Few useful commands in Linux

There are a lot of commands which I use regularly. Some of them are very frequently used. The problem is rarely I remember all the required syntax and probable options for many of them. There is a very popular comic on xkcd on the same. tarSometimes I check man and sometimes I google them. So to help myself I’m accumulating some of them here.
Not adding up all of the explanations here for those options. They are known and self-explanatory mostly. The main purpose is to keep them handy. This list is infinite and the counter will increase regularly.

Find and replace text within file(s)

$ find . -name "*.ext" -type f | xargs sed -i 's/a/b/g'

tar

$ tar -czvf crawler.tar.gz crawler
$ tar -C /home/kuntal/Desktop/ -xzvf crawler.tar.gz

xargs

Won’t repeat here. Have already noted this down in detail here.

kill

Regular kill works with pid but process can be killed using regex too
$ pkill -f "string"

Check port status

$ netstat -tln | grep 27017 or find by a process
$ netstat -plunt | grep java

How data packets are coming through

$ mtr google.com

Faketime

Very useful to test when some process needs to be executed manually at a specific time
$ faketime 'last Friday 5 pm' /bin/date
$ faketime "Fri Nov 11 02:00:00 UTC 2016" [command]

How long a process is running

$ ps -p "$$" -o etime=

Observe logging

$ tail -f /var/log/somelogfile.log | grep -C4 "Process completed"

Print value of a symbolic link or canonical file name

$ readlink -f $(which java)

Strip header and merge files

$ sed 1d file2.csv > file2noheader.csv
$ cat file1.csv file2noheader.csv > outputfile.csv

Read file to find out specific word appearance and print the newline counts

$ cat nohup.out | grep "Fetching xyz data for abc" | wc -l

Grep pattern to find out count

$ cat nohup.out | grep -P '(Some specific string:\s[0-9])' | wc -l

CPU details

$ lscpu

Wireless connection details

$ iwconfig

Set SSH client alive

$ vim ~/.ssh/config
ServerAliveInterval 30

Refresh shell

$ source ~/.bashrc

Find application location

$ whereis python
python: /usr/bin/python3.5m /usr/bin/python /usr/bin/python3.5 /usr/bin/python2.7 /usr/lib/python3.5 /usr/lib/python2.7 /etc/python /etc/python3.5 /etc/python2.7 /usr/local/lib/python3.5 /usr/local/lib/python2.7 /usr/include/python3.5m /usr/share/python /usr/share/man/man1/python.1.gz

Message output color

$ RED='\033[0;31m'
$ NC='\033[0m'
$ printf "${RED}This is in red.${NC} This is in no color."

Append stderr and stdout to a file. More on redirection
command [$1] 2>&1 | tee -a $LOG

Get details of executed process PID


$ command & echo $!
$ command & export COMMAND_PID=$!
$ command & echo $! >> /tmp/command.pid
$ command > /tmp/command.log 2> /tmp/command.error.log & echo $! >> /tmp/command.pid
$ echo "command PID: $(cat /tmp/command.pid)"

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