The LEGENDARY grep command

the man page starts as follows...
 
SYNOPSIS
       grep [OPTIONS] PATTERN [FILE...]
       grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

DESCRIPTION
       grep  searches  the named input FILEs (or standard input if no files are named, or if a single hyphen-minus (-) is given as file name) for lines containing a match to the given PATTERN.  By default, grep prints the matching lines.

       In addition, two variant programs egrep and fgrep are available.  egrep is the same as  grep -E.   fgrep  is  the  same  as grep -F.   Direct  invocation as either egrep or fgrep is deprecated, but is provided to allow historical applications that rely on them to run unmodified.
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The following are some practical uses of grep from my own experience  and some sourced from the net.

1. Search file contents

Search for the word PAGE (case sensitive) in the file application.xml;
$grep "PAGE" /opt/tomcat7/webapps.labs/primetest/WEB-INF/configs/application.xml

For case insensitive match use the -i switch as follows;

$grep -i "PAGE" /opt/tomcat7/webapps.labs/primetest/WEB-INF/configs/application.xml

2. Search inside a directory

Search for the word drvierClassName inside the directory META-INF

$ grep -R "driverClassName" META-INF/

You will see result on a separate line preceded by the name of the file in which it was found.

3. Find whole words only

When you search for class, grep will match class, className etc.

$ grep -w "class" WEB-INF/context.xml

4. Multiple word search

Use the egrep command as follows:
$ egrep -w 'word1|word2' /path/to/file

5. Count occurences

The grep can report the number of times that the pattern has been matched for each file using -c (count) switch:

$ grep -R -c "driverClassName" META-INF/

6. Show line numbers

Pass the -n switch to precede each line of output with the number of the line in the text file from which it was obtained:

$ grep -R -n "driverClassName" META-INF/

7. Names of matching files only

Use the -l (list) switch as follows;

$ grep -R -l "driverClassName" META-INF/


8. Grep invert match

You can use -v option to print inverts the match; that is, it matches only those lines that do not contain the given word. For example print all line that do not contain the word bar:

$ grep -v bar /path/to/file

NB: for coloured output (if not on by default) you can add --color=auto to your grep command

Other options:

• -A x or -B x (where x is a number) --- display “x” lines After or Before the section where the particular word is found.

Periodicaly restart dead process in Linux

The following is (one way of ) how to automaticaly check for and restart a dead process running in a linux host

Part A

/bin/netstat -ln | /bin/grep ":80 " | /usr/bin/wc -l | /bin/awk '{if ($1 == 0) system("/sbin/service httpd restart") }'

This checks if there is any service running on port 80, if not it runs the command to start the service (apache httpd in this example)

Part B
Create a cron job to periodicaly do the above

#monitors the service every 10 minutes
*/10 * * * * /usr/sesame/bash/service_monitor.sh> /dev/null 2>&1

NB: please note the trailing space after the port number


Tested in Redhat|Fedora|CentOS