tr Command: Tutorial & Examples
Manipulate standard input
tr command, short for translate, is an essential utility in the shell command language. It's
primarily used to translate or delete characters from standard input, writing to standard output.
How it works
tr command works by taking two sets of characters as input. It replaces occurrences of the characters in the first
set with the corresponding character in the second set. If the second set is shorter than the first, the
extends it by repeating its last character.
What it is used for
tr command is versatile and is used for a variety of tasks in server and VM management. This includes translating
lowercase letters to uppercase, deleting unwanted characters from output, and even replacing spaces with new lines. It's
also handy for stripping out control characters, or for converting DOS/Windows text files to a Unix format.
Why it is important
tr command is a powerful tool for manipulating text directly in the shell, without needing to
use a text editing program. This can be particularly useful when processing large amounts of data or when you need to
format data in a specific way for further processing.
How to use it and common command line parameters
tr command is straightforward. You simply need to specify the set of characters to be replaced and the set
of characters to replace them with.
Let's look at some examples:
echo 'Hello, World!' | tr '[:lower:]' '[:upper:]'
This command will output: 'HELLO, WORLD!'. It translates all lowercase letters to uppercase.
echo 'Hello, World!' | tr -d '[:punct:]'
This command will output: 'Hello World'. It deletes all punctuation characters.
There are many options that you can use with
--delete: delete characters in the first set
--squeeze-repeats: replace each input sequence of a repeated character that is listed in the last specified SET with a single occurrence of that character
--complement: use the complement of SET1
Potential problems and pitfalls
One common pitfall when using the
tr command is assuming that it works with strings. However,
tr works with sets of
characters, not strings. For example, the command
tr 'abcd' 'jklm' will replace each 'a' with a 'j', each 'b' with a '
k', and so on.
Also, when using
tr, you should be aware that it reads from standard input and writes to standard output. This means
that if you want to modify a file, you need to use redirection operators.
For example, to replace all lowercase letters with uppercase letters in a file, you would use:
tr '[:lower:]' '[:upper:]' < inputfile > outputfile
Typical problems that can be solved with the tr command
tr command can help solve many text processing issues. For example, you could use
tr to solve the following
Replace all spaces with new lines to count the number of words in a file:
tr ' ' '\n' < file | wc -l
Convert a file from DOS/Windows format to Unix format by removing carriage return characters:
tr -d '\r' < inputfile > outputfile
Remove non-printable characters from a file:
tr -cd '[:print:]' < inputfile > outputfile
In each of these cases,
tr provides a quick and efficient way to process text directly from the command line.