rh135-day11.pdf

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RH135-Day11

Written by Razib Shahriar Rubence

Regular expressions and grep:

A regular expression is a pattern that describes a set of strings. Regular Expressions is nothing

but a pattern to match for each input line. A pattern is a sequence of characters.

The grep command searches the given files for lines containing a match to a given pattern list.

In other words, use the grep command to search words or strings in a text files. When it finds a

match in a file, it will display those line on screen. the command grep supports extended regular

expression

Examples in Practice LAB:

- Search for 'rhce' in /etc/passswd

# grep -color 'rhce' /etc/passwd

Sample outputs:

rhce:x:1000:1000:rhce Gite,,,:/home/rhce:/bin/bash

rhcegite:x:1001:1001::/home/rhcegite:/bin/sh

giterhce:x:1002:1002::/home/giterhce:/bin/sh

- Search rhce in any case (i.e. case insensitive search)

# grep -i -w rhce /etc/passwd

- Search rhce or datapark in any case

# grep -E -i -w 'rhce|datapark' /etc/passwd

The PATTERN in last example, used as an extended regular expression.

-You can use ^ and $ to force a regex to match only at the start or end of a line, respectively.

The following example displays lines starting with the rhce only:

grep '^rhce' /etc/passwd

Sample outputs:

rhce:x:1000:1000:rhce Gite,,,:/home/rhce:/bin/bash

rhcegite:x:1001:1001::/home/rhcegite:/bin/sh

- You can display only lines starting with the word rhce only i.e. do not display rhcegite, rhceg

etc:

grep -w '^rhce' /etc/passwd

- Find lines ending with word rhce:

grep 'rhce$' filename

- Match line only containing rhce:

grep '^rhce$' filename

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RH135-Day11

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-You can search for blank lines with the following examples:

grep '^$' filename

-You can search for users who has Shell

grep ':/bin/bash$' /etc/passwd

-Match rhce or Rhce:

grep '[rR]hce' filename

-You can also match digits (i.e match rhce1 or rhce2 etc):

grep -w '[rR]hce[0-9]' filename

-You can match two numeric digits (i.e. match rhce11, rhce12 etc):

grep 'rhce[0-9][0-9]' filename

-Display all the lines containing either a "w" or "n" character:

grep [wn] filename

- How Do I do OR with grep?

grep 'word1|word2' filename

How Do I do AND with grep?

grep 'word1' filename | grep 'word2'

The following will match both "col" and "cool":

egrep 'co{1,2}l' filename

How Do I Show Only The Matches, Not The Lines?

grep -o regex filename

Find Command:

Apart from the basic operation of looking for files under a directory structure, you can also

perform several practical operations using find command that will make your command line

journey easy.

01. Find files using name

This is a basic usage of the find command. This example finds all files with name —

MyCProgram.c in the current directory and all it’s sub-directories.

# find -name "MyCProgram.c"

02. Other options with find commnand

find /home -user datapark

Find every file under the directory /home owned by the user datapark.

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RH135-Day11

Written by Razib Shahriar Rubence

find /usr -name *park

Find every file under the directory /usr ending in "park".

find /usr -name data*

Find every file under the directory /usr starting with "data".

find /var/spool -mtime +60

Find every file under the directory /var/spool that was modified more than 60 days ago.

Pipelines and Redirection

- ">" is used to redirect Standard Output to a file which will overrite all text (if any) in that file.

grep "root" /etc/passwd > /tmp/newfile

- ">>" is used to redirect Standard Output to a file which will add new lines with this output in

that file

grep "data" /etc/passwd >> /tmp/newfile

- "2>" is used to redirect Standard Error to a file

find /etc -name passwd 2> /tmp/errorfile

find /etc -name passwd > /tmp/outputfile 2> /tmp/errorfile

- "2> /dev/null" is used to discard the error messages by redirecting to /dev/null

find /etc -name passwd > /tmp/outputfile 2>/dev/null

- "2>&1" is used to combine OUTPUT and and ERROR

find /etc -name passwd /tmp/all 2>&1

- | (pipe) is used to send OUTPUT from one command as INPUT of another command

cat /etc/passwd | grep "root"

First command shows all local users information from /etc/passwd the second command (grep

"root") took this information and finaly dispalys only root user.

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