Intro To 'tr' Command In Linux

2023-08-23 - By Robert Elder

I use the 'tr' command to replace or delete single-byte characters in files or streams:

echo "Arch Linus." | tr 's' 'x'

Arch Linux.

Replacing Multiple Characters Using 'tr' (Character Sets)

I can replace all occurrences of the letter 'a' with the letter 'b' like this:

echo "Apple_Banana" | tr 'a' 'b'

Apple_Bbnbnb

I can perform multiple character replacements at once by listing the search and replace character one after another, like this:

echo "Apple_Banana" | tr 'a_' 'b='

Apple=Bbnbnb

echo "Apple_Banana" | tr 'a_n' 'b=q'

Apple=Bbqbqb

echo "Apple_Banana" | tr 'a_np' 'b=qg'

Aggle=Bbqbqb

Replacing The Complement Of A Character Set

The '-c' flag causes the replacement to act on the complement of the character set. In this command, I'm replacing any non-printable characters in the bash executable binary with a period character:

tr -c '[:print:]' '.' < /bin/bash | less

Deleting Characters

If we start with the file 'nani.txt':

cat nani.txt

Omae
wa
mou
shindeiru

xxd nani.txt

00000000: 4f6d 6165 200a 7761 200a 6d6f 7520 0a73  Omae .wa .mou .s
00000010: 6869 6e64 6569 7275 200a                 hindeiru .

The '-d' flag lets me delete unwanted characters like newlines:

cat nani.txt | tr -d '\n'

Omae wa mou shindeiru

cat nani.txt | tr -d '\n' | xxd

00000000: 4f6d 6165 2077 6120 6d6f 7520 7368 696e  Omae wa mou shin
00000010: 6465 6972 7520                           deiru

Squeezing Repeated Characters Together

The '-s' flag allows you to squeeze repeated characters together. I can correct the repeated quote characters in this JSON document named 'blocks.json':

{
  """inventory""":[
    {
      """type""":"""coal""",
      """count""": 43
    },
    {
      """type""":"""diamond""",
      """count""": 64
    },
    {
      """type""":"""lapis_lazuli""",
      """count""": 9999999
    }
  ]
}

using this command:

cat blocks.json | tr -s '"'

{
  "inventory":[
    {
      "type":"coal",
      "count": 43
    },
    {
      "type":"diamond",
      "count": 64
    },
    {
      "type":"lapis_lazuli",
      "count": 9999999
    }
  ]
}

Caveats Of The 'tr' Command

The tr command has many caveats, such as its use of the less popular POSIX notation to describe character sets:

man tr

...
       [:alnum:]
              all letters and digits

       [:alpha:]
              all letters

       [:blank:]
              all horizontal whitespace

       [:cntrl:]
              all control characters
...

and non-portability of character ranges:

info tr

...
     Many historically common and even accepted uses of ranges are not
     portable.  For example, on EBCDIC hosts using the ‘A-Z’ range will
     not do what most would expect because ‘A’ through ‘Z’ are not
     contiguous as they are in ASCII.  If you can rely on a POSIX
     compliant version of ‘tr’, then the best way to work around this is
     to use character classes (see below).  Otherwise, it is most
     portable (and most ugly) to enumerate the members of the ranges.
...

It also currently (as of 2023) lacks support for multi-byte character replacements:

info tr

...
   Currently ‘tr’ fully supports only single-byte characters.
Eventually it will support multibyte characters; when it does, the ‘-C’
option will cause it to complement the set of characters, whereas ‘-c’
will cause it to complement the set of values.  This distinction will
matter only when some values are not characters, and this is possible
only in locales using multibyte encodings when the input contains
encoding errors.
...

And that's why the 'tr' command is my favourite Linux command.

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