Expresiones varias con grep

Utilizar varias expresiones en la misma linea mediante el parámetro -e.

man grep:

-e PATTERN, –regexp=PATTERN

Use PATTERN as the pattern. If this option is used multiple times or is combined with the -f (–file) option, search for all patterns given. This option can be used to protect a pattern beginning with “-”.

Ejemplo: Eliminar comentarios y lineas en blanco, generalmente para inspeccionar archivos de configuración:

grep -v -e '^#' -e '^\s*$' /etc/adduser.conf

Obtener texto a partir de un patrón ignorando un prefijo mediante el parámetro -o y el uso de \K en una expresión de tipo PCRE:

man grep:

-o, –only-matching

Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.

-P, –perl-regexp

Interpret the pattern as a Perl-compatible regular expression (PCRE). This is experimental and grep -P may warn of unimplemented features.

man pcresyntax:

MATCH POINT RESET

\K reset start of match

\K is honoured in positive assertions, but ignored in negative ones.

Ejemplo: Obtener el tipo de filesystem de /:

grep -oP '^[^#]+\s+/\s+\K\S+' /etc/fstab

Obtener texto a partir de un patrón ignorando prefijo y sufijo usando -o en una expresión de tipo PCRE con -P:

man perlre

“(?<=pattern)”

A zero-width positive lookbehind assertion. For example, “/(?<=\t)\w+/” matches a word that follows a tab, without including the tab in $&.

“(?=pattern)”

A zero-width positive lookahead assertion. For example, “/\w+(?=\t)/” matches a word followed by a tab, without including the tab in $&

Ver: https://unix.stackexchange.com/questions/13466/can-grep-output-only-specified-groupings-that-match