Understanding Linux / UNIX tr command

by Vivek Gite · 12 comments

Q. Can you explain the tr command and how to use it under Linux / UNIX like oses?

A. The tr utility copies the given input to produced the output with substitution or deletion of selected characters. tr abbreviated as translate or transliterate. It takes as parameters two sets of characters, and replaces occurrences of the characters in the first set with the corresponding elements from the other set i.e. it is used to translate characters.

It is commonly used in shell scripts and other application.

tr command syntax

tr [options] "set1" "set2" echo something | tr "set1" "set2" tr "set1" "set2" < input.txt tr "set1" "set2" < input.txt > output.txt

How do I use tr command under Linux / UNIX?

Translate the word 'linux' to upper-case:
$ echo 'linux' | tr "[:lower:]" "[:upper:]" $ echo 'linux' | tr "a-z" "A-Z" $ echo 'I LovE linuX. one is better Than 2' | tr "a-z" "A-Z"
Output:

LINUX
I LOVE LINUX. ONE IS BETTER THAN 2

Create a list of the words in /path/to/file, one per line, enter:
$ tr -cs "[:alpha:]" "\n" < /path/to/file
Where,

-c : Complement the set of characters in string1
-s : Replace each input sequence of a repeated character that is listed in SET1 with a single occurrence of that character

Shell scripting example

In the following example you will get confirmation before deleting the file. If the user responds in lower case, the tr command will do nothing, but if the user responds in upper case, the character will be changed to lower case. This will ensure that even if user responds with YES, YeS, YEs etc; script should remove file:

#!/bin/bash
echo -n "Enter file name : "
read myfile
echo -n "Are you sure ( yes or no ) ? "
read confirmation
confirmation="$(echo ${confirmation} | tr 'A-Z' 'a-z')"
if [ "$confirmation" == "yes" ]; then
   [ -f $myfile ] &&  /bin/rm $myfile || echo "Error - file $myfile not found"
else
   : # do nothing
fi

Remove all non-printable characters from myfile.txt
$ tr -cd "[:print:]" < myfile.txt

Remove all two more successive blank spaces from a copy of the text in a file called input.txt and save output to a new file called output.txt
tr -s ' ' ' ' < input.txt > output.txt

The -d option is used to delete every instance of the string (i.e., sequence of characters) specified in set1. For example, the following would remove every instance of the word nameserver from a copy of the text in a file called /etc/resolv.conf and write the output to a file called ns.ipaddress.txt:
tr -d 'nameserver' < /etc/resolv.conf > ns.ipaddress.txt

Recommended readings:

To check the other options that can be used in the tr command, see the tr command man page, enter:
$ man tr

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{ 12 comments… read them below or add one }

1 Planet Lowyat 12.23.07 at 3:48 pm

Great tips. Typo error -> prodcued

2 Marko 02.13.08 at 3:08 pm

tr -d ‘nameserver’ ns.ipaddress.txt

I think this would delete the word nameserver, but also all occurrences of characters from set {nameserver}, which could probably create big mess.

Proper command should be:
sed ’s/nameserver//g’ ns.ipaddress.txt

3 Maher 08.23.08 at 11:10 am

can you give scripts for these :
Exercise 1: Write a shell script (named checkfiles.sh) that lists
all text files “*.TXT” in a directory and writes the first line of each text-file to a file called Text_Heads.

Exercise 2: Write a shell script (called words-no.sh) that counts the number of distinct words in a text file given as Argument.

Remark: White space characters are spaces, tabs, form feeds, and new lines. Use the tr, sort, grep. wc.

Exercise 3: Write a shell script (called cleancore.sh) that removes unwanted files from a directory and its subdirectories.
Argument 1 is the directory where to start the search. It defaults to your home directory (If there is no directory by this name).
Argument 2 after the directory is standard file-name that match the files that are to be removed, it defaults is ‘core’.

4 heba 09.01.08 at 11:45 am

How can I get the Exercises solve??

Please help me

5 laoa 09.10.08 at 11:02 am

Is there any way to do the following questions

1. Replace many different chars with one same char

ab(cde)fg -> ab~cde~fg
echo ab(cde)fg | tr “()” “~~”

echo a1234b | tr “[0-9]” “A”
a1234b ### hope aAAAAb
echo a1234b | tr “[0-9]” “AAAA”
aAAA4b
echo a1234b | tr “[0-9]” “AAAAA”
aAAAAb

2. replace one char with two chars

a~b -> a==b

Thanks

6 vivek 09.10.08 at 12:21 pm

laoa,

Avoid shell escape, enter:
echo 'ab(cde)fg' | tr "()" "~~"
OR
echo ab\(cde\fg | tr "()" "~~"

7 pham van hai 11.11.08 at 3:39 pm

please help me
convert file in window into file in linux
I try
cat file_linux | tr -s “\n” “\n\r” ?
but it cannot run

8 budoliv 02.06.09 at 10:36 pm

How do you get only the first letter capitalized?

9 Chris de Vidal 03.06.09 at 10:26 pm

pham van hai: Use the unix2dos program instead.
budoliv: I’d suggest you use sed instead but I don’t know the syntax off-hand.

10 Avedo 05.18.09 at 5:43 am

Hey!

Nice Tutorial. I tried to write a small shell script to rename my mp3-files, but it doesn’t work. What is wrong with it? Can someone help me?

Greets, Andy

for i in "*.[mM][pP]3";
do
        # convert uppercase to lowercase
        $n="$(echo ${i} | tr [:upper:] [:lower:])"

        # replace whitespaces with underscores
        $n="$(echo ${n} | tr [:blank:] '_')"

        # replace dots with underscores
        $n="$(echo ${n} | tr '.' '_')"

        # replace hypens with underscores
        $n="$(echo ${n} | tr '-' '_')"

        # replace multiple underscores
        $n="$(echo ${n} | tr -s '_')"

        # remove numbers at the beginning
        $n="$(echo ${n} | sed 's/^[0-9]*//')"

        # remove underscores at the beginning
        $n="$(echo ${n} | sed 's/^\_//')"

        # correct file ending
        $n="$(echo ${n} | sed 's/\_mp3/\.mp3/')"

        # rename file
        mv -v "$i" "$n";
done

11 Vivek Gite 05.18.09 at 10:18 am

Have you tried rename command to rename multiple files? It also support regex.. Try find command to generate a file list:

find /path/to/dir -iname "*.mp3" -print0 | while read -d $'\0' file; do your-script.sh "$file"; done

Create your-script.sh to rename each file:

#!/bin/sh
       i="$@"
        # convert uppercase to lowercase
        $n="$(echo ${i} | tr [:upper:] [:lower:])"

        # replace whitespaces with underscores
        $n="$(echo ${n} | tr [:blank:] '_')"

        # replace dots with underscores
        $n="$(echo ${n} | tr '.' '_')"

        # replace hypens with underscores
        $n="$(echo ${n} | tr '-' '_')"

        # replace multiple underscores
        $n="$(echo ${n} | tr -s '_')"

        # remove numbers at the beginning
        $n="$(echo ${n} | sed 's/^[0-9]*//')"

        # remove underscores at the beginning
        $n="$(echo ${n} | sed 's/^\_//')"

        # correct file ending
        $n="$(echo ${n} | sed 's/\_mp3/\.mp3/')"

        # rename file
        mv -v "$i" "$n";

HTH

12 kiran 06.30.09 at 10:11 am

try this it will work

for i in *.
do
# convert uppercase to lowercase
n=”$(echo ${i} | tr [:lower:] [:upper:])”

# replace whitespaces with underscores
n=”$(echo “${n}” | tr [:blank:] ‘_’)”

# replace dots with underscores
n=”$(echo “${n}” | tr ‘.’ ‘_’)”

# replace hypens with underscores
n=”$(echo “${n}” | tr ‘-’ ‘_’)”

# replace multiple underscores
n=”$(echo “${n}” | tr -s ‘_’)”

# remove numbers at the beginning
n=”$(echo “${n}” | sed ’s/^[0-9]*//’)”

# remove underscores at the beginning
n=”$(echo “${n}” | sed ’s/^\_//’)”

# correct file ending
n=”$(echo “${n}” | sed ’s/\_mp3/\.mp3/’)”

# rename file
mv -v “$i” “$n”;
done

Understanding Linux / UNIX tr command

tr command syntax

How do I use tr command under Linux / UNIX?

Shell scripting example

Recommended readings:

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