Translate, squeeze, and/or delete characters. Copies the standard input to the standard output with substitution or deletion of selected characters.

      tr [-Ccsu] string1 string2
      tr [-Ccu] -d string1
      tr [-Ccu] -s string1
      tr [-Ccu] -ds string1 string2

   -C     Complement the set of characters in string1, that is `-C ab'
	  includes every character except for `a' and `b'.

   -c     Same as -C but complement the set of byte values in string1.

   -d     Delete characters in string1 from the input.

   -s     Squeeze multiple occurrences of the characters listed in the last
	  operand (either string1 or string2) in the input into a single
	  instance of the character.	 This occurs after all deletion and
	  translation is completed.

   -u     Guarantee that any output is unbuffered.

     In the first synopsis form, the characters in string1 are translated into
     the characters in string2 where the first character in string1 is trans-
     lated into the first character in string2 and so on.  If string1 is
     longer than string2, the last character found in string2 is duplicated
     until string1 is exhausted.

     In the second synopsis form, the characters in string1 are deleted from
     the input.

     In the third synopsis form, the characters in string1 are compressed as
     described for the -s option.

     In the fourth synopsis form, the characters in string1 are deleted from
     the input, and the characters in string2 are compressed as described for
     the -s option.

     The following conventions can be used in string1 and string2 to specify
     sets of characters:

     character	Any character not described by one of the following conven-
		tions represents itself.

     \octal	A backslash followed by 1, 2 or 3 octal digits represents a
		character with that encoded value.  To follow an octal
		sequence with a digit as a character, left zero-pad the octal
		sequence to the full 3 octal digits.

		A backslash followed by certain special characters maps to
		special values.

		\a    <alert character>
		\b    <backspace>
		\f    <form-feed>
		\n    <newline>
		\r    <carriage return>
		\t    <tab>
		\v    <vertical tab>

		A backslash followed by any other character maps to that char-

     c-c	Represents the range of characters between the range end-
		points, inclusively.

     [:class:]	Represents all characters belonging to the defined character
		class.	Class names are:

		alnum	  <alphanumeric characters>
		alpha	  <alphabetic characters>
		cntrl	  <control characters>

		digit	  <numeric characters>
		graph	  <graphic characters>
		lower	  <lower-case alphabetic characters>
		print	  <printable characters>
		punct	  <punctuation characters>

		space	  <space characters>
		upper	  <upper-case characters>
		xdigit	  <hexadecimal characters>

		With the exception of the `upper' and `lower' classes,
		characters in the classes are in unspecified order.  In the
		`upper' and `lower' classes, characters are entered in
		ascending order.

		For specific information as to which ASCII characters are
		included in these classes, see ctype(3) and related manual

     [=equiv=]	Represents all characters belonging to the same equivalence
		class as equiv, ordered by their encoded values.

     [#*n]	Represents n repeated occurrences of the character represented
		by #.  This expression is only valid when it occurs in
		string2.  If n is omitted or is zero, it is be interpreted as
		large enough to extend string2 sequence to the length of
		string1.  If n has a leading zero, it is interpreted as an
		octal value, otherwise, it's interpreted as a decimal value.


     The LANG, LC_ALL, LC_CTYPE and LC_COLLATE environment variables affect
     the execution of tr as described in environ(7).

The tr utility has historically been extremely forgiving of syntax errors, for example, the -c and -s options were ignored unless two strings were specified. This implementation will not permit illegal syntax.

The tr utility has historically not permitted the manipulation of NUL bytes in its input and, additionally, stripped NUL's from its input stream. This implementation has removed this behavior as a bug.

Exits 0 on success, and >0 if an error occurs


# Swap case
$ echo "Hello World" | tr "A-Za-z" "a-zA-Z"

# Make lower case
$ echo "Hello World" | tr "[:upper:]" "[:lower:]"
hello world

# As a function
$ toLower() {
 echo $1 | tr "[:upper:]" "[:lower:]"

$ toLower SomeMixEDCaseText

# Split the path into its elements
$ echo $PATH | tr ":" "\n" | sort

Create a list of the words in file1, one per line, where a word is taken to be a maximal string of letters.
$ tr -cs "[:alpha:]" "\n" < file1

Translate the contents of file1 to upper-case.
$ tr "[:lower:]" "[:upper:]" < file1

Strip out non-printable characters from file1.
$ tr -cd "[:print:]" < file1

Remove diacritical marks from all accented variants of the letter 'e':
$ tr "[=e=]" "e"

“Chance is always powerful. - Let your hook be always cast; in the pool where you least expect it, there will be a fish” ~ Ovid


tr man page -
awk - Find and Replace text within file(s)
grep - Search file(s) for lines that match a given pattern

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