Regular Expressions or Regex (Grep):
Globally research a regular expression and print.
To search a string or regular expression in a file(s)
Syntax:
grep [options] PATTERN FILE(S)
grep [options] word to search filename
Examples:
$grep john filename
$grep "it technology" filename
$grep john filename1 filename2 filename3
Search all files in a current directory
$grep john *
Ignore case
$grep -i john filename
Counts number of lines.
$grep -c john filename
Print the lines along with line numbers
$grep -n john filename
List file names only the given pattern
$grep -l john filename
Search the pattern recursively
$grep -r john *
Prints non matching lines
$grep -v john filename
Prints only the given pattern
$grep -o john filename
Prints start with exam pattern
$grep "exam*" filename
$grep "b[a e i o u]ll" filename
Output:
ball
bell
bill
boll
$grep "b..d" filename
Output:
band
book
ba#h
ba-d
Note:
” . ” & ” * ” are wild card characters, it matches any single character.
Awk
Awk is used to search a word or particular string in a file. To Know more about AWK and how it get used see below examples.
The syntax of AWK:
To print the second line:
awk ‘FNR == 2 {print}’
To print the second field:
awk ‘{print $2}’
To print the third field of the fifth line:
awk ‘FNR == 5 {print $3}’
If you wish to list all the lines and columns in a file, execute
Copy$ awk ‘ {print $0}’ file.txt
To match all entries with the letter ‘e’
awk ‘/e/ {print $0}’ file.txt
To print the 2nd and 3rd columns, execute the command below.
$ awk ‘{print $2 “\t” $3}’ file.txt
$ awk ‘/variable_to_be_matched/ {print $0}’ file.txt
Use exit to stop as soon as you’ve printed the required output, there’s no use to process the whole file:
awk ‘FNR == 2 {print; exit}’