Interpret backspaces in file

Question

Is there a way to interpret/"execute" backspaces in a file/line using common bash commands? I have a console program which prints some stuff and than erases it using backspaces and finally writes the end result. What I actually want is the output at the end.

echo -e "Foo\b\b\bBar" | what_goes_here > test.log

My test.log should only contain "Bar". I guess a problem to do this is how much text should be buffered until considered as really printed... In my case a "line buffered" interpreter would be sufficient.

Most utility have a switch which do not print such characters in first place. But the utility at hand has no such switch...

Answer 1

That's what col -b is for:

$ printf 'a\bb\n' | col -b | od -tc
0000000   b  \n
0000002

$ printf 'aaa\b\b\bbb\n' | col -b | od -tc
0000000   b   b   a  \n
0000004

The sed equivalent would be something like:

bs=$(printf '\b')
sed "s/^[^$bs]*/&\
\
/;:1
s/\n.\{0,1\}\(.*\n\)\([^$bs]\)/\2\
\1/;s/\(.\{0,1\}\)\n\(.*\n\)$bs/\
\1\2/;t1
s/\n//g"

Dating to the times of the real tele-typewriters (tty), the x\bx sequence is sometimes found to represent a bold x (x typed over itself), x\b_ or _\bx for an underlined x (x̲), and x\b- or -\bx for strike-through (x̶).

Another useful command to deal with those is the colcrt command.

$ printf '_\bfo\b_o\b_ bar b\b-a\b-z\b-\n' | colcrt
foo bar baz
---

Another option is to convert \b_ and \b- to the Unicode combining characters U+0332 and U+0336:

Here assuming a Unicode locale and zsh, ksh93 or bash:

$ printf 'f\b_o\b_o\b_ bar b\b-a\b-z\b-\n' | sed $'s/\b_/\u0332/g;s/\b-/\u0336/g'
f̲o̲o̲ bar b̶a̶z̶

(pipe to colcrt or col -b to also handle the x\bx bold).

Answer 2

Try sed

if used in ksh93/zsh/bash

printf "Foo\b\b\bBar\n" | sed -e :a -e "s/.\{0,1\}"$'\b'"//;ta" > test.log

or more portable

printf "Foo\b\b\bBar\n" | sed -e :a -e "s/.\{0,1\}$(printf "\b")//;ta" > test.log

or GNU sed:

printf "Foo\b\b\bBar\n" | gsed -re ":a;s/.?\x08//;ta" > test.log

---

Note: this approach treats '\b' as a backward erase instead of a backspace, so there would be a difference if the number of characters after a number of \b is less than the number of \b. (Thanks @StephaneChazelas )