Learn-Bash-Shell-Scripting/Filter Commands: grep command - syntax | options | rules at main · Azad-02/Learn-Bash-Shell-Scripting · GitHub

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> grep command for Scripting

- is a filter command used for searching a string in file

SYNTAX

>> grep [options] "string/pattern" file/files
>> cat file.txt | grep [options] "string/pattern"			# to display file and then search
>> echo "text" | grep [options] "string/pattern"			# to search in simple text

- to search in single file
>> grep "<string>" <filename>

- to search in multiple files
>> grep "<string>" <filename1> <filename2> <..>

- to search in all files present in current directory
>> grep "<string>" *

- to search string in simple text
>> echo "<big string>" | grep "<string to search>"

BASIC OPTIONS

-i : to ignore case for matching or searching / ignores case sensitivity of string to be searched
-w : to match exactly whole word being searched
-v : to display lines which are not having given string or text
-o : to display only the matched parts from matched lines
-n : to display line number and the line matched with the string
-c : to display number of matched lines

Syntax for these 3 options (>>grep [option] Number "string" file)
-A : to display N lines after match
-B : to display N lines before match
-C : to display N lines around match i.e. N lines before and after match

GREP command options for searching in whole directory
-r : for recursive search in current directory as well as all subdirectories (>>grep -r "string" *)
-l : to display only file names where text is matched
-h : to hide filenames and display only content from files that matched

- Apply grep command on output of other command
>> echo "simple string" | grep "string"
>> apache2 -v | grep "version"

ADVANCE OPTIONS

-f : takes searching string/pattern from a file, one per line (>>grep -f <filename where search strings are present> <filename in which all those strings are to be searched>)
-e : to search multiple search strings/patterns directly from command (>>grep -e "string1" -e "string2" -e "string3" file.txt)
-E : must use for searching patterns / simplified -e command (>>grep -E "string1|string2|string3" file.txt)

** PATTERN is nothing but a string consisting of/representing Multiple Strings **

DEFAULT syntax for grep command (mostly to be used) -
>> grep -E[basic options] "pattern" file/files

Rules to create PATTERN

- xy|pq - Matches for xy or pq
- ^xyz  - Matches for lines which are starting with 'xyz'
- xyz$  - Matches for lines ending with 'xyz'
-  ^$   - Match for the empty lines
-   \   - To remove special purpose of any symbol (eg. \^ - searches for ^ character in file, \$ - searches for $ character in file, \.)
-   .   - Matches any one character per dot (eg. >>grep -E "t..s" file.txt - searches for all words having 1st letter 't' and 4th letter 's', '..' can be any 2 char)
-  \b   - Match the empty space at the edge of the string ("\bstring" - single space before 'string', "string\b" - single space after 'string')
-   ?   - Preceeding character is optional and will be matched atmost once ("yf?" - presence of 'f' is optional ZERO times or ONCE just after 'y')
-   *   - Preceeding character will be matched zero or more times ("yf*" - presence of 'f' can be ZERO or MORE times just after 'y')
-   +   - Preceeding character must be present atleast once, can be N number of times ("yf+" - 'f' must be present ATLEAST ONCE)
- [xyz] - Matches for the lines having either x or y or z. ("[abxyz]" = "a|b|x|y|z")
- [a-d] - Matches for the lines haivng either a or b or c or d. ("[abcd]" = "a|b|c|d" = "a-d")
-[a-dv-z] - Matches for the lines having a to d char or v to z. ("[a|b|c|d|v|w|x|y|z]" = "[a-dv-z]" = "[abcdvwxyz]")
- ^[abc]- Matches for the lines starting with either a or b or c
- [^abc]- Matches for the lines which are not starting with either a or b or c
-  {N}  - Preceeding string Matched exactly N times. ("xf{3}" - searches for x followed by 3 f i.e. "xf{3}" = "xfff")
-  {N,} - Preceeding string Matched N or more than N times. ("xf{3,}" - searches for atleast 3 times f and can vary upto any number)
- {N,M} - Preceeding string Matched atleast N times but not more than M times. ("xf{2,4}" - f can be 2,3 or 4 times after x)

- [[:alnum:]] - Searches for lines having Alphanumeric characters - Alphabets and Numbers
- [[:alpha:]] - Alphabetic characters
- [[:blank:]] - Blank characters - space/tabs
- [[:digit:]] - Digits - 0,1,2,3,4,5,6,7,8,9
- [[:lower:]] - Lower-case Letters
- [[:upper:]] - Upper-case Letters
- [[:space:]] - Space Characters - tab, newline, verical tab, form feed, carriage return, & space