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Why (and how) did using cat on binary files mess up the terminal?

What can cause my shell to look like this?Diff of PDF files leaves garbage on next command lineBinary Output from CurlWhy running `cat /etc/localtime` on PuTTY puts a stream of “PuTTY” in the prompt?Why does “head /bin/ls” reset the terminalJumbled interface due to reading binary files in screenColorized `cat` for source and script files?Cannot merge files using catHow to prevent random console output from breaking the terminal?Why does “head /bin/ls” reset the terminalDynamically extract comments from files using catHow are binary files “binary”?What is the difference in using Ctrl+D and Ctrl+C to terminate cat command?How to reverse the content of binary file?Why the output of “cat a.out” is some weird text, instead of binary?How should I output the contents of a file to terminal without using cat?

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ margin-bottom:0;
}

7

If I understand the cat manual correctly:

concatenate files and print on the standard output

cat will take files as argument and print them on standard output.

What I don't get is if I use the command:

cat img.png > copy.png

I will obtain 2 png files identical while if I just

cat img.png  

I have all chance that my terminal get messed up and misinterpret what I type.

• How's that possible?


• Binary values are still binary data. Why it does not simply shows a series of 0 and 1 or the interpretation of those binary data in ASCII or whatever the encoding in terminal is?


• Is this behavior also possible by cating a text file containing strange characters?


• Should a mechanism to prevent this behavior like try{}catch{} statement should be implemented?

terminal cat binary

share|improve this question

edited 1 hour ago

muru

38.5k592168

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Your terminal does not get messed up. It is in a state you forced it into by sending it control characters. That you cannot use it any more after changing the state might not be what you wanted, but it is entirely a result of you not understanding the consequences of your actions. That would be the same as switching your font color to green in a word processor and saying your word processor is messed up, only because you don't know how to switch it back to black font without e.g. exiting the program.
  
  – Anthon
  Mar 13 '14 at 10:30
  
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  a reset command might help sometimes, but this is no miracle solution.
  
  – Ouki
  Mar 13 '14 at 10:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The actual sequence to type is Control-J reset Control-J. Almost always restores sanity.
  
  – Joshua
  Jan 1 '15 at 18:02
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Joshua And what's the difference between a lone reset and a reset between Ctrl-J keypresses? I can't see any (nor any reason to go the more complicated way)
  
  – syntaxerror
  Jan 2 '15 at 1:56
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Because if the terminal was left in RAW mode, Enter generates Ctrl-M instead of Ctrl-J so the shell doesn't see the necessary keystroke to end a line and run the command.
  
  – Joshua
  Jan 2 '15 at 3:37
  

  
  
  
  

add a comment | 

7

If I understand the cat manual correctly:

concatenate files and print on the standard output

cat will take files as argument and print them on standard output.

What I don't get is if I use the command:

cat img.png > copy.png

I will obtain 2 png files identical while if I just

cat img.png  

I have all chance that my terminal get messed up and misinterpret what I type.

• How's that possible?


• Binary values are still binary data. Why it does not simply shows a series of 0 and 1 or the interpretation of those binary data in ASCII or whatever the encoding in terminal is?


• Is this behavior also possible by cating a text file containing strange characters?


• Should a mechanism to prevent this behavior like try{}catch{} statement should be implemented?

terminal cat binary

share|improve this question

edited 1 hour ago

muru

38.5k592168

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Your terminal does not get messed up. It is in a state you forced it into by sending it control characters. That you cannot use it any more after changing the state might not be what you wanted, but it is entirely a result of you not understanding the consequences of your actions. That would be the same as switching your font color to green in a word processor and saying your word processor is messed up, only because you don't know how to switch it back to black font without e.g. exiting the program.
  
  – Anthon
  Mar 13 '14 at 10:30
  
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  a reset command might help sometimes, but this is no miracle solution.
  
  – Ouki
  Mar 13 '14 at 10:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The actual sequence to type is Control-J reset Control-J. Almost always restores sanity.
  
  – Joshua
  Jan 1 '15 at 18:02
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Joshua And what's the difference between a lone reset and a reset between Ctrl-J keypresses? I can't see any (nor any reason to go the more complicated way)
  
  – syntaxerror
  Jan 2 '15 at 1:56
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Because if the terminal was left in RAW mode, Enter generates Ctrl-M instead of Ctrl-J so the shell doesn't see the necessary keystroke to end a line and run the command.
  
  – Joshua
  Jan 2 '15 at 3:37
  

  
  
  
  

add a comment | 

If I understand the cat manual correctly:

concatenate files and print on the standard output

cat will take files as argument and print them on standard output.

What I don't get is if I use the command:

cat img.png > copy.png

I will obtain 2 png files identical while if I just

cat img.png  

I have all chance that my terminal get messed up and misinterpret what I type.

• How's that possible?


• Binary values are still binary data. Why it does not simply shows a series of 0 and 1 or the interpretation of those binary data in ASCII or whatever the encoding in terminal is?


• Is this behavior also possible by cating a text file containing strange characters?


• Should a mechanism to prevent this behavior like try{}catch{} statement should be implemented?

terminal cat binary

share|improve this question

edited 1 hour ago

muru

38.5k592168

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

cat img.png > copy.png

cat img.png  

share|improve this question

edited 1 hour ago

muru

38.5k592168

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

share|improve this question

edited 1 hour ago

muru

38.5k592168

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

edited 1 hour ago

muru

38.5k592168

edited 1 hour ago

muru

38.5k592168

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

asked Mar 13 '14 at 9:54

KiwyKiwy

6,31753862

2 Answers
2

active

oldest

votes

7

cat concatenates files given as arguments on command line to standard output, it reads bytes at a time an as default does not perform any interpretation of the bytes it reads.

In your first example you are redirecting stdout to a file, that's why you get a new file.

In your second example the bytes are written to the terminal, and it is the terminal which is interpreting sequences of characters as control sequences for the terminal, this is why you get unusual behaviour on your terminal. It has nothing to do with cat as such, cat doesn't know what you are going to do with it's output. You might be sending it through a pipe to another program to interpret/process/print or play "Singing in the rain".

So following the unix philosophy,

do one thing, do one thing only, but do it well

cat should not attempt to second guess what you are trying to do.

edit 1 reply to @kiwy's 1st comment below.

Yes and No, let me explain,

No, if you cat to a terminal, because it (the terminal software) is sending the output to your screen or interpreting control sequences (it is emulating an old piece of hardware ie. a teletype device).

but,

Yes if you cat to a pipe and the program recieving can interpret the characters as commands.

look cat this for an example, cat anyOldShellScript | bash bash will interpret what it gets as commands.

share|improve this answer

edited Mar 13 '14 at 10:38

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  does it means that if you cat a binary files that may contains in plain text instruction like rm -rf . this could be interpreted ?
  
  – Kiwy
  Mar 13 '14 at 10:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I accept the answer though I don't really get why terminal can get messed up like this will if I type like dumbass on my keyboard I never manage to get this :D
  
  – Kiwy
  Mar 13 '14 at 10:30
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  And now irony... hum
  
  – Kiwy
  Mar 13 '14 at 10:43
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Kiwy control characters don't exist on your keyboard, but you can make echo output them if you choose. See stackoverflow.com/questions/5947742/… for how to do it and termsys.demon.co.uk/vtansi.htm for some of what is possible
  
  – David Wilkins
  Mar 13 '14 at 12:32
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DavidWilkins hey thanks that's nice, so much things to learn and no time for that :-(
  
  – Kiwy
  Mar 13 '14 at 12:33
  

  
  
  
  

add a comment | 

1

I guess this happens mostly because of non-printable characters with codes below 0x20. Those are special control/escape codes, which are used for keys like Backspace, Delete etc.

share|improve this answer

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628

add a comment | 

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7

cat concatenates files given as arguments on command line to standard output, it reads bytes at a time an as default does not perform any interpretation of the bytes it reads.

In your first example you are redirecting stdout to a file, that's why you get a new file.

In your second example the bytes are written to the terminal, and it is the terminal which is interpreting sequences of characters as control sequences for the terminal, this is why you get unusual behaviour on your terminal. It has nothing to do with cat as such, cat doesn't know what you are going to do with it's output. You might be sending it through a pipe to another program to interpret/process/print or play "Singing in the rain".

So following the unix philosophy,

do one thing, do one thing only, but do it well

cat should not attempt to second guess what you are trying to do.

edit 1 reply to @kiwy's 1st comment below.

Yes and No, let me explain,

No, if you cat to a terminal, because it (the terminal software) is sending the output to your screen or interpreting control sequences (it is emulating an old piece of hardware ie. a teletype device).

but,

Yes if you cat to a pipe and the program recieving can interpret the characters as commands.

look cat this for an example, cat anyOldShellScript | bash bash will interpret what it gets as commands.

share|improve this answer

edited Mar 13 '14 at 10:38

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  does it means that if you cat a binary files that may contains in plain text instruction like rm -rf . this could be interpreted ?
  
  – Kiwy
  Mar 13 '14 at 10:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I accept the answer though I don't really get why terminal can get messed up like this will if I type like dumbass on my keyboard I never manage to get this :D
  
  – Kiwy
  Mar 13 '14 at 10:30
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  And now irony... hum
  
  – Kiwy
  Mar 13 '14 at 10:43
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Kiwy control characters don't exist on your keyboard, but you can make echo output them if you choose. See stackoverflow.com/questions/5947742/… for how to do it and termsys.demon.co.uk/vtansi.htm for some of what is possible
  
  – David Wilkins
  Mar 13 '14 at 12:32
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DavidWilkins hey thanks that's nice, so much things to learn and no time for that :-(
  
  – Kiwy
  Mar 13 '14 at 12:33
  

  
  
  
  

add a comment | 

7

cat concatenates files given as arguments on command line to standard output, it reads bytes at a time an as default does not perform any interpretation of the bytes it reads.

In your first example you are redirecting stdout to a file, that's why you get a new file.

In your second example the bytes are written to the terminal, and it is the terminal which is interpreting sequences of characters as control sequences for the terminal, this is why you get unusual behaviour on your terminal. It has nothing to do with cat as such, cat doesn't know what you are going to do with it's output. You might be sending it through a pipe to another program to interpret/process/print or play "Singing in the rain".

So following the unix philosophy,

do one thing, do one thing only, but do it well

cat should not attempt to second guess what you are trying to do.

edit 1 reply to @kiwy's 1st comment below.

Yes and No, let me explain,

No, if you cat to a terminal, because it (the terminal software) is sending the output to your screen or interpreting control sequences (it is emulating an old piece of hardware ie. a teletype device).

but,

Yes if you cat to a pipe and the program recieving can interpret the characters as commands.

look cat this for an example, cat anyOldShellScript | bash bash will interpret what it gets as commands.

share|improve this answer

edited Mar 13 '14 at 10:38

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  does it means that if you cat a binary files that may contains in plain text instruction like rm -rf . this could be interpreted ?
  
  – Kiwy
  Mar 13 '14 at 10:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I accept the answer though I don't really get why terminal can get messed up like this will if I type like dumbass on my keyboard I never manage to get this :D
  
  – Kiwy
  Mar 13 '14 at 10:30
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  And now irony... hum
  
  – Kiwy
  Mar 13 '14 at 10:43
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Kiwy control characters don't exist on your keyboard, but you can make echo output them if you choose. See stackoverflow.com/questions/5947742/… for how to do it and termsys.demon.co.uk/vtansi.htm for some of what is possible
  
  – David Wilkins
  Mar 13 '14 at 12:32
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DavidWilkins hey thanks that's nice, so much things to learn and no time for that :-(
  
  – Kiwy
  Mar 13 '14 at 12:33
  

  
  
  
  

add a comment | 

cat concatenates files given as arguments on command line to standard output, it reads bytes at a time an as default does not perform any interpretation of the bytes it reads.

In your first example you are redirecting stdout to a file, that's why you get a new file.

In your second example the bytes are written to the terminal, and it is the terminal which is interpreting sequences of characters as control sequences for the terminal, this is why you get unusual behaviour on your terminal. It has nothing to do with cat as such, cat doesn't know what you are going to do with it's output. You might be sending it through a pipe to another program to interpret/process/print or play "Singing in the rain".

So following the unix philosophy,

do one thing, do one thing only, but do it well

cat should not attempt to second guess what you are trying to do.

edit 1 reply to @kiwy's 1st comment below.

Yes and No, let me explain,

No, if you cat to a terminal, because it (the terminal software) is sending the output to your screen or interpreting control sequences (it is emulating an old piece of hardware ie. a teletype device).

but,

Yes if you cat to a pipe and the program recieving can interpret the characters as commands.

look cat this for an example, cat anyOldShellScript | bash bash will interpret what it gets as commands.

share|improve this answer

edited Mar 13 '14 at 10:38

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

share|improve this answer

edited Mar 13 '14 at 10:38

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

answered Mar 13 '14 at 10:08

X TianX Tian

7,95312237

1

I guess this happens mostly because of non-printable characters with codes below 0x20. Those are special control/escape codes, which are used for keys like Backspace, Delete etc.

share|improve this answer

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628

add a comment | 

1

I guess this happens mostly because of non-printable characters with codes below 0x20. Those are special control/escape codes, which are used for keys like Backspace, Delete etc.

share|improve this answer

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628

add a comment | 

I guess this happens mostly because of non-printable characters with codes below 0x20. Those are special control/escape codes, which are used for keys like Backspace, Delete etc.

share|improve this answer

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628

share|improve this answer

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628

answered Mar 13 '14 at 10:06

UVVUVV

2,10511628