Linux and Unix tr command tutorial with examples

Tutorial on using tr, a UNIX and Linux command for translating or deleting characters. Examples of converting uppercase to lowercase, deleting specific characters, squeezing repeating patterns and basic finding and replacing.

The UNIX and Linux tr command
The UNIX and Linux tr command

What is the tr command in UNIX?

The tr command in UNIX is a command line utility for translating or deleting characters. It supports a range of transformations including uppercase to lowercase, squeezing repeating characters, deleting specific characters and basic find and replace. It can be used with UNIX pipes to support more complex translation.

How to convert lower case to upper case

To convert from lower case to upper case the predefined sets in tr can be used. The [:lower:] set will match any lower case character. The [:upper:] set matches any uppercase character. To convert from lower to upper these can be used to translate a string.

echo uppercaseme | tr '[:lower:]' '[:upper:]'

How to search for a complement of a pattern

To search for a complement of a pattern use the -c option. Searching for a complement means searching for the inverse of the set specified. In the following example any character that is not ‘a’ is matched and translated to ‘z’.

echo abcdefghijklmnop | tr -c 'a' 'z'

Searching for a complement pattern can effectively be used to pull out patterns from text. In the following example the -c option is combined with the -d delete option to extract a phone number.

echo 'Phone: 01234 567890' | tr -cd '[:digit:]'

How to delete specific characters

To delete specific characters use the -d option. This option deletes characters in the first set specified. The following example removes any occurrence of ‘up’.

echo 'clean this up' | tr -d 'up'
clean this

Note that the tr is relatively crude. For more complex patterns sed is a better option.

How to squeeze repeating characters

To squeeze repeat occurrences of characters specified in a set use the -s option. This removes repeated instances of a character. In the following example a string with too many spaces is squeezed to remove them.

echo 'too    many    spaces  here' | tr -s '[:space:]'
too many spaces here

How to truncate a search pattern

To truncate the first set to the second set use the -t option. By default tr will repeat the last character of the second set if the first and second sets are different lengths. Consider the following example.

echo 'the cat in the hat' | tr 'abcdefgh' '123'
t33 31t in t33 31t

The last character of the second set is repeated to match any letter from c-h. Using the truncate option limits the matching to the length of the second set.

echo 'the cat in the hat' | tr -t 'abcdefgh' '123'
the 31t in the h1t

Note that this option is not available in the BSD version of tr.

How to find and replace

The tr tool works well for simple find and replace operations where one character should be replaced with another. The following example replaces underscores with spaces.

echo "some_url_that_I_have" | tr "_" "-"

For more complex find and replace operations a tool like sed is recommended.

How to build translate pipelines

The tr can be combined with pipes to build a translation pipeline. In the following example tr is used to clean a file named ‘Bad File nAme.txt’. This removes backslashes, converts spaces to underscores and converts uppercase to lowercase.

echo Bad\ File\ nAme.txt | tr -d '\' | tr ' ' '_' | tr '[:upper:]' '[:lower:]'

Further reading


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