sed for substitution in scripts in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sed command do in a script?

sed is a tool that edits text by searching and replacing patterns. It reads input line by line and can change text automatically.

beginner

How do you write a basic substitution command with sed?

The basic form is sed 's/old/new/'. It means: find old and replace it with new in each line.

beginner

What does the g flag do in sed 's/old/new/g'?

The g flag means "global". It replaces all occurrences of old in each line, not just the first one.

intermediate

How can you use sed to replace text in a file and save the changes back to the file?

Use the -i option: sed -i 's/old/new/g' filename. This edits the file directly.

intermediate

Why is it useful to use different delimiters in sed substitution, like sed 's|old|new|g'?

Using different delimiters helps when the text contains slashes /. It avoids confusion and makes the command easier to read.

What does the command sed 's/apple/orange/' do?

A Adds 'orange' after 'apple'

B Deletes all 'apple' words

C Replaces the first 'apple' with 'orange' in each line

D Prints lines containing 'apple'

This command replaces the first occurrence of 'apple' with 'orange' on each line.

How do you replace all occurrences of 'cat' with 'dog' in a file named pets.txt and save changes directly?

A sed -i 's/cat/dog/g' pets.txt

B sed 's/cat/dog/g' pets.txt

C sed 's/cat/dog/' pets.txt > pets.txt

D sed -r 's/cat/dog/g' pets.txt

The -i option edits the file in place, saving changes directly.

Why might you use sed 's|path/to/old|path/to/new|g' instead of sed 's/path\/to\/old/path\/to\/new/g'?

A Because <code>|</code> is faster

B To avoid escaping slashes <code>/</code> in paths

C Because <code>/</code> is not allowed in sed

D To make the command case-insensitive

Using | as delimiter avoids needing to escape slashes, making the command easier to read.

What happens if you run sed 's/old/new/' without the g flag?

A The command deletes the line

B All occurrences of 'old' are replaced

C No replacement happens

D Only the first occurrence of 'old' per line is replaced

Without g, only the first match per line is replaced.

Which option edits a file without creating a backup when using sed -i?

A sed -i 's/old/new/' file

B sed -i '' 's/old/new/' file

C sed -i.bak 's/old/new/' file

D sed 's/old/new/' file

Using -i alone edits the file in place without backup (behavior may vary by system).

Explain how to use sed to replace all occurrences of a word in a text file and save the changes.

Think about the 's/old/new/g' pattern and how to apply it directly to a file.

Describe why choosing different delimiters in sed substitution commands can be helpful.

Consider when your text contains slashes or other special characters.