grep

grep ⇒ global regular expressions print

grep uses posix regular expressions you should use grep -P to get perl compatible regular expressions

Find lines containing the exact word “root” in the file users.txt. Ensure it matches whole words only, not substrings like “rooted” or “rooter”.

grep -w "root" users.txt
 

Write a command to search for the word “connection” in network.log, but only show the count of matching lines.

grep -c "connection" network.log
 

How would you search for lines containing the word “fail” in application.log, but only show the file name of the files where a match is found (without showing the actual matching lines)?

grep -vP "^\d+$|^.*-->.*$" lesson13.srt | xclip -selection clipboard

Grep Command Options Reference Guide

Basic Pattern Matching Options

-i : Ignore case distinctions in patterns and input data -w : Match only whole words (surrounded by non-word characters) -x : Match only whole lines -v : Invert match (select non-matching lines) -F : Interpret pattern as a fixed string (not a regular expression) -E : Interpret pattern as an extended regular expression -P : Interpret pattern as a Perl regular expression

Output Control Options

-n : Prefix each line of output with line number -H : Print filename with output lines -h : Suppress filename prefix in output -l : Print only names of files containing matches -L : Print only names of files not containing matches -c : Print only count of matching lines per file -o : Show only the matched parts of lines -q : Quiet mode (suppress normal output) -s : Suppress error messages about nonexistent or unreadable files -b : Print byte offset of match -m NUM : Stop after NUM matches

Context Control Options

-B NUM : Print NUM lines of leading context before matching lines -A NUM : Print NUM lines of trailing context after matching lines -C NUM : Print NUM lines of context (before and after matching lines)

File and Directory Handling

-r : Recursively search directories -R : Recursively search directories (follow symbolic links) -d ACTION : Process directories (ACTION: ‘read’, ‘skip’, or ‘recurse’) -D ACTION : Process devices (ACTION: ‘read’ or ‘skip’) -f FILE : Obtain patterns from FILE -e PATTERN : Use PATTERN as the pattern (useful when pattern starts with ’-‘)

Binary File Options

-a : Process binary files as text -I : Process binary files as if not containing matches -U : Process binary files as UTF-8 text -z : Treat input/output data as null-terminated lines

Performance Options

-T : Initial tab alignment -Z : Output null-terminated lines —line-buffered : Flush output on every line —mmap : Use memory-mapped input if possible

Examples of Usage:

1. Case-insensitive search:

   grep -i "pattern" file.txt

2. Search whole words only:

   grep -w "word" file.txt

3. Show line numbers with matches:

   grep -n "pattern" file.txt

4. Recursive search with context:

   grep -R -C 2 "pattern" directory/

5. Count matches in file:

   grep -c "pattern" file.txt

Common Combinations:

1. Recursive case-insensitive search:

   grep -ri "pattern" directory/

2. Search whole words with line numbers:

   grep -wn "word" file.txt

3. Extended regex with context:

   grep -E -C 2 "pattern1|pattern2" file.txt

4. Quiet recursive search with filename only:

   grep -rlq "pattern" directory/

Tips:

• Use -E for extended regular expressions when you need more complex patterns
• Combine -r with -l to find files containing matches
• Use -v with -l to find files NOT containing matches
• When searching binary files, -a or -I might be necessary
• Use -m to limit the number of matches when searching large files

Examples

Command Overview

blkid /dev/sda1 | grep -Po 'UUID="\K[^"]+'

• blkid /dev/sda1: Outputs information about the /dev/sda1 block device, including its UUID, label, type, etc.
• grep -Po:
  • -P: Enables Perl-compatible regular expressions (PCRE), which allow advanced patterns like \K.
  • -o: Outputs only the matching parts of the input (not the entire line).

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Breaking Down the Regular Expression

2. \K

   • “Forget everything matched so far.”
   • \K tells grep to drop the part of the match (UUID=") from the final output. Only the part after \K will be captured in the result.
   • Essentially, this prevents UUID=" from appearing in the output.

3. [^"]+

   • [^"]: Matches any character that is NOT a double quote (").
   • +: Matches one or more of the preceding character (in this case, any non-quote characters).
   • This ensures we capture the UUID value, which consists of alphanumeric characters up to (but not including) the next double quote.

Example Match

Input:

/dev/sda1: LABEL="New Volume" BLOCK_SIZE="512" UUID="F86E68DF6E6897E0" TYPE="ntfs"

Step-by-step matching:

• UUID=": Matches the start of the UUID definition.
• \K: Tells grep to forget about the UUID=" part.
• [^"]+: Matches F86E68DF6E6897E0 (everything up to the next ").

Output:

F86E68DF6E6897E0

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