L in u X ba sics for h acke rs g e t t I n g s t a r t e d w I t h

22
Chapter 2
Numbering the Lines
Sometimes—especially with very long files—we may want the file to display 
line numbers. Since snort.conf has more than 600 lines, line numbers would 
be useful here. This makes it easier to reference changes and come back to 
the same place within the file.
To display a file with line numbers, we use the 
nl
(number lines) com-
mand. Simply enter the command shown in Listing 2-4.
kali >nl /etc/snort/snort.conf
612 #################################################################
613 #dynamic library rules
614 #include $SO_RULE_PATH/bad-traffic.rules
615 #include $SO_RULE_PATH/chat.rules
--snip--
630 #include $SO_RULE_PATH/web-iis.rules
631 #include $SO_RULE_PATH/web-misc.rules
632 #Event thresholding or suppression commands. See threshold.conf
633 include threshold.conf
Listing 2-4: Displaying line numbers in terminal output
Each line now has a number, making referencing much easier. Note that 
this command skips the numbering for the blank lines.
Filtering Text with grep
The command 
grep
is probably the most widely used text manipulation com-
mand. It lets you filter the content of a file for display. If, for instance, you 
want to see all lines that include the word output in your snort.conf file, you 
could use 
cat
and ask it to display only those lines (see Listing 2-5).
kali >cat /etc/snort/snort.conf | grep output
# 6) Configure output plugins
# Step #6: Configure output plugins
# output unified2: filename merged.log, limit 128, nostamp, mpls_event_types, 
vlan_event_types
output unified2: filename merged.log, limit 128, nostamp, mpls_event_types, 
vlan_event_types
# output alert_unified2: filename merged.log, limit 128, nostamp
# output log_unified2: filename merged.log, limit 128, nostamp
# output alert_syslog: LOG_AUTH LOG_ALERT
# output log_tcpdump: tcpdump.log
Listing 2-5: Displaying lines with instances of the keyword or phrase specified by grep
This command will first view snort.conf and then use a pipe (
|
) to send 
it to 
grep
, which will take the file as input, look for lines with occurrences of 
the word output, and display only those lines. The 
grep
command is a very 
powerful and essential command for working in Linux, because it can save 
you hours of searching for every occurrence of a word or command in a file.

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Text Manipulation