Alvin's HISTFILE workaround

Here is the background of my story. In my site, there are 3 hosts (named host-A, host-B & host-C) running AIX 6.1. Each host allows a maximum of 2 telnet login sessions. So, I can at most login 6 telnet sessions.  My $HOME directory is mounted in host-B. Using NFS, all the 3 hosts will share the same $HOME directory. Ideally I would expect each telnet session have its own command history, but this does not happen.

Once login, the korn shell will create a command history file .sh_history in the $HOME.  Then the 6 telnet sessions will share the SAME command history file $HOME/.sh_history. This is really a nightmare.

To solve this problem, I tried to define the HISTFILE variable in the first line of my $HOME/.profile.  This HISTFILE will define an unique filename for each of the 6 telnet sessions. So, each session will have different command history filenames. However, such a HISTFILE setup does not work.

Why the HISTFILE is not functioning? Sometimes even the HISTFILE is defined in the first line of the $HOME/.profile file, it is not functioning at all. Why? It is because of the /etc/profile file. When korn shell is your login shell, once login, the korn shell will firstly execute the system file /etc/profile before executing the $HOME/.profile file. Usually, some third party software (such as file transfer or database software) will modify this /etc/profile to instruct it to run some setup scripts of the software. Commonly those setup script will use some functions. Once there is function declaration in the script, the korn shell will record the function in the command history. Thus, the default command history file $HOME/.sh_history is used. Once the default $HOME/.sh_history file is used, the name of the command history file will not be changed by the HISTFILE variable.

Actually, the command [[ set -o nolog ]] can prevent the korn shell to record the function the command history file. Usually, the nolog option will not be set in the /etc/profile. You can modify the /etc/profile to add this nolog option. However, once the host is already in production, changing the system file /etc/profile requires numerous procedures and approvals. No one will take the risk to approve this kind of system file modifications. In other words, it is very difficult to make this change in a production host. As a result, those third party software setup script and its function in the /etc/profile will force the korn shell to use the command history file $HOME/.sh_history once you login.

To be more complicate, the AIX 6.1 seems cache the command history in the memory. Therefore, when issuing the command [[ fc -l ]], sometimes the result is the command history in the cache memory but sometimes it is a mix of commands from the 6 telnet sessions due to sharing same $HOME/.sh_history file.

The korn shell in AIX 6.1 has a built-in fc command. When issuing the command [[ fc -l ]] in an interactive korn shell, this built-in fc command seems rely on the cached command history.

However, when using [[ fc -l ]] inside a script (which is run under another process), the external /usr/bin/fc or /bin/fc commands may be used (because a process should not use the korn shell process built-in commands). For the external command /usr/bin/fc, it seems that it cannot access the cached command history of the login korn shell. So, it will reply on the default command history file $HOME/.sh_history or the HISTFILE environment variable. When the HISTFILE is defined and exported to the script, it will use it, even though this HISTFILE is not functioning in the login korn shell. This further complicates the situation.

Also, when issuing [[ fc -l & ]] or [[ nohup fc -l & ]] in an interactive korn shell, a separate process is run for the command. This separate process seems behave quite similar to the situation above.

In order to solve the above problems, what we need is an unique command history file for each of the 6 telnet session. But, how to do so ? Here is my solution. I named it as Alvin's HISTFILE workaround.

This workaround is to add the following line at the beginning of the $HOME/.profile file.

if tty -s then      export HISTFILE=~/.sh_history.e`/bin/date +"%s"`.`/bin/hostname`.pts`/bin/tty | /bin/awk -F'/' '{print $NF}'`.r$RANDOM      if [ -e ~/.sh_history ]      then           /bin/ln ~/.sh_history $HISTFILE           /bin/rm -f ~/.sh_history           /bin/find ~ -type f -name '.sh_history.e*r*' -mtime +15 -exec /bin/rm {} \; &      fi fi

Here is a detail description of how this workaround achieve my purpose.

When I telnet login the first session, this workaround is executed

The first line [[ if tty -s ]] is to ensure that this is an interactive shell. In my site, there are scheduled jobs running using root-id to [[ su - ]] to my user-id to execute some programs on behave of me. This action will call my $HOME/.profile and then run some programs in "no terminal" mode. In AIX 6.1, such a "no terminal" situation will make the [[ pts ]] command to give error messages which will spoil the scheduled jobs and also the HISTFILE variable. Anyway, the primary purpose of the workaround is for interactive korn shell, so it is better to have such a [[ if tty -s ]] checking.

The next line is to define the HISTFILE variable. The variable will use the epoch time, the hostname, the pts number and a random number. So, for the 6 telnet login sessions, the HISTFILE will be unique.

Then, the [[ if ]] statement is to double confirm the $HOME/.sh_history is already exist. Such file should be created by the /etc/profile once login.

Then, we use hard-link to make the two files $HOME/.sh_history and the $HISTFILE sharing the same i-node. Now, the content of this i-node file is the command history of this interactive korn shell. This i-node has 2 links : the $HOME/.sh_history and the $HISTFILE files. Now, no matter whether the built-in fc command or the external /usr/bin/fc command is used, each command will refer to the same command history content in that i-node.

The next line is to remove the filename .sh_history in the $HOME directory. Such a file removal will not affect the interactive korn shell. The shell should already get the i-node number for command history processing throughout its life. The i-node will not physically clean-up because there is still a link (the $HISTFILE file) pointing to it. As a result, both the interactive korn shell and the external /usr/bin/fc commands will function as usual.

Now, I login the second telnet session. Since the file .sh_history is already removed from the $HOME directory, the /etc/profile will create a new .sh_history file (with a new i-node number, of course). Then, this second interactive korn shell will work on this new i-node number for command history. Also, a new HISTFILE is defined for this shell. The $HISTFILE filename is different from the first telnet session. So, this new i-node and new $HISTFILE is only for this second telnet shell. Therefore, I have a completely independent command history file in the second login shell.

This process is repeated for the third, fourth, fifth and sixth interactive login korn shell. There will be six $HISTFILE in the $HOME directory, each using a different i-node.

The final [[ find ]] command is to clean-up those $HISTFILE files older than 15 days. It is assumed that you exit the login korn shell at least 15 days after login. If you do not exit the shell after 15 days, a new login process will kill your $HISTFILE and your i-node will be clean-up. Then, un-expected behavior will be resulted. Normally, you should exit the shell as soon as you no longer need it.

Alvin SIU
2013-04-23

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