Know Your Linux Box

Name,OS, Kernel , RAM, CPU, Block & loop devices, users, files-permissions...

hostname #returns name of current box

hostname -b <NEW_NAME> #Sets name to box. also done by editing /etc/hosts and /etc/hostname.

#indicates current os architecture is 32/64 bit

echo $(getconf LONG_BIT) #OR,

uname -m

uname -a #Detailed info about box.

df -h #Displays all mounted partitions and their size/usage/availability/mount-point...

du -sh <DIRECTORY/FILE> #displays size(disk usage) sumed-up and in human readable format.

free -m #shows RAM Usage and availability in MBs.

cat /proc/meminfo #Alternate : lsmem,Displays all info about RAM.

cat /proc/cpuinfo #Alternate : lscpu,Displays all info about CPU.

cat /proc/modules #Alternate : lsmod,Displays all info about loaded kernel modules.

cat /etc/shells #Displays all available shells (ex- sh, bash... ).

bash -version #Displays bash version.

blkid #Alternate : lsblk, Displays all block devices(hardrive-partitions, mounted-cd/dvd).

# List all process, and resource consumed by each

top # Then hit following as required.

c # shows complete path of each path.

F # selecting sort field.

1 # show all cpu cores.

top -p <PID> #displays info about specified processid only.

netstat -tulnp #Displays all open TCP/UDP ports. Use grep to find if port is in use or not.

sync; echo 3 > /proc/sys/vm/drop_caches #clear cache.

#File permissions:

#File Permissons : In linux, access to files are granted based on ownership. whether a user is owner, or in same group as owner or other. Each file holds access-levels for all three type of users.

Access levels- (Read[4], Write[2] & Execute[1]) 4+2+1 = 7

Ownership - (Owner, Group user & Others)

chmod 777 <FILE_NAME> #grants read write & execute permission to everyone.

#View/list files and folders:

*1st character in result of 'ls' indicates type of file.(- is regular file, d is directory, l is link file)

*2-10 characters are permissions in set of 3 for 3 users.(r-read, w-write, x-execute) for File-Owner , User-in-owner-group , other-users .

ls #lists name of current path content(s).

ls -l #lists content(s) of current path with details.

ll #lists content(s) of current path, same as ls -l.

ls -ltr #lists content(s) sorted on modified time in reverse order.

ls -h #lists content(s) in human readable format(e.g-size in kb).

ls -a #list All(including hidden) files/folders in current path.

#User Management:

cut -d: -f1 /etc/passwd #lists all available users.

less /etc/group #lists all available groups.

useradd <USER> #Creates new user. see useradd -h for group assignment & others

passwd [<USER>] #resets self/specified user password.

usermod -aG <GROUP> <USER> #Adds user to group.

visudo #allows to configure sudo rights for users

#ulimit defines various limits. /etc/security/limits.conf

ulimit -n 65536 #sets max number of open files to 65536.

ulimit -u 2048 #sets number of threads.

sysctl -w vm.max_map_count=262144 # virtual memory limits. /etc/sysctl.conf

#enable/disable swap memory

#1. Create file with required swap size.

dd if=/dev/zero of=/application/swapfile1 bs=1024 count=10485760

#2. convert file to swap partition

mkswap /application/swapfile1

#3. change ownership of swapfile

chown root:root /application/swapfile1

chmod 0600 /application/swapfile1

#4. change swap target.

swapon /application/swapfile1

#5. setup fstab to mount automatically this newly created swap partition

vi /etc/fstab #open file for editing, and append following.

/application/swapfile1 swap swap defaults 0 0

#For Terminals:

Enter>~>. #releases stuck terminals due to broken connections.

Ctrl + F #Search in terminal logs..

Shift + PgUP/Dn #scrolls terminal output (even in virtual box with console only installations)

#Visudo

# User privilege specification

root    ALL=(ALL:ALL) ALL

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