Bash scripting skills have become imperative for Linux administrators and developers to expertly harness the power of the shell. As over 96% of the top 1 million servers run Linux – with Bash being the default shell for most distributions – mastering Bash is a major competency for unlocking automation capabilities across server fleets.

However, according to the 2022 Open Source Jobs Report, 57% of employers have difficulty finding skilled Linux talent. This indicates how specialized Bash scripting expertise continues to be rare despite explosive demand. With the Linux admin job market projected to grow 17% from 2020-2030 in the US – understanding Bash variable assignment nuances is more lucrative than ever for unlocking career growth opportunities.

This comprehensive guide aimed at intermediate Linux users demystifies efficient, secure variable assignment in Bash – best practices I‘ve repeatedly relied upon for authoring cross-platform scripts managing 1000s of servers.

foundational tenets of effectively using variables in bash

Before diving into specifics around declaration and manipulation, we must level-set on some key guiding tenets related to robust variable usage in Bash:

Security Trumps Convenience

Linux servers are highly privileged backends with potential access to an organization‘s most sensitive data. Any Bash script vulnerabilities can expose massive risk as scripts execute with elevated OS-level permissions.

Hence scripting best practices should optimize for security – even at the cost of minor developer convenience. Things like:

  • Enforcing strict resource access controls
  • Hardening environment variable contexts
  • Validating all external and user inputs

Often trump raw developer speed or comfort.

Plan First, Code Later

Like building architecture blueprints before construction – planning variable usage, lifecycle and scope before writing business logic is key for reslient scripts. Rather than adopting an ad-hoc approach led by experimentation (i.e coding first, refactoring later), thoughtfully ideating variables binding to core functionality paves the way for smoother sailing during active development.

Prevent Side-Effects Through Isolation

As Bash variables have process-wide scope, any changes impact the entire script environment. Limit variable mutations by isolating functionality into functions that locally scope variables. With rigorous input validation and return values, these function islands prevent side effects whilst retaining modularity.

Now equipped with this defensive mindset, let‘s explore variable syntax, assignment and manipulation features natively available within Bash.

Variable Declaration and Assignment Basics

Per statistics shared at Red Hat Summit 2022, ~30% of Bash scripts carry risky variable handling issues like using uninitialized or unset variables. However, correctly declaring variables is straightforward: 

variable_name=value

For example: 

username=JohnDoe

This assigns the string JohnDoe to a variable called username.

To reference the value of a variable, prepend it with a $ sign. For the above this gives: 

echo $username
# Prints JohnDoe

Unlike programming languages like Python, Bash does not enforce strict data types – everything resolves to a string. But typecasting features do enable operating on numbers and floats.

You can compactly declare multiple variables on one line:

url="www.example.com" page="/home"

However per Bash style guide from Google, Red Hat and W3C – declare one variable per line for enhanced readability, especially in collaborative scripts.

Securely Reading User Input

The read builtin allows capturing input from the user and saving it into a variable:

read username
# Prompts user to enter value for $username

However, this exposes security risks as users can input unexpected values including spaces, quotes and special symbols wreaking parsing havoc.

There are two approaches here.

Validating and Sanitizing Input

Firstly, instantly validate and sanitize any user inputs. For example to permit only alphanumeric user names:

read username
if ! [[ "$username" =~ ^[a-zA-Z0-9_]+$ ]]; then
   echo "Invalid username"
   exit 1
fi

The regex pattern passed to =~ operator checks if $username matches alphanumeric strings. Exit if invalid.

You may also apply string manipulation (a separate topic) to sanitize inputs.

Use Double Quotes for Safety

Even if allowing complex values, referencing the input via double quotes prevents splitting on spaces or expanding glob patterns:

read -r username 
echo "Entered username is $username"

Similarly, use double quotes everywhere the variable is referenced in later code.

So in summary, make user input safe through:

  • Validation against allowed values
  • Sanitization via string manipulation
  • Quoting references to handle edge cases

This guarantees script stability.

Assign Command Output to Variables

Beyond user input, variables can capture output of other commands too.

For example to set a variable to the current working directory:

path="$(pwd)"
echo "Working directory is $path"

The $(pwd) syntax executes pwd, takes its output and assigns into path.

Take care when nesting commands this way:

files=($(lsDocuments/old)) # Space breaks this

The space between ls and directory will cause issues. So understanding when nesting demands quoting is important.

You can also store command output in arrays natively supported by Bash:

files=($(ls)) 
echo "First file is ${files[0]}"

So powerful one-liners are possible but need defensive coding practices.

Modifying Variables

Initialized variables can be updated to alternates values by re-assigning: 

count=0 
count=5
echo "Count updated to $count" # Prints 5

Special variables called environment variables central to Bash operation like PATH, LANG etc can be updated too:

PATH="$PATH:/opt/bin/" # Appends a dir to PATH
LANG="en_US.UTF-8" # Modifies LANG

Brace expansions around variable names empower elegent modifications without spawning subshells:

url="https://oldsite.com"
echo "${url/old/new}" # URL old -> new

Here leading matching old is replaced with new via slash syntax.

++, — operations natively increment/decrement values:

((count++)) # Increments by 1
echo $count 

((count--)) # Decrements by 1

Even full-blown arithmetic is supported for modification:

((count += 5)) # Increment by 5 
((value *= 2)) # Double the value

So Bash variables, unlike traditional programming languages, can be re-used and mutated in situ. While this enables implementing logic flows linearly, it demands rigor so changes do not lead to unintended side-effects elsewhere.

Hence where possible, enforce immutability for key variables.

Bash Variable Scoping 101

Like other languages, variables have a defined scope controlling their visibility to rest of the script:

Global Scope

By default variables are globally scoped – meaning visible and mutable anywhere in script or child processes:

# Global variable
count=0 

function increment {
  ((count++))  
}

increment # Mutates global $count
echo $count # Changed here too

This should only be done for variables truly needed application-wide.

Function Local Scope

For modular code, use function-local scopes to prevent side effects:

# Global variable
app_name="MyApp"

function set_count {
  local count=0 # Local only to this function

  ((count++)) 
  echo "$app_name: $count" # Can access globals 
}

set_count
echo $count # Error - not found

Here count is local to set_count function thanks to local keyword. This best practice prevents functions tampering application state.

While this section used simple examples – when dealing with 1000s of variables in enterprise scripts spanning 1000s+ LoC – implementing sound scoping strategies is imperative.

Securely Working with Null and Empty Variables

Unlike languages like JavaScript, undefined variables in Bash are not explicitly detected and can cause problems:

Referencing Uninitialized Variables

# $foo not defined
echo $foo # No output!

Without rigor, this silent lack of errors leads to bugs down the line.

hence why you must mandate compulsory variable initialization through:

1. Nounset Bash Option

Using set -o nounset ensures referencing unbound variables raises errors and fails fast: 

set -o nounset
echo $foo # bash: foo: unbound variable

2. Check Undefined Variables

Alternately manually check if critical variables are defined:

if [ -z "$foo" ]; then
   echo "Variable undefined"
   exit 1
fi

Here the -z flag checks length is zero.

3. Enabling Pipefail

For variables consumed in pipes, enable pipefail to catch failures mid-pipeline:

set -o pipefail
# Rest of script

Now if variables in pipe commands unset, pipeline exits early.

Empty / Null Variable Handling

Conceptually different from undefined variables, empty values require their own handling:

name="" # empty string 

if [ -z "$name" ]; then
   echo "Empty string" 

elif [ "$name" = "null" ]; then
   echo "null value"
fi

Here -z checks 0 length, and explicit check for "null" string.

Bottom line: Whether undefined, empty or null – fail fast to catch issues early.

Final Words

Robust Bash scripting mandates hardening variable usage, scope control and state management.

While this article focused specifically on declaration, assignment and manipulation – complementary competencies like proper scoping, sanitization and error handling all tie together into a fail safe development approach needed for configuring Linux production environments securely and at scale.

Internalizing these lessons will serve you well on your journey toward Bash skills mastery while debunking the myth that shell scripting somehow ranks lower than more mainstream languages. The industry needs and rewards this platform expertise now more than ever.

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