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UNIX for Dummies Questions & Answers
^H characters appear when opening text file using vi - RHEL
Post 302819231 by Don Cragun on Monday 10th of June 2013 01:02:38 PM
06-10-2013
In addition to what bakunin and in2nix4life have said, using x\bx\bx (where x is any printing character and \b is a backspace character) is a remnant of the days before CRT terminals. On a hardcopy device, that sequence literally printed the character 3 times in (almost) the same position (making it appear as bold text). Some CRT displays also recognized this sequence and highlighted characters presented this way. Some line printers also printed lines containing backspaces multiple times such that each overstruck character was printed in the appropriate position as many times as it appeared in the line (again producing bold text because hard copy devices were never accurate enough to double or triple print a character at exactly the same position).
Another common sequence is _\bx which produces underlined text on hardcopy output devices and on soft copy devices that recognize the sequence to produce underlined text.
Back in the early days, nroff was used for character addressable device output and troff was use to produce output for typesetting hardware. Later, troff device tables allowed it to be used with any type of output device. Historically, the col utility was used to get rid of several nroff artifacts that made it hard for humans to read the text files containing the overstrikes on a device that showed all of the characters in a line without performing overstrike processing (such as on a CRT when using vi to view the contents of the file). The col utility could also be used to convert tabs to sequences of spaces and vice versa, and also had options for dealing with half-line motions (that nroff used to display subscripts and superscipts on terminals and printers like the DASI 300s daisy wheel printers and terminals). Some later *roff implementations provided deroff in addition to or instead of col; but most systems that have an nroff utility also have a col utility (look for man pages for these utilities to determine the choices you have on your system).
Note also that without options, the cat utility doesn't do anything to get rid of overstrikes; it just copies data to the output device and the overstrikes and when an output device handles overstrikes by displaying the last character "printed" in a given output position, the overstrikes become invisible.
Thus endeth this morning's ancient history lesson on overstriking.
Another common sequence is _\bx which produces underlined text on hardcopy output devices and on soft copy devices that recognize the sequence to produce underlined text.
Back in the early days, nroff was used for character addressable device output and troff was use to produce output for typesetting hardware. Later, troff device tables allowed it to be used with any type of output device. Historically, the col utility was used to get rid of several nroff artifacts that made it hard for humans to read the text files containing the overstrikes on a device that showed all of the characters in a line without performing overstrike processing (such as on a CRT when using vi to view the contents of the file). The col utility could also be used to convert tabs to sequences of spaces and vice versa, and also had options for dealing with half-line motions (that nroff used to display subscripts and superscipts on terminals and printers like the DASI 300s daisy wheel printers and terminals). Some later *roff implementations provided deroff in addition to or instead of col; but most systems that have an nroff utility also have a col utility (look for man pages for these utilities to determine the choices you have on your system).
Note also that without options, the cat utility doesn't do anything to get rid of overstrikes; it just copies data to the output device and the overstrikes and when an output device handles overstrikes by displaying the last character "printed" in a given output position, the overstrikes become invisible.
Thus endeth this morning's ancient history lesson on overstriking.
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