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

Concept Flow - sed for substitution in scripts

Start script

↓

Read input text

↓

Apply sed substitution

↓

Replace matching text

↓

Output modified text

↓

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.

Practice

1. What does the sed command sed 's/cat/dog/' file.txt do?

easy

A. Prints lines containing 'cat' from file.txt

B. Replaces all occurrences of 'cat' with 'dog' in the entire file.txt

C. Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt

D. Deletes all lines containing 'cat' from file.txt

Solution

1. Step 1: Understand the sed substitution syntax

   The command sed 's/cat/dog/' file.txt uses s/old/new/ which replaces only the first match per line.

2. Step 2: Apply to the file content

   Each line in file.txt will have its first 'cat' replaced by 'dog', other occurrences remain unchanged.

3. Final Answer:

   Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt -> Option C

4. Quick Check:

   sed s/old/new/ replaces first match per line [OK]

Hint: Remember: no 'g' means only first match replaced per line [OK]

Common Mistakes:

• Thinking it replaces all matches without 'g'
• Confusing substitution with deletion
• Assuming it prints only matching lines

2. Which of the following is the correct syntax to replace all occurrences of 'apple' with 'orange' in a file using sed?

easy

A. sed 's/apple/orange/' file.txt

B. sed 'replace apple with orange' file.txt

C. sed 's/apple/orange//g' file.txt

D. sed 's/apple/orange/g' file.txt

Solution

1. Step 1: Identify the global flag usage

   To replace all occurrences on each line, the 'g' flag is needed after the substitution pattern.

2. Step 2: Check each option

   sed 's/apple/orange/g' file.txt uses s/apple/orange/g which correctly replaces all 'apple' with 'orange'. sed 's/apple/orange/' file.txt replaces only first occurrence. sed 'replace apple with orange' file.txt is invalid syntax. sed 's/apple/orange//g' file.txt has extra slashes causing syntax error.

3. Final Answer:

   sed 's/apple/orange/g' file.txt -> Option D

4. Quick Check:

   Use 'g' flag for all matches [OK]

Hint: Add 'g' after last slash to replace all matches [OK]

Common Mistakes:

• Omitting the 'g' flag to replace all matches
• Adding extra slashes causing syntax errors
• Using incorrect command words like 'replace'

3. What will be the output of the following command if the input file contains the line apple apple apple?

sed 's/apple/orange/' file.txt

medium

A. orange orange orange

B. orange apple apple

C. apple orange apple

D. apple apple orange

Solution

1. Step 1: Understand sed substitution without 'g'

   Without the 'g' flag, sed replaces only the first occurrence of 'apple' per line.

2. Step 2: Apply substitution to the input line

   The first 'apple' becomes 'orange', the rest remain unchanged, resulting in 'orange apple apple'.

3. Final Answer:

   orange apple apple -> Option B

4. Quick Check:

   First match replaced only without 'g' [OK]

Hint: No 'g' means only first match changes per line [OK]

Common Mistakes:

• Assuming all occurrences change without 'g'
• Confusing order of words after substitution
• Thinking sed changes input file by default

4. You wrote this sed command to replace 'foo' with 'bar' globally: sed 's/foo/bar/' file.txt. But only the first 'foo' per line is replaced. What is the fix?

medium

A. Add the 'g' flag: sed 's/foo/bar/g' file.txt

B. Use double quotes instead of single quotes

C. Add a space after 'bar/'

D. Use sed -i option to edit in place

Solution

1. Step 1: Identify missing global flag

   The command lacks the 'g' flag, so only first occurrence per line is replaced.

2. Step 2: Add the 'g' flag to fix

   Adding 'g' after the substitution pattern makes sed replace all occurrences per line.

3. Final Answer:

   Add the 'g' flag: sed 's/foo/bar/g' file.txt -> Option A

4. Quick Check:

   Global flag 'g' replaces all matches [OK]

Hint: Add 'g' to replace all matches, not just first [OK]

Common Mistakes:

• Thinking quotes affect substitution behavior
• Adding spaces inside substitution pattern
• Confusing in-place edit with substitution flags

5. You want to replace the word 'error' with 'warning' only on lines containing 'log' in a file. Which sed command achieves this?

hard

A. sed '/log/ s/error/warning/g' file.txt

B. sed 's/error/warning/g /log/' file.txt

C. sed 's/error/warning/g if /log/' file.txt

D. sed 's/error/warning/g' file.txt | grep log

Solution

1. Step 1: Understand conditional substitution in sed

   To apply substitution only on lines matching a pattern, use /pattern/ s/old/new/flags syntax.

2. Step 2: Analyze options

   sed '/log/ s/error/warning/g' file.txt correctly applies substitution only on lines containing 'log'. Options A and C have invalid syntax. sed 's/error/warning/g' file.txt | grep log replaces everywhere then filters lines, which does not limit substitution.

3. Final Answer:

   sed '/log/ s/error/warning/g' file.txt -> Option A

4. Quick Check:

   Use address before s/// to limit lines [OK]

Hint: Put pattern before s/// to restrict substitution lines [OK]

Common Mistakes:

• Placing substitution after pattern incorrectly
• Trying to use if statements inside sed
• Filtering lines after substitution instead of limiting it