tr (translate characters) in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the tr command do in Linux?

The tr command translates or replaces characters from input text. It reads from standard input and writes to standard output.

beginner

How do you use tr to change all lowercase letters to uppercase?

Use tr 'a-z' 'A-Z'. This replaces every lowercase letter with its uppercase version.

intermediate

What happens if the second set of characters in tr is shorter than the first?

The last character of the second set is repeated to match the length of the first set.

beginner

How can you delete characters using tr?

Use the -d option with the characters to delete. For example, tr -d 'aeiou' deletes vowels.

intermediate

Explain the use of tr -s option.

The -s option squeezes repeated characters into one. For example, tr -s ' ' replaces multiple spaces with a single space.

What does tr '0-9' ' ' do?

A Reverses digits

B Deletes digits

C Converts digits to asterisks

D Replaces digits with spaces

The command replaces all digits (0-9) with spaces.

Which option deletes characters in tr?

A -d

B -s

C -c

D -r

The -d option deletes specified characters.

How do you convert uppercase letters to lowercase using tr?

A tr 'A-Z' 'a-z'

B tr 'a-z' 'A-Z'

C tr '0-9' 'a-z'

D tr -d 'A-Z'

Use tr 'A-Z' 'a-z' to convert uppercase to lowercase.

What does tr -s '\n' do?

A Deletes newlines

B Replaces newlines with spaces

C Squeezes multiple newlines into one

D Duplicates newlines

The -s option squeezes repeated newlines into a single newline.

If the second set is shorter in tr, what happens?

A Error occurs

B Second set is repeated to match first

C Characters are deleted

D No change happens

The last character of the second set repeats to match the first set's length.

Describe how to use tr to replace all vowels with asterisks in a text.

Think about specifying vowels as first set and '*' as second set.

Explain the difference between tr -d and tr -s options with examples.

Consider how each option changes the input text differently.

Practice

1. What does the tr command do in Linux?

easy

A. It translates or deletes characters from input text.

B. It lists files in a directory.

C. It compiles source code.

D. It displays the current directory path.

Solution

1. Step 1: Understand the purpose of tr

   The tr command is used to translate or delete characters from input text streams.

2. Step 2: Compare with other options

   Options A, C, and D describe other commands like ls, compilers, and pwd, which are unrelated to tr.

3. Final Answer:

   It translates or deletes characters from input text. -> Option A

4. Quick Check:

   tr = translate/delete chars [OK]

Hint: Remember: tr = translate characters [OK]

Common Mistakes:

• Confusing tr with ls or pwd commands
• Thinking tr compiles code
• Assuming tr lists files

2. Which of the following is the correct syntax to replace all lowercase 'a' characters with 'b' using tr?

easy

A. tr -replace a b

B. tr a-z b

C. tr 'a' 'b'

D. tr 'a' to 'b'

Solution

1. Step 1: Recall tr syntax

   The correct syntax uses single quotes around the sets of characters: tr 'a' 'b'.

2. Step 2: Evaluate options

   tr a-z b uses a range which translates all lowercase letters to 'b' (padded), incorrect for only 'a'. Options A and C use invalid syntax for tr.

3. Final Answer:

   tr 'a' 'b' -> Option C

4. Quick Check:

   Use quotes around sets in tr [OK]

Hint: Use quotes around characters in tr command [OK]

Common Mistakes:

• Using range 'a-z' for single character
• Using invalid options like -replace
• Adding extra words like 'to'

3. What is the output of the command:
echo "hello world" | tr 'a-z' 'A-Z'?

medium

A. HELLO WORLD

B. hello world

C. HELLO world

D. Error: invalid range

Solution

1. Step 1: Understand the command

   The command converts all lowercase letters (a-z) to uppercase (A-Z) in the input text "hello world".

2. Step 2: Apply translation

   All letters become uppercase, so "hello world" becomes "HELLO WORLD".

3. Final Answer:

   HELLO WORLD -> Option A

4. Quick Check:

   Lowercase to uppercase translation = HELLO WORLD [OK]

Hint: tr 'a-z' 'A-Z' converts lowercase to uppercase [OK]

Common Mistakes:

• Expecting partial uppercase
• Confusing input and output
• Thinking it causes an error

4. Identify the error in this command:
echo "123abc" | tr '0-9' 'a-j'

medium

A. Ranges in sets are mismatched length causing error

B. No error, command works correctly

C. Output will be empty

D. Quotes are missing around sets

Solution

1. Step 1: Analyze the command

   The command replaces digits 0-9 with letters a-j. Both sets have equal length (10 characters).

2. Step 2: Check for syntax or runtime errors

   Quotes are present, ranges are valid and equal length, so no error occurs.

3. Final Answer:

   No error, command works correctly -> Option B

4. Quick Check:

   Equal length ranges with quotes = valid command [OK]

Hint: Ensure equal length sets in tr to avoid errors [OK]

Common Mistakes:

• Assuming mismatched lengths cause errors here
• Missing quotes around sets
• Expecting output to be empty

5. You want to remove all vowels (a, e, i, o, u) from a text file named input.txt. Which command correctly does this using tr?

hard

A. tr -remove 'aeiou' < input.txt

B. tr 'aeiou' < input.txt

C. tr -r 'aeiou' < input.txt

D. tr -d 'aeiou' < input.txt

Solution

1. Step 1: Understand the goal

   The goal is to delete vowels from the input text file.

2. Step 2: Identify correct tr option

   The -d option deletes specified characters. So tr -d 'aeiou' removes vowels.

3. Step 3: Check other options

   tr 'aeiou' < input.txt lacks the second character set, causing an error. Options C and D use invalid options.

4. Final Answer:

   tr -d 'aeiou' < input.txt -> Option D

5. Quick Check:

   Use -d to delete chars in tr [OK]

Hint: Use tr -d to delete characters from input [OK]

Common Mistakes:

• Missing second character set operand
• Using invalid options like -r or -remove
• Forgetting to redirect input