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This article introduces the character conversion command tr. Tr is a pipeline command that can be used to delete text in a message or replace text information!

tr [options] ... SET1 [SET2]

The commonly used options of tr are as follows

-d delete the string SET1 in the information

# echo root:x:0:0:root:/root:/bin/bash | tr –d root
:x:0:0::/:/bin/bash

We see that the root has been deleted.

-s removes repeated characters

# echo root:x:0:0:root:/root :/bin/bash | tr –s [o] rot:x:0:0:rot:/rot:/ bin/bash

When -d is not used and SET1 and SET2 appear in the command at the same time, it means that SET1 is replaced by SET2.

# echo root:x:0:0:root:/root:/bin/bash | tr [o] [s]
rsst:x:0:0:rsst:/rsst:/bin/bash

We can see that o is replaced by s. If we use it with option -s, it will remove duplicates first and then replace.

# echo root:x:0:0:root:/root:/bin/bash | tr –sos
rst:x:0:0:rst:/rst:/bin/bash

It should be noted here that when the length of the SET1 string is greater than the length of SET2, tr will repeatedly append the last character of SET2 until it reaches the length of SET1.

# echo root:x:0:0:root:/root:/bin/bash | tr bash bin
root:x:0:0:root:/root:/bin/binn

Let's see if the result is to output the last character n of bin repeatedly.

Another situation is that if there are repeated characters in SET1, the corresponding character will be found in SET2 according to the position of the last character of the repeated characters in SET1, and all the repeated characters in SET1 will be replaced with this character.

# echo root:x:0:0:root:/root:/bin/bash | tr root bin

Now let's not rush to tell the result, let's analyze it step by step. For r in the first position, it is replaced by b in the first position of bin, which is no problem. For o in the second position, it should normally correspond to i in the second position of bin. But because o is repeated, the last o is the third position, so it corresponds to n in the third position of bin. Therefore, both o should be replaced by n. T in the fourth position is very simple, because bin has only three digits, so t is also replaced by the last digit. So the result of the above command should be to replace root with bnnn.

bnnn:x:0:0:bnnn:/bnnn:/bin/bash

Yes, that's the result. Well, I believe I have made it clear. Let's look at an example using regular expressions:

# echo root:x:0:0:root:/root:/bin/bash | tr [az] [AZ] //Convert all lowercase letters to uppercase
ROOT:X:0:0:ROOT:/ROOT:/BIN/BASH

That's all about the tr command. You can use man tr to view more information about regular expressions in tr.

I hope this article is helpful to you.