set

Understanding set’s Core Functionality

At its heart, set modifies the shell’s environment. This includes setting shell variables, enabling or disabling shell options, and positional parameters (arguments passed to a script). The basic syntax is straightforward:

set [option] [parameter]...

Let’s look at the key aspects:

Setting Shell Variables

The most common use of set is assigning values to shell variables. Unlike export, which makes variables accessible to child processes, set typically limits the scope to the current shell instance.

set my_variable="Hello, world!"
echo $my_variable  # Output: Hello, world!

You can also set multiple variables simultaneously:

set name="John Doe" age=30 city="New York"
echo "Name: $name, Age: $age, City: $city"

Manipulating Positional Parameters

Positional parameters ($1, $2, etc.) represent the arguments passed to a script or function. set allows you to directly manipulate these:

#!/bin/bash

set -- "apple" "banana" "cherry"  # Reassign positional parameters

echo "First argument: $1"  # Output: apple
echo "Second argument: $2" # Output: banana
echo "All arguments: $*"    # Output: apple banana cherry

This example overrides the original positional parameters, replacing them with “apple”, “banana”, and “cherry”.

Enabling and Disabling Shell Options

set also controls shell options, which modify the shell’s behavior. For example, -e exits the script immediately upon encountering an error:

#!/bin/bash

set -e

false  # This will cause the script to exit

echo "This line won't be executed"

Other useful options include:

• -x: Enables tracing – displays each command before execution.
• -v: Enables verbose mode – displays each line of the script as it’s read.
• -u: Treats unset variables as errors.

Example using -x and -v:

#!/bin/bash
set -xv

my_var="hello"
echo "$my_var"

Running this script will show you each command before its execution and the script’s lines as they are being read.

Unsetting Variables

While unset is the dedicated command, set can indirectly unset variables by reassigning them to null:

set my_variable=""
echo "$my_variable"  # Output: (empty string)

Note: This doesn’t entirely remove the variable; it simply sets its value to an empty string.

Advanced set Usage: Array Handling

While set isn’t the primary tool for array manipulation in Bash (associative arrays are preferred), it can be used to simulate arrays:

set -- one two three four
echo "$1" # one
echo "$2" # two
echo "$@" # one two three four

This shows how positional parameters can act as a rudimentary array. However, for array handling, consider using Bash arrays with the declare -a command.

set with Special Parameters

set interacts with special shell parameters such as $# (number of arguments), $? (exit status of the last command), and others. While you won’t directly assign values to these with set, their behavior is affected by set’s manipulation of positional parameters.