sed for substitution in scripts in Bash Scripting - Step-by-Step Execution

Concept Flow - sed for substitution in scripts

Start script

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Read input text

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Apply sed substitution

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Replace matching text

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Output modified text

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End script

The script reads input text, applies sed substitution to replace matching text, then outputs the modified text.

Execution Sample

Bash Scripting

echo "Hello World" | sed 's/World/Bash/'

This command replaces 'World' with 'Bash' in the input text and outputs the result.

Execution Table

Step	Input Text	sed Command	Substitution Result	Output
1	"Hello World"	s/World/Bash/	Hello Bash	Hello Bash
2	"No more input"	No substitution needed	No more input	No more input
3	"End of input"	No substitution needed	End of input	End of input

💡 All input lines processed, script ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
input_text	"Hello World"	"Hello Bash"	"No more input"	"End of input"
output_text		"Hello Bash"	"No more input"	"End of input"

Key Moments - 3 Insights

Why does sed only replace the first match on a line by default?

By default, sed's 's/pattern/replacement/' replaces only the first occurrence per line, as shown in execution_table step 1 where only 'World' is replaced once.

How to replace all occurrences of a pattern in a line?

Add the 'g' flag like 's/pattern/replacement/g' to replace all matches on a line, which is not shown here but changes the substitution behavior.

What happens if the pattern is not found in the input?

sed outputs the line unchanged, as seen in execution_table steps 2 and 3 where no substitution occurs.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 1?

A "Hello Bash"

B "Hello World"

C "Bash World"

D "Hello"

💡 Hint

Check the 'Output' column in execution_table row for step 1

At which step does sed not perform any substitution?

A Step 1

B Step 2

C Both Step 2 and Step 3

D Step 3

💡 Hint

Look at the 'Substitution Result' column in execution_table for steps 2 and 3

If we change the sed command to 's/World/Bash/g', how would the output change for input 'World World'?

A "Bash World"

B "Bash Bash"

C "World Bash"

D "World World"

💡 Hint

The 'g' flag replaces all occurrences on the line, not just the first

Concept Snapshot

sed substitution syntax: s/pattern/replacement/
By default, replaces first match per line
Add 'g' flag to replace all matches
Used in scripts to modify text streams
Outputs modified text line by line

Full Transcript

This example shows how sed is used in scripts to substitute text. The script reads input lines, applies the substitution command s/World/Bash/, which replaces the first occurrence of 'World' with 'Bash' on each line, then outputs the result. If the pattern is not found, the line is output unchanged. Adding the 'g' flag to the command replaces all occurrences on a line. The execution table traces each input line, the sed command applied, the substitution result, and the output. The variable tracker shows how input and output text change step by step. Key moments clarify common confusions about sed's default behavior and flags. The visual quiz tests understanding of the substitution results and effects of flags.