I have a file with multiple lines:

line1
line2
line3
line_before
line4
line5
line6

and 3 patterns:

line2
line4
line6

Expect:

line2
line_before
line4
line6

I try use grep -B 1 but it show line before any patterns.

Thank you for any help!

CodePudding user response：

With awk...

In my exemple, your file with multiple lines is named "file.txt" and the file with 3 patterns is filem.txt.

The awk filter.awk program:

#! /bin/awk -f

NR == FNR && NR != 2 {
    # First file: Save 1st en 3rd tokens
    m[$0]=1
    next
}
NR == FNR && NR == 2 {
    # First file: Save 2nd token
    b[$0]=1
    next
}

NR != FNR {
    # Second file

    # Search line in tokens
    for (e in m) {
        if ($0 == e) {
            # Print current line
            print
            old = $0
            next
        }
    }
    for (e in b) {
        if ($0 == e) {
            # Print previous line
            if (FNR > 1) print prev
            # Print current line
            print
            old = $0
            next
        }
    }
}
{
    # Save current line
    prev = $0
}

Execute like this (files order is very important):

awk -f filter.awk filem.txt file.txt

Output:

line2
line_before
line4
line6

CodePudding user response：

I am assuming you want -B1 for only -e line4. (I would phrase that as: "How can I grep for multiple patterns, including the line before only one?")

Suggestions with grep -zPo:

grep -zPo '\v\K(line[26]|\V*\vline4)(?=\v)' file

or (same regex, different syntax):

grep -zPo '(?<=\v)(line[26]|\V*\vline4)(?=\v)' file

Both output this:

line2
line_before
line4
line6

Explanation:

• -z: consume entire file
• -P support perl regexes
• -o: output only matching strings (one line at a time)
• \v: vertical whitespace character
• \v\K or (?<=\v) : Require vertical whitespace before match, but do not include it in match
• (?=\v): require vertical whitespace after match, but do not include it in the match
• \V*\vline4 : for the line4 pattern, include the preceeding line (all not-vertical-whitespace-chars, a linebreak, and then the matching line -- to mimic grep -B1 line4