sed for substitution in scripts in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this sed substitution command?

Given a file colors.txt containing:

red
blue
green
blue

What will be the output of this command?

sed 's/blue/yellow/' colors.txt

A

red
yellow
green
yellow

B

red
blue
green
blue

C

red
yellow
green
blue

D

red
yellow
yellow
yellow

💡 Hint

Remember, sed by default replaces only the first match per line.

The sed command replaces only the first occurrence of 'blue' on each line. 'blue' appears once on lines 2 and 4, so both are replaced with 'yellow'.

💻 Command Output

intermediate

What does this sed command output?

Given a file data.txt with content:

apple apple apple
banana apple
apple banana

What is the output of:

sed 's/apple/orange/g' data.txt

A

orange orange orange
banana orange
orange banana

B

orange apple apple
banana apple
apple banana

C

orange orange orange
banana apple
apple banana

D

apple apple apple
banana apple
apple banana

💡 Hint

The 'g' flag replaces all occurrences on each line.

The 'g' flag in sed means global replacement on each line, so all 'apple' words are replaced with 'orange'.

🔧 Debug

advanced

Why does this sed substitution fail to replace text?

Consider this command:

echo 'hello world' | sed 's/Hello/Hi/'

Why does it output hello world unchanged?

A The command is missing the -i flag for in-place editing

B sed is case-sensitive and 'Hello' does not match 'hello'

C The pipe symbol '|' is used incorrectly

D The substitution syntax is invalid

💡 Hint

Check the letter case in the pattern and input.

sed matches patterns exactly by default, so 'Hello' does not match 'hello' due to case difference.

🚀 Application

advanced

How to replace only the second occurrence of a word in each line using sed?

You want to replace only the second occurrence of 'cat' with 'dog' in each line of a file. Which sed command achieves this?

A sed 's/cat/dog/1' filename

B sed 's/cat/dog/g2' filename

C sed 's/cat/dog/2' filename

D sed 's/cat/dog/3' filename

💡 Hint

sed supports a number after the substitution to specify which occurrence to replace.

The number after the substitution command specifies which occurrence to replace. '2' means the second occurrence only.

🧠 Conceptual

expert

What is the output of this complex sed script?

Given a file file.txt with content:

one two three two one
four two five two four

What is the output of:

sed -E 's/(two)/[\1]/2g' file.txt

A

one [two] three [two] one
four [two] five [two] four

B

one two three [two] one
four [two] five [two] four

C

one two three two one
four two five two four

D

one two three [two] one
four two five [two] four

💡 Hint

The '2g' after the substitution means replace the second and subsequent occurrences per line.

When a number followed by 'g' is given after 's///', sed replaces that occurrence and subsequent ones per line. Here, only the second 'two' per line is replaced with [\1] (backreference to the match).