tr / col / expand

tr [options] set1 [set2]
expand [options] [file...]
unexpand [options] [file...]

The following examples demonstrate how each command works.

sample_tr_col_expand.sh

# Example input text:
# "hello world"       → convert to uppercase
# "my file name.txt"  → replace spaces
# "abc123def456"      → delete characters
# "too    many   spaces" → squeeze repeated characters

Converts all lowercase letters to uppercase.

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

Replaces spaces with underscores (useful for normalizing filenames).

echo "my file name.txt" | tr ' ' '_'
my_file_name.txt

Uses -d to delete specified characters. This example removes all alphabetic characters, leaving only digits.

echo "abc123def456" | tr -d '[:alpha:]'
123456

Uses -s to squeeze consecutive repeated characters into one.

echo "too    many   spaces" | tr -s ' '
too many spaces

Combines -s with cut to extract fields from data with irregular spacing.

echo "  user1  18  Tokyo  " | tr -s ' ' | cut -d' ' -f2-
user1 18 Tokyo

Uses -c (complement) to replace all non-alphanumeric characters with spaces.

echo "hello, world! 123" | tr -c '[:alnum:]\n' ' '
hello  world  123

Removes \r from Windows line endings (\r\n) to convert to Unix format.

tr -d '\r' < windows_file.txt > unix_file.txt

Uses expand -t N to convert tabs to N spaces.

printf "name\tage\ncity\tcode\n" | expand -t 8
name    age
city    code

Common Mistake 1: Passing a filename to tr as an argument does not work

tr only reads from standard input. Passing a filename as an argument produces an error.

tr 'a-z' 'A-Z' file.txt
tr: extra operand 'file.txt'

Use input redirection to feed the file to tr.

tr 'a-z' 'A-Z' < file.txt

Common Mistake 2: Mismatched character set lengths in -d mode drops unintended characters

When using tr SET1 SET2, if SET1 is longer than SET2, the extra characters in SET1 all map to the last character of SET2. Use -d to delete characters rather than mapping them to avoid unexpected output.

echo "hello world" | tr 'aeiou' 'A'
hAllA wArld
（e, i, o, u all map to A because SET2 is shorter）

To delete vowels, use -d.

echo "hello world" | tr -d 'aeiou'
hll wrld

tr does not accept filenames as arguments — it only reads from standard input. To process a file, redirect it using tr < file.

tr is commonly used to convert line endings when working with Windows text files on Linux (\r\n → \n). For more advanced text transformation, see also sed.