Understanding Global vs Local Variables in Bash

Bash variable scoping often confuses even experienced shell users. Variables feel local when they are written inside a function, but in Bash, they are global by default. This blog walks through that behavior step by step using simple, practical examples.

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1. Defining a Function That Uses a Global Variable

Let’s start with a simple script that defines two functions. Notice that the variable name is assigned inside the function without using local.

[root@oel01db Shell-Scripting]# cat ./greetings

#!/bin/bash

greet() {

        name=$1

        echo "hello $name"

}

goodbye() {

        name=$1

        echo "goodbye $name"

}

[root@oel01db Shell-Scripting]#

At this point, nothing unusual yet. The real behavior shows up when we source this file and call the function from another script.

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2. Creating Another Script That Uses the Function

Here is a second script that sources greetings and defines its own variable named name.

[root@oel01db Shell-Scripting]# cat ./global-local-var.sh

. ./greetings

name='Mahesh'  # global variable

echo "before calling function greet, name = $name"

greet Ahana

echo "after calling function greet, name  = $name"

[root@oel01db Shell-Scripting]#

Now let’s execute it.

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3. Executing the Script

[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

hello Ahana

after calling function greet, name  = Ahana

[root@oel01db Shell-Scripting]#

What Just Happened?

As you can see, the greet function modified the global variable name.

This happens because:

• Variables in Bash are global by default

• Even variables assigned inside a function are global unless explicitly declared local

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4. Step-by-Step Breakdown

• . ./greetings
  You source the file. The greet function is now loaded into your current shell.

• name='Mahesh'
  You create a global variable called name.

• greet Ahana
  Inside the function, the line name=$1 executes.

• Bash looks for a variable named name. It finds the global one and overwrites it.

Since name is not declared local, Bash effectively says:
“Oh, this is the global variable. I’ll overwrite it.”

• When the function exits, the global variable has already been changed to Ahana.

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5. How to Fix This Problem

There are multiple correct ways to prevent functions from modifying global variables. Let’s explore them one by one.

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Fix 1: Declare the Variable as local

The safest and most recommended solution.

[root@oel01db Shell-Scripting]# cat greetings

#!/bin/bash

greet() {

        local name=$1

        echo "hello $name"

}

goodbye() {

        name=$1

        echo "goodbye $name"

}

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

hello Ahana

after calling function greet, name  = Mahesh

[root@oel01db Shell-Scripting]#

Why This Works

• local restricts the variable’s scope to the function

• Once the function exits, the variable disappears

• The global name remains untouched

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Fix 2: Define the Function Using Parentheses ()

Functions defined with parentheses run in a subshell.

[root@oel01db Shell-Scripting]# cat greetings

#!/bin/bash

greet() (

        name=$1

        echo "hello $name"

)

goodbye() {

        name=$1

        echo "goodbye $name"

}

[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

hello Ahana

after calling function greet, name  = Mahesh

[root@oel01db Shell-Scripting]#

Why This Works

• Parentheses create a subshell

• Variable changes do not propagate back to the parent shell

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Fix 3: Call the Function Inside a Subshell

Instead of changing the function definition, you can run it inside a subshell.

[root@oel01db Shell-Scripting]# cat greetings

#!/bin/bash

greet() {

        name=$1

        echo "hello $name"

}

goodbye() {

        name=$1

        echo "goodbye $name"

}

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# cat ./global-local-var.sh

. ./greetings

name='Mahesh'

echo "before calling function greet, name = $name"

(greet Ahana)

echo "after calling function greet, name  = $name"

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

hello Ahana

after calling function greet, name  = Mahesh

[root@oel01db Shell-Scripting]#

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6. Calling the Function Inside Curly Braces {}

Now let’s look at a common misconception.

[root@oel01db Shell-Scripting]# cat ./global-local-var.sh

. ./greetings

name='Mahesh'

echo "before calling function greet, name = $name"

{ greet Ahana; }

echo "after calling function greet, name  = $name"

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

hello Ahana

after calling function greet, name  = Ahana

[root@oel01db Shell-Scripting]#

Why the Variable Changed Again

• {} runs in the current shell, not a subshell

• So global variables are modified

The curly brace { is a reserved word, not a control operator. Because it is a word, it must be separated by spaces.

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7. Using a Pipe Inside {}

[root@oel01db Shell-Scripting]# cat ./global-local-var.sh

. ./greetings

name='Mahesh'

echo "before calling function greet, name = $name"

{ greet Ahana | tr h m; }

echo "after calling function greet, name  = $name"

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

mello Amana

after calling function greet, name  = Mahesh

[root@oel01db Shell-Scripting]#

Why Didn’t the Global Variable Change This Time?

Because:

Any command that is part of a pipeline runs in a subshell

Even though {} runs in the current shell, the presence of a pipe forces the function to execute in a subshell.

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8. Pipeline with {} and an Outer Pipe

[root@oel01db Shell-Scripting]# cat ./global-local-var.sh

. ./greetings

name='Mahesh'

echo "before calling function greet, name = $name"

{ greet Ahana | tr h m; } | nl

echo "after calling function greet, name  = $name"

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

     1  mello Amana

after calling function greet, name  = Mahesh

[root@oel01db Shell-Scripting]#

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9. Using () with a Pipeline

[root@oel01db Shell-Scripting]# cat ./global-local-var.sh

. ./greetings

name='Mahesh'

echo "before calling function greet, name = $name"

(greet Ahana | tr h m ) | nl

echo "after calling function greet, name  = $name"

[root@oel01db Shell-Scripting]#
[root@oel01db Shell-Scripting]# ./global-local-var.sh

before calling function greet, name = Mahesh

     1  mello Amana

after calling function greet, name  = Mahesh

[root@oel01db Shell-Scripting]#

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Key Takeaways

• Bash variables are global by default

• Use local inside functions whenever possible

• () creates a subshell

• {} runs in the current shell

• Any pipeline forces a subshell, even inside {}

If you remember just one rule:

If you don’t want to modify global state, use local or a subshell.