1. grep

The grep command is used for searching and filtering text in files based on patterns (regular expressions) or simple strings.
grep stands for "Global Regular Expression Print"
We can use grep anywhere like with files, searching for file, directories etc.

Syntax
```
  grep [OPTION] .. Pattern [File] ..
```

15 Cases of grep command

Reference video

https://youtu.be/bKNAYemzC6E?si=3x08KgpifTIt4sMy

File which is being used

To ignore the upper and lower case while searching

No results cuz its case sensitive

Use : -i → ignore
```
 grep -i <keyword> <file-name>
```
Can search patterns also
To search everything except given pattern/keyword :
```
 grep -v <keyword> <file-name>
```
To print how many times (count) given keyword present in file
```
 grep -c <keyword> <file-name>
```
Counts the number of lines in <file-name> that contain <keyword> and prints the count.
To search for exact match of given keyword in a file
```
 grep -w <keyword> <file-name>
```
To print the line number of matches of given keyword in a file
```
 grep -n <keyword> <file-name>
```

To search a given keyword in multiple files

 grep -n <keyword> <file-name> <file-name>

To suppress file names while search a given keyword in multiple files
```
 grep -h <keyword> <file-name> <file-name>
```
To search multiple keywords in a file

grep -e <keyword1>  -e <keyword2> <file-name>

To search multiple keywords in multiple file

 grep -e <keyword1>  -e <keyword2> <file-name>

To only print file names which matches given keywords
```
grep -l <keyword> <file-name1> <file-name>
```
To get the keywords/pattern from a file and match with a another file

-f option allows you to specify a file containing patterns to search for.

If you want to search a lot of keywords then rather than typing all the keywords on terminal , just create a file with those keywords and use the option -f

Syntax

grep -f pattern_file target_file

To print the matching line which start with given keyword

grep "^keyword" file

“^” → caret

To print the matching line which end with given keyword

grep "keyword$" file

Suppose we have 100 files in a directory and we need to search a keyword in all the files

grep -R -f <file-name> <directory-name>/

Can also search a word

We can use egrep command for the multiple keywords search

egrep "key1|key2|key3" <file-name>

If you just wanna search but don't want to print on terminal
```
grep -q "keyword" <file-name>
```

echo $?

This command is used to display the exit status of the previously executed command or script.

0 - successful

Non zero - indicate different types of errors or problems encountered during the execution.

If you want to suppress error messages.

ls | grep filename

2.awk

It is particularly useful for processing structured text data, such as tabular data log files, CSV, or space-separated files**.**
It is designed to process text line by line and allows you to perform various tasks.
Syntax
```
  awk 'pattern {action}' file
```
pattern → Defines a condition (optional).
action → Specifies what to do when the pattern matches.
file → Input file to process.The file that is to be used for this example

Sample file: data

Commands

Print Entire File
```
 awk '{print}' <file-name>
```
Print a Specific Column
```
 awk '{print $1}' <file-name>
```

To print multiple columns

 awk '{print $1, $3}' <file-name>

Use Case:

 ls -l | awk '{print $NF}'

Filter Lines Matching a Pattern
```
 awk '/pattern/ {print}' <file-name>
```

Use Conditions (if statements)

 awk '{if($4 >= 600000) print $0}' <file-name>

Count Lines in a File
```
 awk 'END {print NR}' <file-name>
```
NR (Number of Records) → A built-in awk variable that keeps track of the current line number..

END Ensures the action is performed after reading all lines in the file
To get last column
```
 awk '{print $NF}' <file-name>
```
NF is a built-in awk variable that stands for Number of Fields in a line. It represents the total number of columns (fields) in the current line.

$NF → Refers to the last field of each line.
Using delimiter
```
  awk '{print $2 ":" $NF}' <file-name>
```

Sum a Column

 awk '{sum += $4} END {print "Total salary:", sum}' <file-name>

To get a specific row
```
awk 'NR==3 {print}' <file-name>
```
Range of lines
```
awk 'NR==4, NR==5 {print}' <file-name>
```
Print the line numbers of empty lines.
```
awk 'NF==0 {print NR}' file_name
```
NF==0 → Checks if the line has zero fields (i.e., an empty line).
Updating a file

The changes made by the awk command will not be permanent in the file. The awk command reads the file and makes modifications in memory but does not alter the original file.
-F option

The -F option in the awk command is used to specify the field separator (delimiter) in the input data. It tells awk how to split each line of input into fields.
```
awk -F, '{print $2 " " $5}' <file-name>
```
-F, means you're specifying a comma (,) as the field separator. This is often used when working with CSV (Comma-Separated Values) files, where each field is separated by a comma

How to use for loop in AWK command?

awk 'BEGIN {for(i=0;i<=10;i++) print i;}'

How to use while loop in AWK command?

awk 'BEGIN {while(i<10){ i++; print "Num is " i;}}'

SED command

The sed (Stream Editor) command is used for searching, replacing, deleting, and modifying text in a file or input stream.

To print a specific line
```
 sed -n '3p' file-name
```
To print the last line
```
 sed -n '$p' file-name
```
'$p': Prints only the last ($) line of the file.
To print a range of lines.
```
  sed -n '1,4p' file-name
```
To print the lines that contain a specific word or pattern
```
 sed -n '/pattern/p' file-name
```
To print only the specified lines.
```
 sed -n -e '3p' -e  '5p' file-name
```
To print only the lines that contain specified patterns
```
 sed -n -e'/pilot/p' -e'/BTech/p' file-name
```
To print a specified line and the next few lines from the given file.
```
 sed -n '3,+3p' file-name
```
To print every alternate line starting from the given line.
```
 sed -n '3~2p' file-name
```
To read expression from external file.

ext.txt
```
 sed -n -f ext.txt file-name
```

To replace a word in a file and print

sed 's/old-string/new-string/g' file-name

To replace all occurrences of <string_to_change> with <new_string> only on the specified line of file_name, leaving other lines unchanged.
```
sed '3s/BTech/MTech/g' file-name
```
```
sed '7! s/BTech/MBA/g' data
```

To delete a line

sed '1d' file-name

Deleting a range of lines

sed '1,3d' file-name

Text Processing Tools