What do t and b do

Question

I expect this simple line of code  printf  foo b tbar n      to replace  o  with   t  and to produce the following output  fo     bar    assuming that tab stop occurs every 8 characters   On the contrary I get  foo    bar   It seems that my shell interprets  b as  move the cursors one position back  and  t as  move cursor to the next tab stop   Is this behaviour specific to the shell in which I m running the code  Should I expect different behaviour on different systems

User · Answer

No  that s more or less what they re meant to do   In C  and many other languages   you can insert hard-to-see type characters using   notation     a is alert bell  b is backspace rubout  n is newline  r is carriage return  return to left margin   t is tab   You can also specify the octal value of any character using  0nnn  or the hexadecimal value of any character with  xnn    EG  the ASCII value of   is octal 137  hex 5f  so it can also be typed  0137 or  x5f  if your keyboard didn t have a   key or something  This is more useful for control characters like NUL   0  and ESC   033    As someone posted  then deleted their answer before I could  1 it   there are also some less-frequently-used ones     f is a form feed new page  eject page from printer   v is a vertical tab  move down one line  on the same column    On screens   f usually works the same as  v  but on some printers teletypes  it will go all the way to the next form sheet of paper

User · Answer

Backspace and tab both move the cursor position   Neither is truly a  printable  character   Your code says    print  foo  move the cursor back one space move the cursor forward to the next tabstop output  bar     To get the output you expect  you need printf  foo b  tbar     Note the extra  space    That says    output  foo  move the cursor back one space output a      this replaces the second  o    move the cursor forward to the next tabstop output  bar     Most of the time it is inappropriate to use tabs and backspace for formatting your program output   Learn to use printf   formatting specifiers   Rendering of tabs can vary drastically depending on how the output is viewed   This little script shows one way to alter your terminal s tab rendering   Tested on Ubuntu   gnome-terminal      bin bash tabs -8  echo -e   tnormal tabstop  for x in  seq 2 10   do   tabs  x   echo -e   ttabstop  x   done  tabs -8 echo -e   tnormal tabstop    Also see  man setterm and regtabs   And if you redirect your output or just write to a file  tabs will quite commonly be displayed as fewer than the standard 8 chars  especially in  programming  editors and IDEs   So in otherwords   printf   -8s s    foo    bar       this will ALWAYS output  foo     bar     printf  foo tbar       who knows how this will be rendered      IMHO  tabs in general are rarely appropriate for anything   An exception might be generating output for a program that requires tab-separated-value input files  similar to comma separated value    Backspace   b  is a different story    it should never be used to create a text file since it will just make a text editor spit out garbage   But it does have many applications in writing interactive command line programs that cannot be accomplished with format strings alone   If you find yourself needing it a lot  check out  ncurses   which gives you much better control over where your output goes on the terminal screen   And typically  since it s 2011 and not 1995  a GUI is usually easier to deal with for highly interactive programs   But again  there are exceptions  Like writing a telnet server or console for a new scripting language

User · Answer

The C standard  actually C99  I m not up to date  says   Alphabetic escape sequences representing nongraphic characters in the execution character set are intended to produce actions on display devices as follows   b  backspace  Moves the active position to the previous position on the current line         t  horizontal tab  Moves the active position to the next horizontal tabulation position on the current line         Both just move the active position  neither are supposed to write any character on or over another character  To overwrite with a space you could try  puts  quot foo b  tbar quot    but note that on some display devices - say a daisy wheel printer - the o will show the transparent space

User · Answer

t is the tab character  and is doing exactly what you re anticipating based on the action of  b - it goes to the next tab stop  then gets decremented  and then goes to the next tab stop  which is in this case the same tab stop  because of the  b

User · Answer

This behaviour is terminal-specific and specified by the terminal emulator you use  e g  xterm  and the semantics of terminal that it provides  The terminal behaviour has been very stable for the last 20 years  and you can reasonably rely on the semantics of  b

[c] What do \t and \b do?

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