CSH subshell behaviour in Red Hat Enterprise Linux 5 differs from previous releases

Solution Verified - Updated -


  • Red Hat Enterprise Linux 5
  • tcsh-6.14-12.el5 or newer


Subshells created via `cmd` may be launched after forking twice. This can cause programs that depend on information from getppid() to fail.


This issue should be resolved in tcsh-6.14-17.el5_5.2, released by Advisory RHBA-2010:0823-1.   To resolve this issue, please upgrade to the latest version of tcsh available for Red Hat Enterprise Linux 5.

Root Cause

The following bit of code was removed from sh.glob.c``:

    861             if (t)
    862               t->t_dflg |= F_NOFORK;

The code above prevents tcsh from forking a new process when evaluating command substitutions in backquotes ("").  Removal of that code causedtcsh` to fork an extra process before running the command.

Diagnostic Steps

Run a command like the following in tcsh:

echo `pstree -p $$`

In Red Hat Enterprise Linux 4 (tcsh-6.13), something like the following should be output:


Red Hat Enterprise Linux 5 (tcsh-6.14) should print something like the following:


The output from the command in Red Hat Enterprise Linux 5 shows that tcsh is forking twice before running the pstree command, whereas the output in Red Hat Enterprise Linux 4 shows that tcsh is only forking once.

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