Linux Ask! is a Q & A web site specific for Linux related questions. Questions are collected, answered and audited by experienced Linux users.
How to sum a particular column of a CSV file with awk
Answer:
Assume you have a CSV file like the following (only contain number)
1,2,3 2,3,4 3,4,5 ...
How do you sum the nth column of this file? It is easy with the use of awk
e.g. Assume you want to sum the 2nd column:
# awk -F "," '{ sum += $2 } END { print sum }' test.txt
File content redirection without using cat
Answer:
It is not uncommon that people have abused the command `cat` for many purpose, e.g. file I/O redirection
cat data.txt | sed 's/foo/bar/s'You can avoid the cat command by the following commands:
data.txt < sed 's/foo/bar/s'
sed 's/foo/bar/s' < data.txtBoth directions are okay.
In fact, when you better understand the usage of some commands, you might find out that redirection is completely not needed.
e.g.
sed 's/foo/bar/s' data.txt
How to check how many established connection of my web server?
Answer:
Assume you server is listening on port 80, you can check how many established connection to this port with the following command:
netstat -an | grep :80 | grep ESTABLISHED | wc -l
4919
Rewriting last Git commit
Answer:
Sometimes, when you regret your last commit and before it is pushed to remote, it is easy with the following commands:
git add file1.txt file2.txt
git commit -m 'Add some files'
# Now you regret to include file2.txt
git rm file2.txt
git commit --amend # Edit the commit message in the editor
How to trace DNS from the root servers
Answer:
Use the command `dig`, e.g.
dig +trace mit.edu
;; global options: +cmd
. 19927 IN NS d.root-servers.net.
. 19927 IN NS e.root-servers.net.
. 19927 IN NS m.root-servers.net.
. 19927 IN NS l.root-servers.net.
. 19927 IN NS a.root-servers.net.
. 19927 IN NS k.root-servers.net.
. 19927 IN NS f.root-servers.net.
. 19927 IN NS j.root-servers.net.
. 19927 IN NS b.root-servers.net.
. 19927 IN NS h.root-servers.net.
. 19927 IN NS g.root-servers.net.
. 19927 IN NS i.root-servers.net.
. 19927 IN NS c.root-servers.net.
;; Received 228 bytes from 8.8.8.8#53(8.8.8.8) in 12 ms
edu. 172800 IN NS a.edu-servers.net.
edu. 172800 IN NS c.edu-servers.net.
edu. 172800 IN NS d.edu-servers.net.
edu. 172800 IN NS f.edu-servers.net.
edu. 172800 IN NS g.edu-servers.net.
edu. 172800 IN NS l.edu-servers.net.
;; Received 260 bytes from 199.7.83.42#53(199.7.83.42) in 223 ms
mit.edu. 172800 IN NS usw2.akam.net.
mit.edu. 172800 IN NS asia1.akam.net.
mit.edu. 172800 IN NS asia2.akam.net.
mit.edu. 172800 IN NS use2.akam.net.
mit.edu. 172800 IN NS ns1-37.akam.net.
mit.edu. 172800 IN NS ns1-173.akam.net.
mit.edu. 172800 IN NS eur5.akam.net.
mit.edu. 172800 IN NS use5.akam.net.
;; Received 404 bytes from 192.31.80.30#53(192.31.80.30) in 548 ms
mit.edu. 20 IN A 23.46.134.151
;; Received 41 bytes from 63.150.131.81#53(63.150.131.81) in 4 ms
