Question

I often find this in scripts (and, I have to admit, write it myself):

a=`echo $x | sed "s/foo/bar/"`

or

if echo $x | grep foo
then
    ...
fi

Consider "foo" to include some regex stuff.

I feel that there should be - and most likely is - a better way to phrase this, one that does not involve two commands and a pipe but wraps the thing into some more compact expression.

I just can't find it. Anybody?

Answer 1

Unless you assume a specific shell, there is no better way to do this than “pipe echo to tool”. That is all that is all you can count on in the traditional Bourne shell and/or the POSIX shell. If you consider other shells, then there are some other built-in possibilities.

ksh has

• extra patterns: ?(pattern-list), *(pattern-list), {n}(pattern-list), {n,m}(pattern-list), @(pattern-list), !(pattern-list);
• the %P printf specifier to convert a extended regular expression into a pattern (and %R for extended regular expression to pattern);
• the expr == pattern condition in [[ expr ]] tests;
• the ${param/pattern/replacement} parameter expansion;

bash has

• the extglob option to enable most of the extra patterns of ksh (no {n} and {n,m});
• the expr == pattern condition (in [[ expr ]] tests);
• the ${param/pattern/replacement} parameter expansion;
• (in newer versions) the expr =~ extregexp condition (in [[ expr ]] tests) that can match against extended regular expressions
  • with parenthesized subexpressions and the BASH_REMATCH parameter, sed-style replacements could be done.

zsh has

• its own extended patterns with the EXTENDED_GLOB option;
• ksh-like extended patterns with the KSH_GLOB option;
• the expr == pattern condition (in [[ expr ]] tests);
• the ${pattern/pattern/replacement} parameter expansion;
• the expr =~ extregexp condition (in [[ expr ]] tests), can use PCRE if the RE_MATCH_PCRE option is set
  • with parenthesized subexpressions and the MATCH and match parameters (or BASH_REMATCH with the BASE_REMATCH option set), sed-style replacements could be done;
• the zsh/pcre module that offers pcre_compile, pcre_study, and pcre_match commands and the -pcre-match test condition (in [[ expr ]] tests);
• the zsh/regex module that offers the -regex-match test condition (in [[ expr ]] tests);

Answer 2

To replace the sed line, do something like

${a/foo/bar} or ${a//foo/bar}

In the first form, only the first instance is replaced. The second form is a global search & replace.

In your case, it would be

Instead of:

if echo $x | grep foo
then
    ...
fi

Consider using:

if [ $x =~ foo ]
then
    ...
fi

Where foo is a regular expression.

Answer 3

sed 's/foo/bar/' <<< "foobie"

grep 'this' <<< "this or that"

While Dan McG's answers are good, they won't give exactly the same regexp support if you need it.