The use of ? in grep command.. And practical use

grepregex

I tried to find the actual practical use of ? i.e. for e.g. "egrep a? filename" but was not able to find any.. It returns all results..

So, Please help me out wherein i could know the actual use of egrep ? command..

If i use 'a?', it returns all result i.e. strings or lines. which has 0 a's, 1 a's, 2 a's and so on.. i.e. i am not able to find the use of the same..

Thanks

Best Answer

Say you wanted to match numeric assignment expressions like this in a script:

x=1234

where some numbers are negative and have a minus sign:

x=-5678

You could use this:

grep -E "x=-?[0-9]+" *

The question mark makes the minus optional.

(I don't think plain grep supports ? or +, hence -E).

If your files contain no null bytes

In this case, you can use grep alone:

grep -Plz "termA.*termB|termB.*termA" *

How it works:

The Perl Compatible Regular Expressions termA.*termB and termB.*termA search for strings which have both terms in it.
The combined PCRE termA.*termB|termB.*termA matches all strings containing both terms.
Finally, the -z switch makes data lines end in null bytes instead of newlines.

By the way, there's no need to use -P. If you prefer to continue using POSIX Basic Regular Expressions, the syntax is similar:

grep -lz "termA.*termB\|termB.*termA" *

If your files contain null bytes

In this case, you'll need auxiliary tools:

(grep -l termA * ; grep -l termB *) | sort | uniq -d

How it works:

grep -l termA * ; grep -l termB * displays all files containing either of the terms. Files that contain both terms will be displayed twice.
sort sorts the output.
uniq -d only displays duplicate lines (required lines to be sorted).

Linux – How to Use Grep to Include and Exclude Terms

To and expressions with grep you need two invocations:

grep -Ei "search term" | grep -Eiv "exclude term"

If the terms you are searching for are not regular expressions, use fixed string matching (-F) which is faster:

grep -F "search term" | grep -Fv "exclude term"

Best Answer

Related Solutions

Linux – Search for files with more than one term (grep, awk?)

If your files contain no null bytes

If your files contain null bytes

Linux – How to Use Grep to Include and Exclude Terms

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