When and how is the variable $HOSTNAME populated ?
I am asking because I had a weird situation that just happened.
I have created backup scripts that do backups on different servers. Each servers has different folders to be backed up. So I had included a check on $HOSTNAME to find out which folders to back up.
But tonight, when I started the backup on one server, the backup did not start because it said I was on the wrong server (my coding). I turned out aht the content of the $HOSTNAME was empty. All the other servers had no problems starting the same exact backup script as it $HOSTNAME was containing the right servename.
Here are the steps I did immediatlly after:
A) Checking the content of the $HOSTNAME variable:
- I logged with my own operator userid and did "echo $HOSTNAME". It had the servername in it
- I did a "sudo echo $HOSTNAME" and the content was empty
- I did a "sudo su -" then a "echo $HOSTNAME and the variable had the server name in it.
B) I opened up another window and did the same steps as in A) and all 3 steps showed that $HOSTNAME had a servername content.
I had to resort doing a "sudo su -" and start the backup. Otherwise, I would have had to change the script as a quick workaround.
Somehow, on that specific xterm window I had opened, the content of the $HOSTNAME variable was not populated in all cases. I want to know how this happened and why as it may happen again with another operator (they don't know how to script).
Any variable in the shell comes into existence by your shell's startup scripts...
I would start there. Not sure about ksh, but in bash it would be /etc/bashrc....start there, and that file should lead you to other files that might get "sourced" (processed) within that file. Then there is your local user shell startup file....for instance, "~/.bashrc"
If you know the files, you could "grep -n HOSTNAME" on those files, to show the line numbers where the string HOSTNAME occurs.
funksen and zaxxon are both correct. Never depend on any variables set outside of a script but "bring with you what you need to work with".
The number 1 problem (by a large margin) with cron-scripts is: "i have written a script which ran fine in my shell, then i put it into crontab and now it doesn't work any more. It still runs when i test it." The reason is that your login shell inherits all kinds of environment variables but scripts started by cron don't. If you implicitly rely on - for instance - $PATH being set the same way it is set in your interactive shell you are in for a bad surprise.
Several variables are set in your "~/.profile" and in your "~/.kshrc", but some system-wide variables are set in "/etc/environment". All these files do NOT apply to scripts started by cron.
This is the reason why i have ALL my scripts use their own environment, regardless of them being run in cron or not. The beginning of my scripts always looks like this:
Code:
#! /bin/ksh
if [ -z "$DEVELOP" ] ; then # set environment
. /usr/local/lib/ksh/f_env
else
. ~/lib/f_env
fi
typeset SCRIPT_INTERN_VAR1=""
typeset SCRIPT_INTERN_VAR2=""
... etc., etc. ...
where "/usr/local/lib/ksh/f_env" is a ksh function which declares all the variables common to all the scripts. It looks like this:
Code:
unset ENV # clear the environment
#---------------------------------------------------- set basic environment
typeset -x OS=$(uname -a | cut -d' ' -f1) # find out the OS
# read in standard environment
case "$OS" in
AIX)
. /etc/environment
;;
Linux)
. /etc/profile
;;
*)
. /etc/environment
;;
esac
# set default TERM variable
case "$OS" in
AIX)
TERMDEF="$(termdef)"
;;
Linux)
TERMDEF="$TERM"
;;
*)
TERMDEF="$TERM"
;;
esac
typeset -x TERM=${TERMDEF:-'wyse60'} # set default TERM variable
typeset -x LANG=C # default language environment
typeset -x EDITOR=vi # what else ? ;-))
typeset -x VISUAL=$EDITOR
typeset -x PATH="/usr/bin" # set the path
PATH="$PATH:/bin"
PATH="$PATH:/etc"
PATH="$PATH:/usr/sbin"
PATH="$PATH:/usr/ucb"
PATH="$PATH:/sbin"
PATH="$PATH:/usr/bin/X11"
PATH="$PATH:/usr/local/bin" # tools, home for scripts
PATH="$PATH:/usr/local/sbin" # -"-
typeset -x chTmpDir="" # path for temporary files
... etc., etc. ...
I never have to worry about a script being run in cron or not, being run by a specific user or even on a specific system - it won't fail because of the environment being set wrongly.
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When/how is $HOSTNAME populated ?