Linux disk partitions using file and loop device

1. Create a 1 GB file:

• dd if=/dev/zero of=imgfile bs=1M count=1024

2. Mount the file onto a loop device

• losetup -f  imgfile
• losetup –a (OR -l)

3. Create Partitions:

• fdisk /dev/loop0 (OR /dev/sda)
• Press ‘n’ for new
• Press ‘p’ for primary and ‘e’ for extended partition
• Enter partition size for example as +256M
• Press ‘p’ to print the partition table
• Press ‘w’ to save and exit

4. Reload and list partitions:

• partprobe /dev/loop0
• cat /proc/partitions
• ls -l /dev/loop0*
• fdisk -l /dev/loop0
• partprobe -s /dev/loop0

5. Format partitions:

• mkfs -t ext4 /dev/loop0

6. Mount partition:

• mkdir mntp1
• mount -o loop /dev/loop0p1 mntp1
• df -hT
• umount mntp1 (Unmount a partition)

sed and awk commands

1. sed:

• sed -i s/string1/string2/g file.txt (Replace all occurrences of string1 with string2 and save the file)
• sed s/pattern/replace_string/ file.txt (Replace first occurrence of string1 with string2)
• sed 1,3s/pattern/replace_string/g file.txt (Replace all occurrences of string1 with string2 from lines 1 to 3)
• sed -e ‘s/01/JAN/’ -e ‘s/02/FEB/’ -e ‘s/03/MAR/’ file.txt

2. awk:

• awk ‘{ print $0 }’ /etc/passwd      (Prints entire file)
• awk -F: ‘{ print $1 }’ /etc/passwd  (Prints first field of every line)
• awk -F: ‘{ print $1 $6 }’ /etc/passwd (Prints first and sixth field of every line)

Apache mod_rewrite

• Apache mod_rewrite Introduction:

https://httpd.apache.org/docs/current/rewrite/intro.html

• URL Rewriting Guide:

https://httpd.apache.org/docs/2.2/en/rewrite/

• Apache Rewrite cheatsheet:

http://borkweb.com/story/apache-rewrite-cheatsheet

chmod, chown, chgrp commands in Linux

• Octal -> Read(4), Wite (2), Execute (1)
• Symobolic -> o+rwx, g+rwx, u+rwx = o=owner, g=group, u=user/other

chmod:

• chmod ou+x,g-w file   (‘owner’ and ‘user’ -> execute and group -> remove write)
• chmod 755 file             (Owner -> rwx (7); Group-> rx; Others-> rw)

chown:

• chown user afile (Changes user owner)
• chown user:grp file (Changes user and group ownership togather)
• chown -r user:grp dir (Changes user and group on subdirectores)

chgrp:

• chown grp file (Changes group owner)

getfacl:

• getfacl file (View permission assigned on a file)
• getfacl dir (View permission assigned on a directory)

setfacl:

• setfacl -m u:user:rx /tmp/file

User and group management in Linux

User commands:

• useradd user1 (-d /var/user1 changes home directory)
• userdel -r user1 (removes user and its home/mail folders)
• usermod -a -G grp1, grp2 user1 (Adds users to grp1 and grp2)
• chage -l user1 (List user details)
• Files: /etc/passwd and /etc/shadow
• cut -f 1,6 -d “:” /etc/passwd (List all users and their home directory)

Group commands:

• groupadd grp1
• groupdel grp1
• groupmod
• groups user1 OR id -Gn (List groups of a user)
• /etc/group and /etc/gshadow

Ubuntu Advanced Packaging Tool commands

1. apt-cache:

-> Search the repository for a package

• apt-cache search apache2
• apt-cache -n kernel
• apt-cache pkgnames kernel

-> Display info of a package

• apt-cache show apache2
• apt-cache showpkg apache2

-> List dependent packages

• apt-cache depends apache2

-> Search package based on file

• apt-file search apache2.conf
• apt-file list apache2

2. apt-get
-> Update: Synchronize the package index files with the repository source:

• apt-get update

-> Apply all available updates to packages already installed:

• apt-get upgrade (-u switch will show upgraded packages)
• apt-get dist-upgrade (Remove obsolete packages)

-> Install

• apt-get install apache2

-> Uninstall or Remove

• apt-get remove apache2
• apt-get –purge remove apache2 (Removes config files also)
• apt-get autoremove (Remove package not needed anymore)

-> Clean cache

• apt-get clean

RPM commands

– Install a package

• rpm -ivh httpd-2.4.6-45.el7.centos.4.x86_64

– Uninstall or Remove a package

• rpm -e httpd

– Upgrade a package

• rpm -Uvh httpd-2.4.6-45.el7.centos.4.x86_64

– Query package

• rpm -qa | grep httpd*
• rpm -q httpd

– Search based on config file

• rpm -qf /etc/httpd/conf.d/welcome.conf

– List files installed by the package

• rpm -ql httpd
• rpm -qil httpd

– Show info a the package

• rpm -qi rpm

– Verify packages

• rpm -Va
• rpm -V httpd

– The standard naming format:

• <name>-<version>-<release>.<distro>.<architecture>.rpm
• sed-4.2.1-10.e16.x86_64.rpm

Yum commands

---

1. Install or update a particular package

• yum install httpd
• yum update httpd

2. Check and install updates

• yum check-update
• yum list update
• yum update

3. List available or installed packages

• yum list installed
• yum list available
• yum list installed “kernel*”
• yum list “kernel*”

3. Search a package

• yum search httpd

4. Get details of a package

• yum info httpd

5. List dependencies of a package

• yum deplist httpd

6. Group of packages

• yum grouplist
• yum groupinfo “GNOME Desktop”
• yum groupinstall “GNOME Desktop”
• yum groupremove “”GNOME Desktop”

7. Repository list

• yum repolist

7. Config files

• /etc/yum.conf
• /etc/yum.repos.d/

Script to check connectivity between single or multiple hosts – telnet

Alternate to telnet commands to check single host:

• echo > /dev/tcp/{host}/{port} && echo “SUCCESS” || echo “FAILED”
• timeout 1 bash -c ‘(If output 0 then Sucess else Failed)

Script to check connectivity between multiple hosts:

#!/bin/bash
SRC_HOSTS=(‘src1’ ‘src2’ ‘src3’ ‘src3’ ‘src4’)

DEST_HOSTS=(‘dest1’ ‘dest2’)
PORT=1521
TIMEOUT=1

for src in ${SRC_HOSTS[@]}; do
echo “—Testing connection from $src—”

for dest in ${DEST_HOSTS[@]}; do
ssh root@$src “timeout $TIMEOUT bash -c ‘ /dev/null 2>&1

if [[ $? -eq 0 ]]; then
echo “SUCCESS: $dest”
else
echo “FAILED: $dest”
fi
done
done
exit 0

Note: You could use IP address instead of hostnames in SRC_HOST and DEST_HOST variables.

No supported authentication methods available (Server sent: publickey)

First of all make sure you have setup ssh keys correctly by following the post:

https://websphereissues.wordpress.com/2014/11/25/add-ssh-keys-for-a-user-in-linux/

In /var/log/secure, if you noticed the error “Authentication refused bad ownership or modes for directory“, then you need to set “StrictModes no” in /etc/ssh/sshd_config file and run “service sshd” restart command to resolve this issue.