Linux

Introduction

This article mainly records the problems & solutions I’ve met when operating Linux systems

Common Commands

1. For a specific instruction please refer to here

Command	Description
*File Commands:*
ls	Directory listing
ls -al	Formatted listing with hidden files
ls -lt	Sort the Formatted listing by time modification
cd	Change to home directory
pwd	Show current working directory
mkdir dir	Create a directory dir
cat >file	Places the standard input into the file
more file	Output the contents of the file
tail -f file	Output the contents of file as it grows, starting with the last 10 lines
touch file	Create or update file
rm -r dir	Delete the directory recursively
rm -f file	Force to remove the file
rm -rf dir	Force to remove the directory dir
cp file1 file2	Copy the contents of file1 to file2
cp -r dir1 dir2	Copy dir1 to dir2; create dir2 if not present
mv file1 file2	Rename or move file1 to file2, if file2 is an existing directory
*Process management:*
ps	To display the currently working processes
top	Display all running process
kill pid	Kill the process with given pid
killall proc	Kill all the process named proc
bg	List stopped or background jobs, resume a stopped job in the background
fg n	Brings job n to the foreground
*Searching:*
grep pattern file	Search for pattern in file
grep -r pattern dir	Search recursively for pattern in dir
command \| grep pattern	Search pattern in the output of another command
locate file	Find all instances of file
find . -name filename	Searches in the directory (represented by “.”) and below it, for files and directories with names starting with filename
pgrep pattern	Searches for all the named processes, that matches with the pattern and, by default, returns their ID
*System Info:*
w	Display who is on line
whoami	Who you are logged in as
finger user	Display information about user
uname -a	Show kernel information
cat /proc/cpuinfo	CPU information
cat /proc/meminfo	Memory information
free	Show memory and swap usage
whereis app	Show possible locations of app
which app	Show which applications will be run by default
*Compression:*
tar cf file.tar file	Create tar named file.tar containing file
tar xf file.tar	Extract the files from file.tar
tar czf file.tar.gz files	Create a tar with Gzip compression
tar xzf file.tar.gz	Extract a tar using Gzip
gzip file	Compresses file and renames it to file.gz
gzip -d file.gz	Decompresses file.gz back to file
*Network:*
ping	Ping host and output results
whois domain	Get whois information for domains
dig domain	Get DNS information for domain
wget file	Download file
wget -c file	Continue a stopped download
*Shortcuts:*
ctrl+z	Stops the current command, resume with `fg` in the foreground or `bg` in the background
ctrl+r	Type to bring up a recent command
*Shutdown:*
shutdown -h	Shut down immediately after services are stopped
shutdown -r	Restart after services are stopped

Vim

Vim is a popular and build-in text editor of Linux systems. In most of the cases I use it to modify configuration files of networking or apps

1. Three modes

Command mode: Default mode, can be used to move the cursor to view content
Insert mode: Press the “i” button to enter, and edit the text
Bottom-line mode: Press the “:” button to enter, and to save and exit

2. Commands in the bottom-line mode

Command	Description
:w	Write to disk
:w!	Force writing to disk when the file is read-only
:q	Leave
:q!	Force leaving without saving
:wq	Leave after writing to disk
:wq!	Leave after forcing writing to disk

Network Configuration

In case we have to construct a network includes Linux systems, usually we have to config static IP addresses for them. Here I use “Ubuntu 18.04” as an example (configurations on “CentOS” and “Fedora” are similar)

1. System UI

2. CommandLine

Use ifconfig to check the name of targeted network card

Use sudo vi /etc/network/interfaces or sudo gedit /etc/network/interfaces to open the configuration file

Insert your modification based on the following template, and save the changes

auto ens33
iface ens33 inet static
address 192.168.4.200
netmask 255.255.255.0
gateway 192.168.4.1

Use one of the following commands to restart the network or simply restart the system to activate the new configuration:

/etc/init.d/networking restart
sudo service network-manager restart
sudo service networking restart

Change Permission

1. Problem

I used to install the SysmonForLinux on my ubuntu18, and the system logs that Sysmon generates are stored in the “/var/log/syslog”. Later I wanted the transmission tool Nxlog to transmit those logs to my ELK mainframe. However, the permission of “syslog” is read-only, Nxlog cannot retrieve any data from it

2. Rationale

Insufficient privilege

3. Solution1

A common thought to address the problem aforementioned is to change the permission of the targeted file using sudo chmod 777 /var/log/syslog . It does work at first, nevertheless, I find that the permission of “syslog” will turn back to read-only each time I restart the system

4. Solution2

Another way to solve the problem is to grant the application a higher permission to access the target. Using sudo gedit /etc/passwd we can not only grant apps but also users. To do this, simply change the corresponding number after the “x:” to “0”

In the highlighted part of the pic above, I’ve granted Nxlog the highest permission as the changed number is actually the UID, and the UID of root user is “0”

Remove File Lock

1. Problem

The error below often occurs when installing applications using the apt command on Ubuntu:

2. Rationale

File locks are used to prevent two or more processes from using the same data. When apt or apt-get commands are running, they create lock files in a few places. If the previous apt command was not terminated properly, the lock files are not deleted and hence they prevent any new instances of apt-get or apt commands

3. Solution

Use ps -e | grep apt to find out related processes
Use sudo kill PID to terminate processes if returned
Use the following commands to safely remove the locks:

sudo rm /var/lib/apt/lists/lock
sudo rm /var/cache/apt/archives/lock
sudo rm /var/lib/dpkg/lock