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Linux Tr Command with Examples

Tr is a command line utility in Linux and Unix systems that translates, deletes and squeezes characters from the standard input and writes the result to the standard output.

The tr command is usually used in combination with other commands through piping and can perform operations like removing repeated characters, converting uppercase to lowercase, and basic character replacing and removing.

In this tutorial, we will show you how to use the tr command through practical examples and detailed explanations of the most common options.

How to Use the Tr Command

The syntax for the tr command is as follows:

tr OPTION... SET1 [SET2]

tr accepts two sets of characters, usually with the same length and replaces the characters of the first sets with the corresponding characters from the second set.

A SET is basically a string of characters including the special backslash-escaped characters.

In the following example, tr will replace all characters from the standard input linuxize by mapping the characters from the first set with the matching ones from the second set.

echo 'linuxize' | tr 'lin' 'red'

tr will replace each occurrence of l with r, i with e, and n with d.

reduxeze

The character sets can also be defined using character ranges. For example instead of writing:

echo 'linuxize' | tr 'lmno' 'wxyz'

you can use:

echo 'linuxize' | tr 'l-n' 'w-z'

When -c (--complement) option is used tr will replace all characters that are not in SET1.

In the example below all of the characters that are not in SET1 will be replaced with the last character from SET2:

echo 'linuxize' | tr -c 'li' 'xy'

As you may have noticed the output has one more visible character than the input. This is because the echo command prints an invisible newline character \n that is also replaced with y. To echo a string without a new line use the -n option.

liyyyiyyy

The -d (--delete) option tells tr to delete characters specified in SET1.

The command below will delete all of the characters specified in SET1. When deleting characters without squeezing you can specify only one set.

echo 'Linuxize' | tr -d 'liz'

The L character will not be deleted because the input includes an uppercase L while the l character in the set in lowercase.

Lnuxe

The -s (--squeeze-repeats) option replaces a sequence of repeated occurrences with the character set in the last SET. In the following example tr will remove repeated space characters:

echo "GNU     \    Linux" | tr -s ' '
GNU \ Linux

When SET2 is used the sequence of the character specified in SET1 is replaced with SET2.

echo "GNU     \    Linux" | tr -s ' ' '_'
GNU_\_Linux

The -t (--truncate-set1) option forces tr to truncate SET1 to the length of SET2 before doing further processing.

By default, if SET1 is larger than SET2 tr will reuse the last character of SET2. Here is an example:

echo 'Linux ize' | tr 'abcde' '12'

The output will show that the e character from SET1 is matched with the latest character of SET2, which is 2:

Linux iz2

Now use the same command with the -t option:

echo 'Linux ize' | tr -t 'abcde' '12'

You can see that in this case the last three characters of the SET1 are removed. SET1 becomes ‘ab’, the same length as SET2.

Linux ize

Combining options

The tr command also allows you to combine its options. For example, the following command will replace all characters except i with 0 and then it will squeeze the repeated 0 characters:

echo 'Linux ize' | tr -cs 'i' '0'
0i0i0

Tr Command Examples

In this section, we’ll cover a few examples of common uses of the tr command.

Convert lower case to upper case

Converting lower case to upper case or reverse is one of the typical use cases of the tr command. [:lower:] matches all lower case characters and [:upper:] matches all uppercase characters.

echo 'Linuxize' | tr '[:lower:]' '[:upper:]'
LINUXIZE

Instead of character classes, you can also use ranges:

echo 'Linuxize' | tr 'a-z' 'A-Z'

To convert upper case to lower case simply switch the places of the sets.

Remove all non-numeric characters

The following example will remove all non-numeric characters from the input passed to the tr command:

echo "my phone is 123-456-7890" | tr -cd [:digit:]

[:digit:] stands for all digit characters, and by using the -c option the command removes all non-digit characters. The output will look like this:

1234567890

Put each word in new line

To put each word in a new line we need to match all non alpha numerical characters and replace them with a new line:

echo 'GNU is an operating system' | tr -cs '[:alnum:]' '\n'
GNU
is
an
operating
system

Remove blank lines

To delete the blank lines simply squeeze the repetitive newline characters:

tr -s '\n' < file.txt > new_file.txt

In the command above we are using the redirection symbol < to pass the content of the file.txt to the tr command as input. The redirection > writes the output of the command to the file.txt.

The $PATH environmental variable is a colon-delimited list of directories that tells the shell which directories to search for executable files when you type a command.

To print each directory on a separate line we need to match the colon (:) and replace it with the new line:

echo $PATH | tr  ':' '\n'
/usr/local/sbin
/usr/local/bin
/usr/sbin
/usr/bin
/sbin
/bin

Conclusion

By now you should have a good understanding of how to use the Linux tr command. Although very useful, tr can work only with single characters. For more complex pattern matching and string manipulation, you should use sed or awk.