The difference of .bash_profile, .bashrc

Brief

Detail about .bash_profile and .bashrc on CentOS


The differences of .bashrc and .bash_profile

When login to Linux, both $HOME/.bash_profile and $HOME/.bashrc are executed.

When a shell is run, an only .bahsrc is executed.

So if you want to run only when login, write the process to $HOME/.bash_profile
If you want to run every time bash is run, write the process to $HOME/.bashrc

How to refurbish tmux

Brief
Surmise how to refurbish tmux

Requirements
OS: CentOS, Ubuntu

How to refurbish

Create $HOME/.tmux.conf

The blow is a sample of .tmux.conf
https://gist.github.com/cocoa-maemae/5e00a4d696d69e7b410d133714771d5d


Debug

If $HOME/.tmux.conf is not reflected, kill a tmux process then run tmux again.

e.g.

# Look into a tmux process

ps aux | grep tmux

25718 ? Ss 1:37 tmux
18937 pts/8 S+ 0:00 grep tmux

# kill a tmux process

kill -9 25718

How to use tmux

What is tmux?
Multiple virtual console tools

How to install

(CentOS)
sudo yum install -y tmux
Basic Usage
# Create windows
Ctrl-b,c

# Move to a next window
Ctrl-b,n

# Move to a previous window
Ctrl-b,p

# Divide a pane
Ctrl-b,%

# Move a pane
Ctrl-b,o

How to add an original yum repository

Add **.repo file under /etc/yum.repos.d/

e.g.
[new-repo] //a repository name
name=nae repository RPM packages // a repository explanation
baseurl=http://new.repository/repo/ // a repository url
gpgcheck=0 // Abbreviate a signature validation
enabled=1 // Abbreviate --enablerepo=***

notice:
Before yum install, "createrepo" execution is on a yum repository server side,

How to use gcc

Basic syntax

gcc <options> <file name>

options

# Enables all compiler's warning messages
-Wall

# Just compile file. not execute linkage
-c

# Just create execute file, link compiled files.
-o <an execute file> <a compiled file> <a compiled file>

# Adds include directory of header files
-I <a directory name>

# Looks in directory for library files
-L <a directory name>

# Links with a library file(.so), however, library name does not include lib.
-l <a library name>

# Display an include target directory
-v <a compile target file>

How to use yum

Basic syntax

yum <options> <package names>

options

# install specified packages
install

# uninstall specified packages
remove

# Search packages which are enable to install

search

# search packages and show which packages are root
whatprovides

# Respond as yes against questions
-y

# Specify a yum configuration file
-c <file path>

e.g.

# Install packages
yum install <options> <package names>

# Uninstall packages
yum remove <package names>

# Downgrade packages
yum downgrade <package name>

# Display packages installed
yum list installed

# Clear cache

yum clear all

# Install vim and emacs
sudo yum install -y vim emacs

# Uninstall vim and emacs
sudo yum remove vim emacs

How to use supervisor

Brief
Surmise about supervisor

What is Supervisor?
make any modules a daemon

Environment 
CentOS

How to install
# add epel repository
sudo yum -y install epel-release

# install
sudo yum -y install supervisor

# start supervisor

sudo systemctl start supervisord

How to configure
# make a module a daemon
sudo supervisorctl start <module name>

Create a configuration file under /etc/supervisor.d/

e.g.
/etc/supervisor.d/xxx.ini

# xxx is a module name which is registered
cat /etc/supervisor.d/xxx.ini

[program:xxx]
command=xxx
user=***
autostart=false
autorestart=true
log_stdout=true
log_stderr=true
redirect_stderr=true
stdout_logfile_maxbytes=**MB
stdout_logfile_backups=**
stdout_logfile=***

How to use rpm

Basic syntax

rpm <options> <package name>

options

Moved to Gist.
https://gist.github.com/cocoa-maemae/571240f0ea22abe40e203658dc7c28d8

How to use du

What's du?
Reports the size of directory trees

Basic syntax

du <options> <file or directory name>

options

# Show capacity
-h

# Show amount of files or directories

-s

# Specify tree depth
--max-depth

e.g.

# Show capacities from root path tree
du -sh /

How to clear cache on CentOS

clear page cache
sudo echo 1 > /proc/sys/vm/drop_caches
or
sudo sysctl -w vm.drop_caches=1

clear directory entry and inode
sudo echo 2 > /proc/sys/vm/drop_caches
or
sudo sysctl -w vm.drop_caches=2

clear page cache and directory entry and inode
sudo echo 3 > /proc/sys/vm/drop_caches
or
sudo sysctl -w vm.drop_caches=3

Linux short cut command cheat sheet

# Go back to the line head
Ctrl+a


# Go to the line end
Ctrl+e

# Remove till end-of-line
Ctrl+k


# Remove characters toward line head by each words
Ctrl+w


# Reset a terminal
Ctrl+l

User management on CentOS

Brief
Surmise about user management on CentOS

See the below regarding basic commands
https://gist.github.com/cocoa-maemae/fd29e2e91d0a75226b09b89e2a367231

# How to add user groups
On centos, add a file under /etc/sudoers.d

e.g.
sudo cat /etc/sudoers.d/hoge
%hoge ALL=(ALL) NOPASSWD: ALL

In this example, the left hoge is a group name.

FYI
If sudo visudo is executed, the following can be seen.
## Read drop-in files from /etc/sudoers.d (the # here does not mean a comment)
#includedir /etc/sudoers.d

includedir is commented out, but this is an unique spec, files under /etc/sudoers.d are loaded automatically.

How to use ssh-keygen

Brief
Surmise about ssh-keygen

What is ssh-keygen?
create ssh key pairs, a secret key and a public key

Environment 
CentOS

Basic Syntax
ssh-keygen <options>

e.g.
# create a secret key and a public key
# The keys are created in the folder of /root/.ssh/ at the default
ssh-keygen -t 'algorithm name' -m 'comment'

# create a public key from a secret key
ssh-keygen -yf ${private_key_file} > ${public_key_file}

Linux file permission

Brief
Surmise about Linux file permission

If you run 'ls -alt', and then the file permission is displayed.
ls -alt
drwxr-xr-x 2 root root 6 Jul 24 14:03 tmp

The status from left means
# File type. d means a direcory.
d

# a file mode of a file owner
rwx

# a file mode of a file group
r-x

# a file mode of another group users
r-x

# the number of link to files
2

# a owner name of a file or a directory
root

# a group name of a file or a directory
root

# used bytes
6

# last updated date
Jul 24 14:03

# a file name or a directory name
tmp

The meanings of r and w and x is like the below.
r: enable to refer
w: enable to change
x: enable to execute

r and w and x are allocated numbers.
r=4
w=2
x=1

How to do ssh login by root on Linux

Brief

It can't be done ssh login by root on Linux default configuration.
The below is the solution.

Environment

CentOS[6-7]

Procedure

1. Put id_rsa under /root/.ssh/ on a client side and authorized_key under /root/.ssh/ on a server side

2. Comment the below portion on /etc/ssh/sshd_config
#PermitRootLogin no

3. Add the below on /etc/ssh/sshd_config
PermitRootLogin without-password

4. Restart sshd
(CentOS7)
sudo systemctl restart sshd

(CentOS6)
sudo service restart sshd

How to create or remove symbolic link

What's symbolic link?
Symbolic link is a special kind of link that points to another file.
After created, a link target file is replaced as source file.

How to create symbolic link

ln -s <source file name> <link target file name>

e.g.
$ ln -s /usr/local/bin/ruby /usr/bin/ruby

$ ls -alt /usr/bin/ruby
/usr/bin/ruby -> /usr/local/bin/ruby

How to remove symbolic link

use unlink or rm

e.g.
unlink /usr/bin/ruby
rm /usr/bin/ruby

Linux ssh management

Brief
Surmise ssh management on Linux

How to create a ssh key pair
execute ssh-keygen command and then $HOME/.ssh directory and $HOME/.ssh/id_rsa and $HOME/.ssh/id_rsa.pub are created automatically by default.

# .ssh folder permission
chmod 700 $HOME/.ssh
chmod 600 $HOME/.ssh/id_rsa


How to add a public key on a ssh server
# add a public key
cat <public key file> >> ~/.ssh/authorized_keys

# change permission
chmod 600 ~/.ssh/authorized_keys

# If from is used, the access origin can be limited
e.g.
from="xxxx.xxxx.xxxx.xxx",no-pty ssh-rsa ***

How to configure ssh authentication
// Modify /etc/ssh/sshd_config and comment out the blow portions.
#RSAAuthentication yes
#PubkeyAuthentication yes
#AuthorizedKeysFils .ssh/authorized_keys

// To make a password authentication off
PasswordAuthentication no

After modified, restart sshd