Question

18

How does tr translate one word to another?

rated 0 times [ 18] [ 0] / answers: 1 / hits: 6106 / 2 Years ago, tue, july 5, 2022, 1:04:25

I have a file ma.txt and it contains the output of ls -l; when I run the tr command (tr "nik-pc" "root") I get this output:

nik-pc@nik:~$ cat ma.txt 

total 52

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar 11 11:33 Desktop

lrwxrwxrwx 1 nik-pc nik-pc    2 Mar  8 22:54 di -> hd

drwxr-xr-x 3 nik-pc nik-pc 4096 Mar 13 13:28 Documents

drwxr-xr-x 7 nik-pc nik-pc 4096 Mar 14 18:21 Downloads

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar 11 09:39 dwhelper

-rw-r--r-- 1 nik-pc nik-pc 2134 Mar 13 17:40 hd

-rw-r--r-- 1 nik-pc nik-pc    3 Mar 13 15:34 m

-rw-r--r-- 1 nik-pc nik-pc    0 Mar 17 19:48 ma.txt

drwxr-xr-x 3 nik-pc nik-pc 4096 Mar 13 14:58 Music

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar  5 12:30 Pictures

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar  5 11:44 Public

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar 13 15:58 sd

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar  5 11:44 Templates

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar  5 11:44 Videos

drwxr-xr-x 2 nik-pc nik-pc 4096 Mar 11 11:33 xdm-helper



nik-pc@nik:~$ tr "nik-pc" "root" < ma.txt 

tttat 52

drwxr-xr-x 2 too-tt too-tt 4096 Mar 11 11:33 Desottt

trwxrwxrwx 1 too-tt too-tt    2 Mar  8 22:54 do -> hd

drwxr-xr-x 3 too-tt too-tt 4096 Mar 13 13:28 Dttutetts

drwxr-xr-x 7 too-tt too-tt 4096 Mar 14 18:21 Dtwtttads

drwxr-xr-x 2 too-tt too-tt 4096 Mar 11 09:39 dwhetter

-rw-r--r-- 1 too-tt too-tt 2134 Mar 13 17:40 hd

-rw-r--r-- 1 too-tt too-tt    3 Mar 13 15:34 t

-rw-r--r-- 1 too-tt too-tt    0 Mar 17 19:48 ta.txt

drwxr-xr-x 3 too-tt too-tt 4096 Mar 13 14:58 Musot

drwxr-xr-x 2 too-tt too-tt 4096 Mar  5 12:30 Pottures

drwxr-xr-x 2 too-tt too-tt 4096 Mar  5 11:44 Pubtot

drwxr-xr-x 2 too-tt too-tt 4096 Mar 13 15:58 sd

drwxr-xr-x 2 too-tt too-tt 4096 Mar  5 11:44 Tetttates

drwxr-xr-x 2 too-tt too-tt 4096 Mar  5 11:44 Vodets

drwxr-xr-x 2 too-tt too-tt 4096 Mar 11 11:33 xdt-hetter

At line one it replaced "nik" with "too" and the spelling of "Desktop" became "Desottt".

Why is this? What is logic behind it?

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uxuriousrnal

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answered 2 Years ago socelebrate · Accepted Answer

tr translates a string character-wise. It searches the letters from the first set and replaces them by those form the second set.

You had nik-pc as first set. tr expands the k-p part in that to all letters in the range from "k" to "p", so the set is equal to niklmnopc.

Your second set was root.

What tr now does is to search all occurrences of the first character in the (evaluated) first set and replace them with the first character of the second set. When there is no more character in set 2, it simply repeates its last character. See the table below:

n --> r

i --> o

k --> o

l --> t

m --> t

n --> t

o --> t

p --> t

c --> t

So now it's clear why e.g. "Desktop" becomes "Desottt". The behaviour is fully correct and intended this way.

What you're looking for instead can be achieved using sed:

sed 's/nik-pc/root/g' ma.txt

The syntax is this:

sed 's/SEARCH_PATTERN/REPLACE_STRING/FLAGS' INPUT_FILE

So we let it search for the pattern "nik-pc" and replace the entire match with "root". We need to add the "g" flag to enable global replacing. Without that, it would only replace each first match per line.