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.
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:]' UPPERCASEME
To search for a compliment of a pattern use the
-c option. Searching for a compliment 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' azzzzzzzzzzzzzzzz
Searching for a compliment 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:]' 01234567890
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.
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
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 repeat 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 on the BSD version of
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 "_" "-" some-url-that-I-have
For more complex find and replace operations a tool like
sed is recommended.
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:]'
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