Configuration file reading in Bash Scripting - Step-by-Step Execution

Concept Flow - Configuration file reading

Start script

↓

Open config file

↓

Read line

↓

Is line empty or comment?

Yes→Skip line

No↓

Parse key=value

↓

Store in variable

↓

More lines?

Yes→Read line

No↓

Use variables

↓

End script

The script reads each line of the config file, skips empty or comment lines, extracts key-value pairs, stores them, and then uses these variables.

Execution Sample

Bash Scripting

#!/bin/bash
while IFS='=' read -r key value; do
  [[ $key =~ ^#.*$ || -z $key ]] && continue
  declare "$key=$value"
done < config.cfg

echo "User: $user"
echo "Port: $port"

Reads a config file line by line, skips comments and empty lines, assigns variables, then prints them.

Execution Table

Step	Line Read	Condition (Comment/Empty?)	Action	Variables Set	Output
1	user=alice	No	Set user=alice	user=alice
2	# This is a comment	Yes	Skip line	user=alice
3	port=8080	No	Set port=8080	user=alice, port=8080
4		Yes	Skip line	user=alice, port=8080
5	mode=active	No	Set mode=active	user=alice, port=8080, mode=active
6	EOF	N/A	End of file	user=alice, port=8080, mode=active	User: alice Port: 8080

💡 Reached end of config file, all variables set, script prints values.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
user	unset	alice	alice	alice	alice	alice	alice
port	unset	unset	unset	8080	8080	8080	8080
mode	unset	unset	unset	unset	unset	active	active

Key Moments - 3 Insights

Why does the script skip lines starting with # or empty lines?

Lines starting with # are comments and empty lines have no data, so skipping them avoids errors and only processes valid key=value pairs, as shown in steps 2 and 4 of the execution_table.

How does the script assign variables from the config file?

It reads each line, splits at '=', then uses declare to create variables dynamically, shown in steps 1, 3, and 5 where variables user, port, and mode are set.

What happens if the config file has spaces around '='?

The script may include spaces in keys or values causing incorrect variable names or values; trimming spaces before assignment is needed but not shown here.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'port' after step 3?

A unset

B 8080

C alice

D active

💡 Hint

Check the 'Variables Set' column at step 3 in the execution_table.

At which step does the script skip a line because it is a comment?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Condition' and 'Action' columns in the execution_table for skipped lines.

If the config file had an empty line at step 3 instead of 'port=8080', what would happen to the 'port' variable?

A It would remain unset

B It would be set to '8080'

C It would be set to empty string

D Script would error out

💡 Hint

Refer to how empty lines are handled in the execution_table and variable_tracker.

Concept Snapshot

Read config file line by line with 'while read'.
Skip empty lines and comments starting with '#'.
Split lines by '=' to get key and value.
Use 'declare' to set variables dynamically.
Use variables later in the script as needed.

Full Transcript

This script reads a configuration file line by line. It skips lines that are empty or start with a comment symbol (#). For each valid line, it splits the line into a key and a value at the '=' sign. Then it creates a variable named after the key and assigns it the value. After reading all lines, the script uses these variables, for example, printing the user and port values. This process helps scripts get settings from files easily.