Every once in a while, you might find yourself in a situation where your environment variables are not accessible in bash scripts. This can be a frustrating experience, especially when your scripts rely heavily on these variables. But why does this happen? This article aims to demystify this issue and provide a comprehensive solution.
To make environment variables accessible in bash scripts, they need to be exported. This can be done either by exporting the variables in the .bashrc file or by exporting them within the bash script itself. Without exporting, the variables will not be accessible within the script.
Understanding Bash and Environment Variables
Before diving into the problem, it’s important to understand what bash and environment variables are. Bash (Bourne Again SHell) is a command processor that typically runs in a text window, allowing the user to interact with the system.
Environment variables, on the other hand, are dynamic-named values that can affect the way running processes behave on a computer. They exist in every operating system, and Linux is no exception. They provide a simple way to share configuration settings between multiple applications and processes in Linux.
The Problem: Inaccessible Environment Variables in Bash Scripts
When running a bash script, the script runs in its own bash instance, which is a child shell of the current one. By default, variables are not passed to child shells, unless they are exported. This is where the problem arises. If your environment variables are not exported, they will not be accessible within your bash scripts.
For instance, let’s assume you have an environment variable HADOOP_HOME set as /home/me/dist/hadoop. You have a bash script /tmp/sample.sh that echoes the value of HADOOP_HOME. If HADOOP_HOME is not exported, running /tmp/sample.sh will not display the correct value of HADOOP_HOME.
The Solution: Exporting Environment Variables
To make your environment variables accessible in bash scripts, you need to export them. This can be done in two ways:
Method 1: Exporting Variables in .bashrc
The .bashrc file is a shell script that Bash runs whenever it is started interactively. You can put any command in that file that you could type at the command prompt.
You can export your environment variables in this file. To do this, open the .bashrc file and add the following line:
export HADOOP_HOME=/home/me/dist/hadoopAfter saving the file, run the following command to apply the changes:
source ~/.bashrcThe source command reads and executes commands from the file specified as its argument in the current shell environment.
Now, when you run the /tmp/sample.sh script, the HADOOP_HOME variable will be accessible and will display the correct value.
Method 2: Exporting Variables in the Bash Script
Alternatively, you can modify the script itself to export the variable before using it. Update the /tmp/sample.sh script as follows:
#! /bin/bash
export HADOOP_HOME
echo $HADOOP_HOMEThe export command in a shell script marks an environment variable to be exported with any newly forked child processes and thus it allows a child process to inherit all marked variables.
Save the script and then run it again. This time, the HADOOP_HOME variable will be exported within the script’s environment, and you will see the correct value being echoed.
Conclusion
Remember that exporting a variable makes it available to child shells and processes. Without exporting, the variable will only be accessible within the current shell session. By understanding this, you can ensure that your environment variables are always accessible in your bash scripts.
Environment variables are not accessible in bash scripts by default because they are not automatically passed to child shells. To make them accessible, you need to export them using the export command.
There are two ways to export environment variables in bash. You can either add the export command followed by the variable name and its value in the .bashrc file and then use the source command to apply the changes, or you can explicitly export the variables within the bash script itself using the export command.
The .bashrc file is a shell script that Bash runs whenever it is started interactively. It allows you to set up your shell environment by defining aliases, setting environment variables, and running other shell commands.
You can edit the .bashrc file using a text editor of your choice. For example, you can use the nano editor by running the command nano ~/.bashrc. Make the necessary changes, save the file, and then use the source command to apply the changes to your current shell session.
Yes, you can export multiple environment variables in a single command by separating them with spaces. For example, you can use the command export VAR1=value1 VAR2=value2 VAR3=value3 to export multiple variables at once.
You can use the env command in bash to display all the currently exported environment variables. Running env will list all the variables along with their values.
Yes, you can unset an exported environment variable using the unset command followed by the variable name. For example, you can use the command unset VAR_NAME to unset the variable named VAR_NAME.
No, if you have exported the environment variables in the .bashrc file, they will be automatically exported every time you open a new terminal session. However, if you only export the variables within a specific bash script, you will need to run that script or export the variables manually in each new session.
Yes, you can export environment variables for all users on a system by adding the export commands to the /etc/environment file. This file is read by all users during the login process and can be used to set system-wide environment variables.
Yes, environment variables in bash are case-sensitive. VAR_NAME, var_name, and Var_name would be treated as three separate variables.
