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When using tr -t command, string1 should be truncated to the length of string2, right? 

tr -t abcdefghijklmn 123          # abc... = string1, 123 = string2
the cellar is the safest place    # actual input
the 3ell1r is the s1fest pl13e    # actual output

'to truncate' is another word for 'to shorten', right?tr translates, according to the pattern, completely ignoring the -t option. If I autocomplete to --truncate-set1 [to assure that I use the correct option] produces the same output.

Question: what am I doing wrong here?

I work in BASH, on a Debian based Distro.

UPDATE

Please note that this is a copy of a comment I made below

I thought tr -t means: shorten string1 to the length of string2. I see that a is translated to 1, that b would be translated to 2 and that c is translated to 3. This has nothing to do with shortening. 'To truncate' seems to mean something different than I thought. [I'm no native speaker]

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  • What other output are you expecting instead? tr -t abcdefghijklmn 123 is equivalent of tr abc 123.
    – manatwork
    Apr 26, 2013 at 14:54
  • What are you trying to do? It is hard to understand your question.
    – Rany Albeg Wein
    Apr 26, 2013 at 15:17
  • I think you're confusing "string1" with STDIN. string1 (an argument; called SET1 in GNU man page) is truncated, input is not.
    – depquid
    Apr 26, 2013 at 15:30
  • @RanyAlbegWein: I seem to completely have misunderstood the meaning of 'to truncate'. I've commented on this under the answer below [and still don't get it].
    – erch
    Apr 26, 2013 at 19:34
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    @cellar.dweller : It might be better if you think of the the two args to tr as "sets" or "patterns" of characters and not strings. Those corresponding sets/related patterns are used in the translation of the input. So the -t in your example means that the first set is now only 3 characters, and that's applied in the translation, which is why the exact same example without -t does, in fact, produce different output.
    – goldilocks
    Apr 26, 2013 at 20:24

1 Answer 1

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5

When using tr -t command, string1 should be truncated to the length of string2, right?

Isn't that what happened?

abcdefghijklmn
123

Notice which letters are and are not swapped:

the 3ell1r is the s1fest pl13e

'a' and 'c', but not e, f, i, or l, which were in the original (non-truncated) set 1.

Without the -t, you get:

t33 33331r 3s t33 s133st p3133

This is because (from man tr), "SET2 is extended to length of SET1 by repeating its last character as necessary." So without the -t to truncate set 1, what you have is the same as 

tr abcdefhijklmn 1233333333333

Let's consider another example, but using the same "the cellar is the safest place" as input.

> input="the cellar is the safest place"
> echo $input | tr is X
the cellar XX the XafeXt place

This is because the 2nd set is automatically extended to cover all of the first set. -t essentially does the opposite of that; it truncates the first set rather than extending the 2nd one:

> echo $input | tr -t is X
the cellar Xs the safest place

Which is the same as:

> echo $input | tr i X
the cellar Xs the safest place

Since the 's' was truncated from the first set. If the two sets were the same length, then using -t won't make any difference:

> echo $input | tr is XY
the cellar XY the YafeYt place
> echo $input | tr -t is XY
the cellar XY the YafeYt place
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  • [please note that this comment is a copy of the update that I made on the original post]. I thought tr -t means: shorten string1 to the length of string2. I see that a is translated to 1, that b would be translated to 2 and that c is translated to 3. This has nothing to do with shortening. 'To truncate' seems to mean something different than I thought.
    – erch
    Apr 26, 2013 at 20:12
  • @cellar.dweller Truncate means to shorten, yes, but tr isn't short for truncate, it's short for translate. The -t switch, however, is for truncating set 1 to the length of set 2 (and think: it would be meaningless the other way around). So, what I was trying to point out is that set 1 has been shortened: you get the exact same output as if you used tr abc 123. Everything past c is no longer included in the translation. I added a paragraph better explaining the difference between that and tr abcdefhijklmn 123.
    – goldilocks
    Apr 26, 2013 at 20:13

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