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About bash : How-to-write-a-bash-script-to-set-global-environment-variable

Question Detail

Recently I wrote a script which sets an environment variable, take a look:

#!/bin/bash

echo "Pass a path:"
read path
echo $path

defaultPath=/home/$(whoami)/Desktop

if [ -n "$path" ]; then
    export my_var=$path
else
    echo "Path is empty! Exporting default path ..."
    export my_var=$defaultPath
fi

echo "Exported path: $my_var"

It works just great but the problem is that my_var is available just locally, I mean in console window where I ran the script.

How to write a script which allow me to export global environment variable which can be seen everywhere?

Question Answer

Just run your shell script preceded by “.” (dot space).

This causes the script to run the instructions in the original shell. Thus the variables still exist after the script finish

Ex:

cat setmyvar.sh
export myvar=exists

. ./setmyvar.sh

echo $myvar
exists

Each and every shell has its own environment. There’s no Universal environment that will magically appear in all console windows. An environment variable created in one shell cannot be accessed in another shell.

It’s even more restrictive. If one shell spawns a subshell, that subshell has access to the parent’s environment variables, but if that subshell creates an environment variable, it’s not accessible in the parent shell.

If all of your shells need access to the same set of variables, you can create a startup file that will set them for you. This is done in BASH via the $HOME/.bash_profile file (or through $HOME/.profile if $HOME/.bash_profile doesn’t exist) or through $HOME/.bashrc. Other shells have their own set of startup files. One is used for logins, and one is used for shells spawned without logins (and, as with bash, a third for non-interactive shells). See the manpage to learn exactly what startup scripts are used and what order they’re executed).

You can try using shared memory, but I believe that only works while processes are running, so even if you figured out a way to set a piece of shared memory, it would go away as soon as that command is finished. (I’ve rarely used shared memory except for named pipes). Otherwise, there’s really no way to set an environment variable in one shell and have another shell automatically pick it up. You can try using named pipes or writing that environment variable to a file for other shells to pick it up.

Imagine the problems that could happen if someone could change the environment of one shell without my knowledge.

Actually I found an way to achieve this (which in my case was to use a bash script to set a number of security credentials)

I just call bash from inside the script and the spawned shell now has the export values

export API_USERNAME=abc
export API_PASSWORD=bbbb
bash

now calling the file using ~/.app-x-setup.sh will give me an interactive shell with those environment values setup

The following were extracted from 2nd paragraph from David W.’s answer: “If one shell spawns a subshell, that subshell has access to the parent’s environment variables, but if that subshell creates an environment variable, it’s not accessible in the parent shell.”

In case a user need to let parent shell access your new environment variables, just issue the following command in parent shell:

source <your_subshell_script>

or using shortcut

. <your_subshell_script>

You got to add the variable in your .profile located in /home/$USER/.profile

Yo can do that with this command:

echo 'TEST="hi"' >> $HOME/.profile

Or by edit the file with emacs, for example.
If you want to set this variable for all users, you got to edit /etc/profile (root)

There is no global environment, really, in UNIX.

Each process has an environment, originally inherited from the parent, but it is local to the process after the initial creation.

You can only modify your own, unless you go digging around in the process using a debugger.

write it to a temporary file, lets say ~/.myglobalvar and read it from anywhere

echo "$myglobal" > ~/.myglobalvar

Environment variables are always “local” to process execution the export command allow to set environment variables for sub processes. You can look at .bashrc to set environment variables at the start of a bash shell. What you are trying to do seems not possible as a process cannot modify (or access ?) to environment variables of another process.

You can update the ~/.bashrc or ~/.bash_profile file which is used to initialize the environment.

Take a look at the loading behavior of your shell (explained in the manpage, usually referring to .XXXshrc or .profile). Some configuration files are loaded at login time of an interactive shell, some are loaded each time you run a shell. Placing your variable in the latter might result in the behavior you want, e.g. always having the variable set using that distinct shell (for example bash).

If you need to dynamically set and reference environment variables in shell scripts, there is a work around. Judge for yourself whether is worth doing, but here it is.

The strategy involves having a ‘set’ script which dynamically writes a ‘load’ script, which has code to set and export an environment variable. The ‘load’ script is then executed periodically by other scripts which need to reference the variable. BTW, the same strategy could be done by writing and reading a file instead of a variable.

Here’s a quick example…

Set_Load_PROCESSING_SIGNAL.sh

#!/bin/bash
PROCESSING_SIGNAL_SCRIPT=./Load_PROCESSING_SIGNAL.sh
echo "#!/bin/bash" > $PROCESSING_SIGNAL_SCRIPT
echo "export PROCESSING_SIGNAL=$1" >> $PROCESSING_SIGNAL_SCRIPT
chmod ug+rwx $PROCESSING_SIGNAL_SCRIPT

Load_PROCESSING_SIGNAL.sh (this gets dynamically created when the above is run)

#!/bin/bash
export PROCESSING_SIGNAL=1

You can test this with
Test_PROCESSING_SIGNAL.sh

#!/bin/bash
PROCESSING_SIGNAL_SCRIPT=./Load_PROCESSING_SIGNAL.sh
N=1
LIM=100
while [ $N -le $LIM ]
do
# DO WHATEVER LOOP PROCESSING IS NEEDED
echo "N = $N"
sleep 5
N=$(( $N + 1 ))

# CHECK PROCESSING_SIGNAL
source $PROCESSING_SIGNAL_SCRIPT
if [[ $PROCESSING_SIGNAL -eq 0 ]]; then
# Write log info indicating that the signal to stop processing was detected
# Write out all relevent info
# Send an alert email of this too
# Then exit
echo "Detected PROCESSING_SIGNAL for all stop. Exiting..."
exit 1
fi
done

~/.bin/SOURCED/lazy script to save and load data as flat files for system.

[ ! -d ~/.megadata ] && mkdir ~/.megadata

function save_data {
[ -z "$1" -o -z "$2" ] && echo 'save_data [:id:] [:data:]' && return
local overwrite=${3-false}
[ "$overwrite" = 'true' ] && echo "$2" > ~/.megadata/$1 && return
[ ! -f ~/.megadata/$1 ]   && echo "$2" > ~/.megadata/$1 || echo ID TAKEN set third param to true to overwrite
}

save_data computer engine
cat ~/.megadata/computer
save_data computer engine
save_data computer megaengine true

function get_data {
[ -z "$1" -o -f $1 ] && echo 'get_data [:id:]' && return


[ -f ~/.megadata/$1 ]   && cat ~/.megadata/$1 || echo ID NOT FOUND
:
}

get_data computer
get_data computer

Maybe a little off topic, but when you really need it to set it temporarily to execute some script and ended up here looking for answers:

If you need to run a script with certain environment variables that you don’t need to keep after execution you could do something like this:

#!/usr/bin/env sh

export XDEBUG_SESSION=$(hostname);echo "running with xdebug: $XDEBUG_SESSION";[email protected]

In my example I just use XDEBUG_SESSION with a hostname, but you can use multiple variables. Keep them separated with a semi-colon. Execution as follows (assuming you called the script debug.sh and placed it in the same directory as your php script):

$ debug.sh php yourscript.php

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