with a check of the process list:

  # ps ax | grep agent | grep -v grep
  10072 ?        Ss     0:00 ssh-agent

If I switch this to mksh (or other previously mentioned shells), a control-c does what I expect:

  # head -1 dashbug
  #!/bin/mksh

  # ./dashbug uptime
  Agent pid 10193
  Enter passphrase for /root/.ssh/id_ed25519: ^C
  Agent pid 10193 killed
  #

Is this a problem with dash, or a lack in my understanding?


These are the full details on the CentOS 7 EPEL package for dash:

  # rpm -qi dash
  Name        : dash
  Version     : 0.5.10.2
  Release     : 1.el7
  Architecture: x86_64
  Install Date: Wed 11 Aug 2021 01:45:26 PM CDT
  Group       : System Environment/Shells
  Size        : 159742
  License     : BSD and GPLv2+ and Public Domain and Copyright only
  Signature   : RSA/SHA256, Mon 05 Nov 2018 06:00:50 AM CST, Key ID 72f97b74ec551f03
  Source RPM  : dash-0.5.10.2-1.el7.src.rpm
  Build Date  : Mon 05 Nov 2018 06:00:48 AM CST
  Build Host  : x86-ol7-builder-01.us.oracle.com
  Relocations : (not relocatable)
  Vendor      : Oracle America
  URL         : http://gondor.apana.org.au/~herbert/dash/
  Summary     : Small and fast POSIX-compliant shell
  Description :
  DASH is a POSIX-compliant implementation of /bin/sh that aims to be as small as
  possible. It does this without sacrificing speed where possible. In fact, it is
  significantly faster than bash (the GNU Bourne-Again SHell) for most tasks.

next             reply	other threads:[~2022-05-31 20:39 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-31 20:39 charles.fisher [this message]
2022-05-31 21:57 ` EXIT trap does not fire Paul Smith
2022-06-01 12:47   ` Steffen Nurpmeso
  -- strict thread matches above, loose matches on Subject: below --
2022-11-12 11:02 Akbarkhon Variskhanov
2022-11-12 12:02 ` Harald van Dijk

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