How to set the command(s) that get run before every single prompt?

Question

I used to know this but I forgot: I want to execute a command or a couple of commands before every single prompt. When I am in our PROD box, I want a big fat reminder above every prompt that says * YOU ARE IN PROD * and maybe another thing or two. Currently, I think that is how mail is checked but I can't remember how that configuration is set.

Using ksh and Solaris 5.10

Any idea?

Thanks

Answer 1

If all you want is a big fat fixed warning, then simply add it to the prompt:

red=$(tput setaf 1; tput smso; tput bold) reset_color=$(tput sgr0)
PS1='
${red}[THIS IS A PRODUCTION SYSTEM!!]${reset_color}

rest of the prompt$ '

In modern (post 1995) implementations of ksh (ksh93, mksh), $PS1 is subject to command substitution and parameter expansion, so you can do things like:

function mywarning {
  tput setaf 1; tput bold
  date "+it's %T, please remember it's a production system"
  tput sgr0
}

PS1='$(mywarning)# '

In ksh88 (which Solaris 10's /usr/bin/ksh still is), only parameter and tilde expansion are done in the prompt.

You have to revert to clever tricks for anything fancy in the prompt. And then, you're quite limited in what you can do.

While arithmetic expansion is not performed, as you can expand things like ${array[expr]}, you can have arithmetic expressions (here expr) evaluated. Because assignments (though only to numerical values) can be done in arithmetic expressions, that gives some opportunities to do fancy things.

You can use that to post-process dynamic variables like $SECONDS, $PWD, $?, $!, $RANDOM, $_.

For instance, you can get a HH:MM:SS date with:

typeset -Z2 _h _m _s
eval "$(date +'SECONDS=$((%H*3600+%M*60+%S))')"
PS1='${z[(_h=SECONDS/3600)+(_m=(SECONDS/60)%60)+(_s=SECONDS%60)]}${_h}:${_m}:${_s} $ '

You can add your fat warning every 5 prompts by doing:

set -A warn '' '
[THIS IS A PRODUCTION SYSTEM!!]
' '' '' ''
PS1='${warn[warn_count=(warn_count+1)%5]}# '

I don't think you can have a command run before each prompt (the mail check is done internally by ksh), but you can have one run after each command with the DEBUG trap:

trap 'my-command' DEBUG

Note that it's done after every simple command in every subshell.

So for instance, in:

for f in *; do :; done

my-command will be run as many times as there are files in the current directory.

In:

: | :

It is run 3 times... So in effect, it's not really practical.

Answer 2

I'd simply put it into /etc/profile, or /etc/issue. Sometimes displaying the contents of the issue file is turned off, but I'd still think it's a good reminder.

Changing the contents of the $PS1 variable is ok, I guess, you might want to still print some warnings /etc/profile. Since it's a production system, It's ok to edit those files, since here, you want lasting changes in the system. (Normally, I'd advise against changing those files and using user files instead.)

Answer 3

You can see the host, time, locations and git branch with:

HOST='\033[02;36m\]\h'; HOST=' '$HOST
TIME='\033[01;31m\]\t \033[01;32m\]'
LOCATION=' \033[01;34m\]`pwd | sed "s#\(/[^/]\{1,\}/[^/]\{1,\}/[^/]\{1,\}/\).*\(/[^/]\{1,\}/[^/]\{1,\}\)/\{0,1\}#\1_\2#g"`'
BRANCH=' \033[00;33m\]$(git_branch)\[\033[00m\]\n\$ '
PS1=$TIME$USER$HOST$LOCATION$BRANCH

You could modify any of these, e.g. host.

If you want to run a command use backtics - these ``