tr for character transformation in Bash Scripting

Introduction

The tr command changes or deletes characters in text. It helps fix or clean text quickly.

You want to change all lowercase letters to uppercase in a file.

You need to remove all digits from a text stream.

You want to replace spaces with underscores in a filename.

You want to delete all punctuation marks from a sentence.

You want to squeeze repeated characters into one (like multiple spaces into one).

Syntax

Bash Scripting

tr [options] SET1 [SET2]

SET1 is the set of characters to replace or delete.

If SET2 is given, characters in SET1 are replaced by corresponding characters in SET2.

Examples

Converts all lowercase letters to uppercase.

Bash Scripting

echo "hello" | tr 'a-z' 'A-Z'

Deletes all digits from the input.

Bash Scripting

echo "123abc" | tr -d '0-9'

Replaces spaces with underscores.

Bash Scripting

echo "hello  world" | tr ' ' '_'

Sample Program

This script takes a string, changes all lowercase letters to uppercase using tr, and prints the result.

Bash Scripting

#!/bin/bash

# Convert lowercase to uppercase
input="hello world"
output=$(echo "$input" | tr 'a-z' 'A-Z')
echo "$output"

Important Notes

tr works on standard input and outputs to standard output, so use pipes or redirection.

The -d option deletes characters from the input.

The -s option squeezes repeated characters into one.

Summary

tr changes or deletes characters in text streams.

Use it to convert cases, delete unwanted characters, or replace characters.

It reads from input and writes transformed text to output.

Practice

1. What does the tr command do in bash scripting?

easy

A. It transforms or deletes characters from input text streams.

B. It compiles bash scripts into executables.

C. It lists files in a directory.

D. It runs commands in the background.

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 B, C, and D describe unrelated commands or functions in bash.

3. Final Answer:

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

4. Quick Check:

   tr = transform/delete characters [OK]

Hint: Remember: tr means translate characters [OK]

Common Mistakes:

• Confusing tr with file listing commands like ls
• Thinking tr compiles scripts
• Assuming tr runs commands in background

2. Which of the following is the correct syntax to convert lowercase letters to uppercase using tr?

easy

A. echo "hello" | tr 'A-Z' '0-9'

B. echo "hello" | tr 'a-z' 'A-Z'

C. echo "hello" | tr '0-9' 'A-Z'

D. echo "hello" | tr 'A-Z' 'a-z'

Solution

1. Step 1: Identify the correct character ranges for case conversion

   To convert lowercase to uppercase, use 'a-z' as source and 'A-Z' as target.

2. Step 2: Check the syntax correctness

   echo "hello" | tr 'a-z' 'A-Z' correctly uses tr 'a-z' 'A-Z'. Other options either reverse case or use wrong ranges.

3. Final Answer:

   echo "hello" | tr 'a-z' 'A-Z' -> Option B

4. Quick Check:

   Lowercase to uppercase = echo "hello" | tr 'a-z' 'A-Z' [OK]

Hint: Use 'a-z' then 'A-Z' to uppercase letters [OK]

Common Mistakes:

• Swapping source and target ranges
• Using digits instead of letters
• Mixing uppercase and lowercase incorrectly

3. What is the output of this command?

echo "Hello World 123" | tr -d '0-9'

medium

A. Hello World 123

B. HelloWorld123

C. HelloWorld

D. Hello World

Solution

1. Step 1: Understand the -d option in tr

   The -d option deletes characters matching the set given, here digits '0-9'.

2. Step 2: Apply deletion to the input string

   All digits '1', '2', '3' are removed, leaving "Hello World " with spaces intact.

3. Final Answer:

   Hello World -> Option D

4. Quick Check:

   Digits deleted = "Hello World " [OK]

Hint: Use -d to delete characters, here digits removed [OK]

Common Mistakes:

• Expecting digits to be replaced, not deleted
• Removing spaces accidentally
• Confusing -d with character translation

4. Identify the error in this command:

echo "abc123" | tr 'a-z' 'A-Z' -d '0-9'

medium

A. The command will convert letters but ignore digits.

B. The order of options is wrong; -d must come before sets.

C. You cannot combine translation and deletion in one tr command.

D. The syntax is correct and will work as expected.

Solution

1. Step 1: Understand tr option usage

   tr does not support combining translation and deletion in one command; they are separate operations.

2. Step 2: Analyze the command

   The command tries to translate and delete simultaneously, which is invalid syntax.

3. Final Answer:

   You cannot combine translation and deletion in one tr command. -> Option C

4. Quick Check:

   Translation + deletion together = invalid [OK]

Hint: tr does either translate OR delete, not both at once [OK]

Common Mistakes:

• Trying to chain translate and delete in one tr call
• Assuming option order fixes the problem
• Expecting partial command success

5. You want to replace all vowels in a string with the character '*', but keep other characters unchanged. Which command achieves this?

hard

A. echo "banana" | tr 'aeiou' '*'

B. echo "banana" | tr 'aeiou' 'AEIOU'

C. echo "banana" | tr -d 'aeiou'

D. echo "banana" | tr 'aeiou' 'ABCDE'

Solution

1. Step 1: Understand character replacement with tr

   To replace vowels with '*', map all vowels to a single character '*'.

2. Step 2: Analyze each option

   echo "banana" | tr 'aeiou' '*' replaces each vowel with '*'. echo "banana" | tr 'aeiou' 'AEIOU' changes vowels to uppercase vowels. echo "banana" | tr -d 'aeiou' deletes vowels. echo "banana" | tr 'aeiou' 'ABCDE' replaces vowels with A,B,C,D,E respectively.

3. Final Answer:

   echo "banana" | tr 'aeiou' '*' -> Option A

4. Quick Check:

   Vowels replaced by '*' = echo "banana" | tr 'aeiou' '*' [OK]

Hint: Map vowels to '*' with single char target in tr [OK]

Common Mistakes:

• Using a target set like 'ABCDE' instead of '*'
• Deleting vowels instead of replacing
• Replacing vowels with uppercase instead of '*'