tr Command

Translate, delete, or squeeze characters from standard input

Syntax: tr [OPTION]... SET1 [SET2]

Description

The tr command is used to translate, delete, or squeeze characters from standard input and write the result to standard output. It's a powerful text processing tool that can perform character-by-character transformations on text streams.

Note: The tr command only reads from standard input and writes to standard output. It cannot read files directly - use input redirection or pipes to process file contents.

Common Options

Option	Description
`-c, --complement`	Use the complement of SET1
`-d, --delete`	Delete characters in SET1, do not translate
`-s, --squeeze-repeats`	Replace each sequence of repeated characters in SET1 with a single occurrence
`-t, --truncate-set1`	First truncate SET1 to length of SET2
`--help`	Display help message and exit
`--version`	Output version information and exit

Character Sets

The tr command supports various character set notations:

Notation	Description	Example
`[:alnum:]`	All alphanumeric characters	a-z, A-Z, 0-9
`[:alpha:]`	All alphabetic characters	a-z, A-Z
`[:digit:]`	All digits	0-9
`[:lower:]`	All lowercase letters	a-z
`[:upper:]`	All uppercase letters	A-Z
`[:space:]`	All whitespace characters	space, tab, newline
`[:punct:]`	All punctuation characters	!"#$%&'()*+,-./:;<=>?@[\]^_`{\|}~
`a-z`	Range of characters	All lowercase letters
`\n`	Newline character	Line break
`\t`	Tab character	Horizontal tab

Examples

Convert lowercase to uppercase:

echo "hello world" | tr '[:lower:]' '[:upper:]'
# Output: HELLO WORLD

echo "hello world" | tr 'a-z' 'A-Z'
# Output: HELLO WORLD

Convert uppercase to lowercase:

echo "HELLO WORLD" | tr '[:upper:]' '[:lower:]'
# Output: hello world

echo "HELLO WORLD" | tr 'A-Z' 'a-z'
# Output: hello world

Delete specific characters:

echo "hello123world456" | tr -d '[:digit:]'
# Output: helloworld

echo "hello world!" | tr -d ' !'
# Output: helloworld

Replace spaces with underscores:

echo "hello world test" | tr ' ' '_'
# Output: hello_world_test

echo "multiple   spaces" | tr -s ' ' '_'
# Output: multiple_spaces

Squeeze repeated characters:

echo "hellooo   world" | tr -s 'o '
# Output: helo world

echo "aabbccdd" | tr -s 'a-z'
# Output: abcd

Replace newlines with spaces:

cat file.txt | tr '\n' ' '
# Converts all newlines to spaces

printf "line1\nline2\nline3" | tr '\n' ' '
# Output: line1 line2 line3

Remove all punctuation:

echo "Hello, World! How are you?" | tr -d '[:punct:]'
# Output: Hello World How are you

echo "[email protected]" | tr -d '@.'
# Output: testexamplecom

Convert tabs to spaces:

cat file.txt | tr '\t' ' '
# Converts all tabs to spaces

printf "col1\tcol2\tcol3" | tr '\t' ','
# Output: col1,col2,col3

Use complement to keep only specific characters:

echo "abc123def456" | tr -cd '[:digit:]'
# Output: 123456 (keeps only digits)

echo "Hello123World456" | tr -cd '[:alpha:]'
# Output: HelloWorld (keeps only letters)

ROT13 encoding:

echo "Hello World" | tr 'A-Za-z' 'N-ZA-Mn-za-m'
# Output: Uryyb Jbeyq

echo "Uryyb Jbeyq" | tr 'A-Za-z' 'N-ZA-Mn-za-m'
# Output: Hello World (decoding)

Advanced Usage

Process file contents:

# Convert file to uppercase
tr '[:lower:]' '[:upper:]' < input.txt > output.txt

# Remove all digits from a file
tr -d '[:digit:]' < input.txt > output.txt

# Replace multiple spaces with single space
tr -s ' ' < input.txt > output.txt

Chain with other commands:

# Count unique words (case-insensitive)
cat file.txt | tr '[:upper:]' '[:lower:]' | tr -s '[:space:]' '\n' | sort | uniq -c

# Extract only alphanumeric characters
cat file.txt | tr -cd '[:alnum:]\n'

# Convert CSV to tab-separated
cat file.csv | tr ',' '\t'

Text cleaning:

# Remove control characters
cat file.txt | tr -d '[:cntrl:]'

# Normalize whitespace
cat file.txt | tr -s '[:space:]' ' '

# Remove non-printable characters
cat file.txt | tr -cd '[:print:]\n'

Common Use Cases

Case conversion: Converting text between uppercase and lowercase
Character replacement: Replacing specific characters or character sets
Text cleaning: Removing unwanted characters from text
Format conversion: Converting between different text formats
Data preprocessing: Preparing text data for further processing
Character encoding: Simple character substitution ciphers
Whitespace normalization: Standardizing spacing in text
File format conversion: Converting between CSV, TSV, and other formats

Tips and Best Practices

Use character classes like [:lower:] instead of ranges like a-z for better portability
The -s option is useful for normalizing repeated characters
Combine -c and -d to keep only specific character types
Use tr with pipes to process command output
Remember that tr works character-by-character, not on words or patterns
Test your tr commands with small samples before processing large files
Use input/output redirection to process files efficiently
Escape special characters in the shell when necessary

Related Commands: sed, awk, cut, sort, uniq

Linux Commands