Useful Unix Commands and Tips - Part 2

This is in continuation to the first part in the series. Let us discuss grep and lsof in part 2:

grep:

1) Case insensitive search:

grep -i "string" FILE  

2) Checking for full words, not for sub-strings using grep -w

grep -w "serachword" file

3) Displaying lines before/after/around the match using grep -A, -B and -C. This is very useful if you want to see lines around search string:

i) grep -A <N> "string" FILENAME # display N lines after match:
ii) grep -B <N> "string" FILENAME # display N lines before match
iii) grep -C 2 "Example" demo_text # display N lines around match

4) Searching in all files recursively using grep -r

grep -r "ramesh" *

5) Invert match using grep -v

grep -v -e "pattern" -e "pattern"

6) Counting the number of matches using grep -c. There is no need to pipe the output to wc.

grep -c "pattern" filename
grep -v -c this demo_file (Count of lines not matching pattern)

7) Display only the file names which matches the given pattern using grep -l

grep -l <string> *

8) Show line number while displaying the output using grep -n

grep -n "go" demo_text

9) grep doesn’t support multiple string search. We need to use egrep.

egrep  'warning|error|critical' /var/log/messages

lsof:

1) To see all files opened by a process:

lsof -p <pid>

2) To list all tcp connections:

lsof -a -i4 -i6 -itcp

3) To find connections to a specific host:

lsof -i@hostname

4) Final all network connections when port on one side is known:

lsof -i :portNumber # Display all connections where one of the ports is known
lsof -i tcp:portNumber # Display all TCP connections where one of the port is portNumber
lsof -i udp:portNumber # Display all UDP connections where one of the port is portNumber

5) To find established connections:

lsof -i -sTCP:ESTABLISHED

6) Files opened by a user:

lsof -u ramesh

7) Files opened by users other than a particular user:

lsof -u ^ramesh

8) lsof can be used to find all resources interacting with it. Like users, processes, etc.

lsof /u01/data