Screen Environment – How to Set Screen Environment Variable from Bash

environment-variablesgnu-screen

Background

I use virtualenv to switch between environments "1.0" and "2.0".
I use screen to manage terminals.
When I work on one environment I want all new screen windows to start with this environment.

Question

Can bash instruct screen to set environment variable on new shell instances, so I could make an alias similar to this one:

alias one='export PRJCT=1.0; screen-magic-setenv PRJCT=1.0; workon 1.0'
alias two='export PRJCT=2.0; screen-magic-setenv PRJCT=2.0; workon 2.0'

and afterwards in ~/.bashrc call:

[[ -z $PRJCT ]] || workon $PRJCT # switch to project if set

I know I can command screen to set environment variable on new shells like below but it doesn't help as it won't work in an alias:

:setenv PRJCT
2.0

Best Answer

If I understand correctly, your problem is you cannot find a way to use a shell alias to interact with screen directly. Instead, you can send commands to a running screen using -X, including setenv of course, e.g.:

$ screen -list
There are screens on:
    25216.pts-45.antiriad      (Attached)

$ screen -S 25216 -X setenv PRJCT 2.0

The variable is then set, and will be inherited by child shells from that point on, exactly as with :setenv. If you use -S to set sensible distinct session names your task may be easier. The variable STY holds the current screen session.

If you run just screen -X setenv PRJCT 2.0 within a screen session, omitting -S, it will apply to that screen instance.

alias one='export PRJCT=1.0; screen -X setenv PRJCT 1.0; workon 1.0'

Related Solutions

Bash – How to Set Environment Variables Dynamically

export $key=$val should work just fine in bash. I suspect your issue is the pipe (|). All commands in a pipeline are executed in a subshell. In your example, the instance of bash that is running your script will fork off 2 subshells: one for cat $1 and another for the while read .... The variables are assigned and exported in the subshell running the while loop, and then all of them are promptly thrown out when the subshell exits. One way to fix this is to not spawn subshells at all. Instead of a useless use of cat, try redirection instead:

while read ...; do
   ...
done < "$1"

BashFAQ 24 explains this in much greater detail.

An alternate approach is to just source the file, with exports thrown in:

. <(sed '/^export/!s/^/export /' "$1")

^{The <( ) is process substitution.}

As an additional note of caution, since you are reading environment variables from an argument, you must make sure that the argument file $1 is a trusted source so it can't do bad things to your script.

How does ‘screen’ impact environment variables

A process inherits the environment variables from the parent, this means the first time you call screen (create a new one) it has a copy of all the environment variables of the parent process. Now screen adjusts/creates some variables like COLUMNS, LINES, TERM, TERMCAP, WINDOW and STY. You can also adjust or delete environment variables in your screenrc with setenv/unsetenv.

On some systems, screen is setuid or setgid in order to update utmp and wtmp; then a few more variables are removed from the environment when screen starts, such as LD_LIBRARY_PATH.

If you attach to an existing screen session your environment variables won't be copied as the screen process already exists and has it own environment variables (from the time when you started the process before). This means your changed environment variables won't be visible in the processes started by screen as they are copied from the parent process which has the old environment variables.