Wednesday, January 4, 2012

Logging Shell Commands in Linux

If you own/administer a Linux machine or server which is being used by multiple users, at some time you must have felt the need to log and record the commands being run in the shell by the users. This can be useful for auditing, tracking and saving the commands being used.

Now the problem is that the built-in logging mechanism in bash i.e. .bash_history file (or the "history" command) is not reliable. First, the command is not logged immediately it is entered into the shell, 2nd and most importantly if the bash session is terminated or gets disconnected abnormally, all the commands for that session would be lost.
 This means that if the linux system crashes, you will never see the commands that were last run in the active bash sessions when the system crashed. Well in bash's defence this feature is never intended to be used for tracking or auditing, it is just command history keeper that keeps track of you last executed commands in case you press the up-arrow-key or use reverse command search (ctrl + r).

There are many methods to log shell commands, some involve recompiling bash and others involve installing third party key logging software. Both of these options are discouraged and are risky especially on production servers.

Recently I discovered a very quick, safe, neat and efficient way of logging bash commands. I am using it on many production machines and it is doing its job well.

Step 1:
Use your favourite text editor to open /etc/bashrc and append the following line at the end:
export PROMPT_COMMAND='RETRN_VAL=$?;logger -p local6.debug "$(whoami) [$$]: $(history 1 | sed "s/^[ ]*[0-9]\+[ ]*//" ) [$RETRN_VAL]"'

Step 2:
Set the syslogger to trap local6 to a log file by adding this line in the /etc/syslog.conf file:
local6.*                /var/log/cmdlog.log
and restart
service syslog restart

Logging bash commands is this easy! Just logoff and log back in and all your commands will be logged in /var/log/cmdlog.log from now on. It is easily customizable what you want to log. Let me first explain how this works

How it works

Well the crucial key behind this logging mechanism is the "PROMPT_COMMAND" variable in bash. According to the bash manual:
If set, the value is executed as a command prior to issuing each primary prompt.
In easy words(tldp):

Bash provides an environment variable called PROMPT_COMMAND. The contents of this variable are executed as a regular Bash command just before Bash displays a prompt.
So once we know that bash will execute the content of this variable, we can do any thing (limited to your imagintion). For logging this is what I am doing:
i.e, save the return code in a variable (before it is lost ;))
logger -p local6.debug "$(whoami) [$$]: $(history 1 | sed "s/^[ ]*[0-9]\+[ ]*//" ) [$RETRN_VAL]"
next I am logging the following (using the logger command):
$(whoami) => Current user name
[$$] => PID of current Shell (helps differentiate b/w multiple sessions by the same user)
$(history 1 | sed "s/^[ ]*[0-9]\+[ ]*//" ) => Last executed command, sed is used to remove the command index number and the whitespaces outputted by the history command
[$RETRN_VAL] => Add the return code at the end in square brackets

Sample Logs:
Jan  2 15:20:01 testdb1 oratest: oratest [5583]: cd /testdata [0]
Jan  2 15:20:02 testdb1 oratest: oratest [5583]: ls -ltr [0]
Jan  2 15:20:15 testdb1 oratest: oratest [5583]: du -sh * [0]
Jan  2 15:21:56 testdb1 oratest: oratest [5583]: clear [0]
Jan  2 15:21:59 testdb1 oratest: oratest [5583]: df -h  [0]
Jan  2 15:28:00 testdb1 oratest: oratest [5583]: uptime [0]
Jan  2 15:28:09 testdb1 oratest: oratest [5583]: top c [0]
Jan  3 10:57:18 testdb1 khizer: khizer [22493]: su - [0]
Jan  3 10:57:26 testdb1 khizer: khizer [22493]: su - [1]
Jan  3 10:57:31 testdb1 khizer: root [22646]: ulimit -c [0]
Jan  3 11:20:49 testdb1 khizer: khizer [808]: ping [0]
Jan  3 11:29:57 testdb1 khizer: root [22646]: crontab -e [0]
Jan  3 11:30:01 testdb1 khizer: root [22646]: service crond relaod [1]
Jan  3 11:30:05 testdb1 khizer: root [22646]: service crond reload [0]
Jan  3 11:30:20 testdb1 khizer: khizer [808]: wq [127]
Jan  3 11:30:43 testdb1 khizer: khizer [808]: q [127]
Jan  3 11:37:13 testdb1 khizer: khizer [808]: date [0]
Jan  3 12:00:05 testdb1 khizer: root [22646]: crontab -e [0]
Jan  3 12:00:06 testdb1 khizer: root [22646]: service crond reload [0]
Jan  4 10:42:10 testdb1 oratest: oratest [23732]: rm -rf rda readme.txt 6249879 [0]
Jan  4 10:42:12 testdb1 oratest: oratest [23732]: ls -ltr  [0]
Jan  4 10:42:17 testdb1 oratest: oratest [23732]: mkdir TEST [0]
Jan  4 10:42:18 testdb1 oratest: oratest [23732]: pwd [0]
Jan  4 10:43:56 testdb1 oratest: oratest [23732]: vi backup.sql [0]
Jan  4 10:44:02 testdb1 oratest: oratest [23732]: vi backup.sql [1]


  1. Hi!

    just one small problem:

    if you hit the "enter" key without a command, the last command is being logged again and again...

  2. Yeah it is like that. Normally sys-log would combine the similar outputs to say "Last Message repeated N times" instead of logging thing again and again.

  3. Thank you. This will really help me to monitor the DBA/APPs Guys what they are doing in our server.

  4. Thank you for your resolution. But, how to logging csh, sh...shel commands ? Please help me!

  5. Hello,

    How can i make mapping between PID of current Shell and ip address associated with this.

  6. I can add $SSH_CLIENT into /etc/bashrc and is ok for the curent user login. But if the user became root via sudo, in the next log the ip address no longer exist.

    Any idea?

    1. That is because when a shell user switches to another user it gets a new shell session along with new shell variables. I can't seem to find a easy way to pass along a variable to the newly spawned shells (via su or sudo). A very quick way of solving this problem would be to keep track of TTY session (which will be consistent through all the sub-shell sessions) and $SSH_CLIENT variable in a file. Here is how I did it:

      #> mkdir -p /var/run/cmd_log
      #> touch /var/run/cmd_log/_dev_pts_{0..100}
      #> chmod -R 777 /var/run/cmd_log

      Set the COMMAND_PROMPT variable in /etc/bashrc something like this:

      export PROMPT_COMMAND='RETRN_VAL=$?;TTY_SESSION=$(tty | tr '/' '_');test -n "$SSH_CLIENT" -a ! -s /var/run/cmd_log/$TTY_SESSION && echo $SSH_CLIENT > /var/run/cmd_log/$TTY_SESSION; printf "\033]0;%s@%s:%s\007" "${USER}" "${HOSTNAME%%.*}" "${PWD/#$HOME/~}";logger -p local6.debug "$(whoami) [$$]: $(history 1 | sed "s/^[ ]*[0-9]\+[ ]*//" ) [$RETRN_VAL] [[$(cat /var/run/cmd_log/$TTY_SESSION)]]"'

      I am sure there must be better ways of doing this! If you figure out a better way, do let me know :)

      I hope this helps.

  7. ¿Does this affects the value of $? ? if you need to use it after a command it is preserved or is the value of the PROMPT_COMMAND?

    1. Good question. No this doesn't affect the value of $? variable.

  8. Not working for me on centos 5.6. even the log file is not getting created

  9. kill -9 $$ and you log nothing :) solve this one ;)

    1. You are right. This script has many more limitations than just that. Just unset the PROMPT_COMMAND variable and it will stop working. This script is just to keep a good time based record of the commands you or your fellow users run on the shell. This script doesn't promise to help you track users with bad intentions.

  10. "service syslog restart" this command needs to be run after the config changes. Missing in the steps above

    1. Yes, thanks for pointing that out. I'll add it in the steps.

  11. Hi Khizer! great tutorial. Just one thing (i don't know if i'm doing this right) with this procedure i only get root user bash logs. I mean, with sudo su will log messages, but if i'm not root, it won't. Is this ok? if it is, how can i add all users bash logs?

    1. As long as you stay in the bash shell, no matter what user you switch to, it should log the commands. I use this extensively in production (mainly on RHEL 5/6). How ever, if the user you are switching to from root has its default shell other than bash, like csh or ksh, it will stop working. (you can check the default shell of the users in /etc/passwd)

  12. lets say an oracle user changes his shell and gets into the sql prompt, is ther a way to track down/log the sql commands the user ran ?

    1. No any thing outside bash won't be logged. Just the command you enter in the bash shell

    2. Depending on your db you can monitor sql commands with builtin auditing for oracle, sybase or write a trigger that would save all needed data in table for postgres, mysql

  13. Is there a way you know I could monitor the commands that are typed on runtime. Lets say I want to monitor my own commands and have the terminal prompt me if I typed "find" instead of "find -name ". So i'd like to stop the execution of the command and prompt back saying "hey you typed this command, please type this and that to execute it" sorts.
    Thanks for any help in the direction. I've been trying to get info on how to do this but haven't been successful yet.

  14. Great tutorial and thank you! I've set this up on multiple rhel servers but would like to have all instances of /var/log/cmdlog.log files written to a single file on one server. Can you assist?

  15. hi,

    I am facing one problem with this .The history 1 command executes when the user login initially and takes the last command executed .

    For example a normal user executed init 6 ( which will fail since it i not root ) and logs init 6 command with return code 1 ( This is fine for me ) and lets consider user logged off for the day.
    Next day when user logging in the last command will be init 6 in history and it logs as init 6 with a return code 0 ( Infact on this day user was not tried the init 6 command ) . But the log says user executed init 6 command and it was a success .

    Could you please help on this ?
    On the next day

  16. You could use two alternative options to achieve similar result:
    - auditd, if your kernel/distro supports it, targeted more at real auditing, or
    - Snoopy Logger ( ), lightweight(-er) solution, if you trust your users and just need a log of commands).

    Disclosure: Snoopy maintainer here.

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  18. This comment has been removed by the author.

  19. Thank you very much, It is very useful for me..............

  20. gracias, me sirvio y tomare en cuenta los comenarios de los demas