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Accepted
Jimm Chen
Asked: 2023-04-04 10:34:48 +0800 CST

Why egrep outputs two blank lines but "redirecting to file" produces actual text

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I'm using Ubuntu 20.04.4 with stock grep version 3.4.

I find something I cannot explain, please help.

[/dev/pts/0(xterm-256color) 2023-04-04 10:28:01 ERR:0]
[chj @Ub20cppdev ~/test]
$ cat input3l.csv 
"C:\Program Files (x86)\Comodo\",223743576,223920128
"C:\Program Files (x86)\Comodo\Dragon\",223743576,223920128
"C:\Program Files (x86)\Comodo\Dragon\dragon_s.dll",127097864,127098880

[/dev/pts/0(xterm-256color) 2023-04-04 10:28:07 ERR:0]
[chj @Ub20cppdev ~/test]
$ cat input3l.csv | egrep '(".+\\"),' 
"C:\Program Files (x86)\Comodo\",223743576,223920128
"C:\Program Files (x86)\Comodo\Dragon\",223743576,223920128

[/dev/pts/0(xterm-256color) 2023-04-04 10:28:19 ERR:0]
[chj @Ub20cppdev ~/test]
$ cat input3l.csv | egrep '(".+\\"),.+' 



[/dev/pts/0(xterm-256color) 2023-04-04 10:28:33 ERR:0]
[chj @Ub20cppdev ~/test]
$ cat input3l.csv | egrep '(".+\\"),.+' > egrep1.txt

[/dev/pts/0(xterm-256color) 2023-04-04 10:28:43 ERR:0]
[chj @Ub20cppdev ~/test]
$ cat egrep1.txt 
"C:\Program Files (x86)\Comodo\",223743576,223920128
"C:\Program Files (x86)\Comodo\Dragon\",223743576,223920128

enter image description here

grep

1 Answers

  1. Best Answer

    What you're seeing is almost certainly because of the interaction between color codes in the grep output and Windows-style (CRLF) line-endings in your input. You don't see the same when you redirect output, because the default egrep alias

    $ alias egrep
alias egrep='egrep --color=auto'

    only colors the output when writing to a terminal. Compare for example:

    $ printf '%s\r\n' 'foobar' | grep --color=always 'foo' | sed l
\033[01;31m\033[Kfoo\033[m\033[Kbar\r$
foobar
$

    versus

    $ printf '%s\r\n' 'foobar' | grep --color=always 'foo.*' | sed l
\033[01;31m\033[Kfoobar\r\033[m\033[K$

$

    You can see that the trailing .* (or in your case, with egrep / grep -E, trailing .+) is capturing the carriage return character \r and emitting it before the closing ANSI color sequence \033[m\033[K, causing it to overtype the matched text foo.

    See also When piping grep after a curl request, regex works very strange

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