Understanding Variables in Unix Shell: A Comprehensive Guide

In the realm of Unix and shell scripting, the concept of temporary variables is often misunderstood. While there are variables and you can use them to hold values temporarily, there is no inherent distinction between 'temporary' and 'persistent' variables. This article aims to provide a deep dive into understanding how variables work in Unix shells, including their scope, manipulation, and the differences between shell and environment variables.

Creating and Using Temporary Variables in a Unix Shell

It is indeed possible to create temporary variables in Unix shell environments such as Bash. These variables can be used for various purposes, like calculations or storing intermediate results, during the terminal session. Here's how you can define, use, and unset them:

Defining a Variable

To define a variable, you simply assign a value to it. This can be done using a straightforward assignment statement:

my_varvalue

Using the Variable

To access the variable, you prefix it with a dollar sign ($):

echo $my_var

Unsetting the Variable

If you need to remove the variable, you can use the unset command:

unset my_var

Complete Example

Here’s a complete example of creating, using, and unsetting a temporary variable:

# Define a temporary variabletemp_varvalue# Use the variableecho $temp_var     # Output: value# Unset the variableunset temp_var# Trying to use it after unsettingecho $temp_var     # Output: (no output)

Scope of Variables

Variables defined in the Unix shell can have different scopes:

Local Scope

Variables defined in the terminal are local to that session. Once the terminal session closes, these variables no longer exist. This is a critical distinction to remember. Here’s an example:

# Define a local variabletemp_varvalue# Check the variableecho $temp_var     # Output: value# Close the terminal session and open a new one# Check the variable in the new sessionecho $temp_var     # Output: (no output)

Subshells

If you create a variable in a subshell (e.g., within a command substitution or in a subshell created by parentheses), it won't affect the parent shell. Here’s an example:

# Create a variable in a subshell(temp_varvalue; echo $temp_var)     # Output: value# Variable is not set in the parent shellecho $temp_var     # Output: (no output)

Understanding Environment Variables

Environment variables in Unix are a special kind of variable used to pass values to programs. They are stored as KEYVALUE pairs in the environment and passed to programs through the ve system call.

Passing Environment Variables

For example, if you set an environment variable and then run a program that accesses the environment, the new program will inherit the variable:

# Set an environment variable and run a Python scriptMYVARvalue export MYVAR python -c import os print(os.environ['MYVAR'])

Unexported Variables

If the variable is not exported, it is not available to the new program:

# Set a shell variable but not export itMYVARvalue python -c import os print(os.environ['MYVAR'])

Note: In the above example, setting a variable with a usage prefix (e.g., MYVARsomevalue) passes the variable to the child process only while export MYVARsomevalue ensures that the variable is passed to the child process as part of the environment.

Shell Variables vs. Environment Variables

It's important to understand the difference between shell and environment variables:

Shell Variables

These are variables defined in the current shell session. They are only accessible to the current shell and any processes started from it. You can define and use them without needing to export them:

# Define a shell variablemy_varvalue# Use the variableecho $my_var     # Output: value# Unset the variableunset my_var

Environment Variables

These are variables that are passed to programs through the environment. They are typically set using the export command and can be accessed through $env_var or $env_var syntax:

# Define an environment variableexport MY_VARvalue# Use the variable in a Python scriptpython -c import os print(os.environ['MY_VAR'])

Local Variables in Shell

Unlike in other programming languages, shell does not support truly 'local' variables. However, you can pretend they are local using the local keyword:

# Define a local variablelocal my_varvalue# Use the variableecho $my_var     # Output: value

Be aware that this only masks over any global variables with the same name. It does not create a fully isolated context.

Conclusion

In summary, you can create and use temporary variables in a Unix shell environment. These variables can be used for various purposes during terminal sessions. Additionally, understanding the difference between shell and environment variables is crucial for writing effective shell scripts.

Resources

Bash Scripting Guide - Detailed guide on Bash scripting, including variable manipulation. Unix Shell Variables - Reference guide on shell variables in Unix. Shell Scripting Tutorial - Comprehensive tutorial on shell scripting, including local and environment variables.