Tổng hợp các lệnh Linux

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Linux Command Query

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System Information

command code	Explanation of notes
arch	Show processor architecture of the machine (1)
uname -m	Processor architecture of the display machine (2)
uname -r	Display the kernel version in use
dmidecode -q	Display hardware system components - (SMBIOS / DMI)
hdparm -i /dev/hda	List the architectural features of a disk
hdparm -tT /dev/sda	Performing a test read operation on disk
cat /proc/cpuinfo	Display information about CPU info
cat /proc/interrupts	Display Interrupt
cat /proc/meminfo	Checking memory usage
cat /proc/swaps	Show which swaps are used
cat /proc/version	Show kernel version
cat /proc/net/dev	Display network adapters and statistics
cat /proc/mounts	Show loaded file systems
lspci -tv	Listing PCI Devices
lsusb -tv	Show USB Devices
date	Display system date
cal 2007	Display of the calendar for 2007
date 041217002007.00	Set date and time - month day hour minute year. Seconds.
clock -w	Saving time changes to BIOS

System shutdown/restart

command code	Explanation of notes
shutdown -h now	Shut down the system
init 0	Shut down the system
telinit 0	Shut down the system
shutdown -h hours:minutes &	Shut down the system at the scheduled time
shutdown -c	Cancellation of scheduled system shutdown
shutdown -r now	reopen
reboot	reopen
logout	deregister

Files and Directories

command code	Explanation of notes
cd /home	Go to the '/ home' directory'
cd ..	Return to previous level of catalog
cd ../..	Return to the previous two levels of the table of contents
cd	Go to your personal home directory
cd ~user1	Go to your personal home directory
cd -	Return to the last directory
pwd	Show work path
ls	Viewing files in a directory
ls -F	Viewing files in a directory
ls -l	Display file and directory details
ls -a	Show hidden files
ls [0-9]	Display file names and directory names that contain numbers
tree	Shows a tree structure of files and directories starting from the root directory
lstree	Shows a tree structure of files and directories starting from the root directory
mkdir dir1	Create a directory called 'dir1' '
mkdir dir1 dir2	Create two directories at the same time
mkdir -p /tmp/dir1/dir2	Creating a Catalog Tree
rm -f file1	Delete a file called 'file1''
rmdir dir1	Delete a directory called 'dir1''
rm -rf dir1	Delete a directory called 'dir1' and its contents.
rm -rf dir1 dir2	Delete two directories and their contents simultaneously
mv dir1 new_dir	Rename/Move a directory
cp file1 file2	Copy a file
cp dir/* .	Copy all files in a directory to the current working directory
cp -a /tmp/dir1 .	Copy a directory to the current working directory
cp -a dir1 dir2	Copy a directory
ln -s file1 lnk1	Create a soft link to a file or directory
ln file1 lnk1	Create a physical link to a file or directory
touch -t 0712250000 file1	Modify the timestamp of a file or directory - (YYMMDDhhmm)
iconv -l	List known codes
iconv -f fromEncoding -t toEncoding inputFile > outputFile	Changing the encoding of characters
find . -maxdepth 1 -name *.jpg -print -exec convert	Batch resize files in the current directory and send them to a thumbnail directory (requires conversion from ImageMagick)

Document search

command code	Explanation of notes
find / -name file1	Search for files and directories in the root filesystem starting with '/'
find / -user user1	Search files and directories belonging to user 'user1'.
find /home/user1 -name \*.bin	Search for files ending in '.bin' in the directory '/ home/user1'.
find /usr/bin -type f -atime +100	Search for executable files that have not been used in the last 100 days
find /usr/bin -type f -mtime -10	Search for files that have been created or modified within 10 days
find / -name \*.rpm -exec chmod 755 '{}' \;	Search for files ending in '.rpm' and define their permissions
find / -xdev -name \*.rpm	Search for files ending in '.rpm', ignoring removable devices such as CD-ROMs, JD disks, etc.
locate \*.ps	Finding files ending in '.ps' - run the 'updatedb' command first
whereis halt	Display the location of a binary file, source code, or man
which halt	Displays the full path to a binary or executable file

Mounting a file system

command code	Explanation of notes
mount /dev/hda2 /mnt/hda2	Mount a disk called hda2 - make sure the directory '/ mnt/hda2' already exists
umount /dev/hda2	Unmount a disk called hda2 - first exit from mount point '/ mnt/hda2'
fuser -km /mnt/hda2	Force uninstallation when device is busy
umount -n /mnt/hda2	Run an uninstall operation without writing to the /etc/mtab file - useful when the file is read-only or when the disk is full
mount /dev/fd0 /mnt/floppy	Mounting a floppy disk
mount /dev/cdrom /mnt/cdrom	Mounting a cdrom or dvdrom
mount /dev/hdc /mnt/cdrecorder	Mounting a cdrw or dvdrom
mount /dev/hdb /mnt/cdrecorder	Mounting a cdrw or dvdrom
mount -o loop file.iso /mnt/cdrom	Mounting a file or ISO image
mount -t vfat /dev/hda5 /mnt/hda5	Mount a Windows FAT32 file system
mount /dev/sda1 /mnt/usbdisk	Mounting a usb flash drive or flash memory device
mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share	Mount a windows network share

disk space

command code	Explanation of notes
df -h	Display the list of mounted partitions
ls -lSr \|more	Sort files and directories by size
du -sh dir1	Estimating the amount of disk space already used by the directory 'dir1' '
du -sk * \| sort -rn	Displays file and directory sizes in order of size.
rpm -q -a --qf '%10{SIZE}t%{NAME}n' \| sort -k1,1n	Shows the space used by installed rpm packages in order of size (fedora, redhat-like systems).
dpkg-query -W -f='${Installed-Size;10}t${Package}n' \| sort -k1,1n	Shows the space used by installed deb packages in terms of size (ubuntu, debian-like systems).

Users and Groups

command code	Explanation of notes
groupadd group_name	Create a new user group
groupdel group_name	Deleting a user group
groupmod -n new_group_name old_group_name	Renaming a user group
useradd -c "Name Surname " -g admin -d /home/user1 -s /bin/bash user1	Create a user belonging to the "admin" user group.
useradd user1	Create a new user
userdel -r user1	Delete a user ('-r' excludes home directories)
usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1	Modifying user attributes
passwd	Modify password
passwd user1	Modify a user's password (root only)
chage -E 2020-12-31 user1	Setting the expiration date of the user password
pwck	Check '/etc/passwd' for file formatting and syntax corrections, and for the presence of user
grpck	Check '/etc/passwd' for file format and syntax corrections, and for the existence of groups
newgrp group_name	Login to a new group to change the preset group for newly created files

file permissions

command code	Explanation of notes
ls -lh	Display Permissions
ls /tmp \| pr -T5 -W$COLUMNS	Divide the terminal into 5 columns for display
chmod ugo+rwx directory1	Set read (r), write (w), and execute (x) permissions for the owner (u), group (g), and others (o) of a directory.
chmod go-rwx directory1	Remove group (g) and others (o) read/write execute permissions on directories.
chown user1 file1	Changing the owner attribute of a file
chown -R user1 directory1	Change the attributes of all the people in a directory and change the attributes of all the files in the directory.
chgrp group1 file1	Changing the group of a document
chown user1:group1 file1	Changing the owner and group attributes of a file
find / -perm -u+s	List all files on a system that use SUID control
chmod u+s /bin/file1	Set the SUID bit of a binary file - the user running the file is given the same permissions as the owner
chmod u-s /bin/file1	Disabling the SUID bit for a binary file
chmod g+s /home/public	Set the SGID bit for a directory - similar to SUID, but directory-specific
chmod g-s /home/public	Disable the SGID bit for a directory
chmod o+t /home/public	Set the STIKY bit for a file - only legal owners are allowed to delete the file
chmod o-t /home/public	Disable the STIKY bit for a directory

Document Properties

command code	Explanation of notes
chattr +a file1	Allow reading and writing of files in append mode only
chattr +c file1	Allow this file to be automatically compressed/decompressed by the kernel.
chattr +d file1	The dump program will ignore this file when making a file system backup
chattr +i file1	Set as immutable files that cannot be deleted, modified, renamed or linked to
chattr +s file1	Allow a file to be securely deleted
chattr +S file1	Once the application performs a write operation to this file, causing the system to immediately write the modifications to disk
chattr +u file1	If a file is deleted, the system will allow you to recover the deleted file later.
lsattr	Show special properties

Packaging and Compression

command code	Explanation of notes
bunzip2 file1.bz2	Extract a file called 'file1.bz2'
bzip2 file1	Compress a file called 'file1'
gunzip file1.gz	Extract a file called 'file1.gz'
gzip file1	Compressing a file called 'file1'
gzip -9 file1	Maximum compression
rar a file1.rar test_file	Create a package called 'file1.rar'
rar a file1.rar file1 file2 dir1	Compress both 'file1', 'file2' and the directory 'dir1'.
rar x file1.rar	Unzip the rar package
unrar x file1.rar	Unzip the rar package
tar -cvf archive.tar file1	Create an uncompressed tarball
tar -cvf archive.tar file1 file2 dir1	Create a file containing 'file1', 'file2' and 'dir1'.
tar -tf archive.tar	Displaying the contents of a package
tar -xvf archive.tar	Release a package
tar -xvf archive.tar -C /tmp	Release the package to the /tmp directory
tar -cvfj archive.tar.bz2 dir1	Creating a bzip2 format archive
tar -jxvf archive.tar.bz2	Unzip a bzip2 format archive
tar -cvfz archive.tar.gz dir1	Create a gzip-formatted archive
tar -zxvf archive.tar.gz	Decompress a gzip-formatted archive.
zip file1.zip file1	Create a zip archive
zip -r file1.zip file1 file2 dir1	Compress several files and directories at once into a single zip package
unzip file1.zip	Extract a zip archive

RPM package

command code	Explanation of notes
rpm -ivh package.rpm	Install an rpm package
rpm -ivh --nodeeps package.rpm	Installing an rpm package and ignoring dependency warnings
rpm -U package.rpm	Updating an rpm package without changing its configuration file
rpm -F package.rpm	Update an rpm package that is sure to be installed
rpm -e package_name.rpm	Removing an rpm package
rpm -qa	Show all installed rpm packages on the system
rpm -qa \| grep httpd	Show all rpm packages with "httpd" in their name.
rpm -qi package_name	Get special information about an installed package
rpm -qg "System Environment/Daemons"	Show rpm packages for a component
rpm -ql package_name	Display a list of files provided by an installed rpm package
rpm -qc package_name	Display a list of configuration files provided by an installed rpm package
rpm -q package_name --whatrequires	Display a list of dependencies on an rpm package
rpm -q package_name --whatprovides	Shows the volume occupied by an rpm package
rpm -q package_name --scripts	Show scripts executed during installation/removall
rpm -q package_name --changelog	Show modification history of an rpm package
rpm -qf /etc/httpd/conf/httpd.conf	Confirm which rpm package provides the given file.
rpm -qp package.rpm -l	Displays a list of files provided by an uninstalled rpm package
rpm --import /media/cdrom/RPM-GPG-KEY	Import public key digital certificates
rpm --checksig package.rpm	Confirming the integrity of an rpm package
rpm -qa gpg-pubkey	Verify the integrity of all installed rpm packages
rpm -V package_name	Checks file size, license, type, owner, group, MD5 check, and last modification time.
rpm -Va	Check all installed rpm packages on your system - use with care!
rpm -Vp package.rpm	Confirm that an rpm package is not yet installed
rpm2cpio package.rpm \| cpio --extract --make-directories bin	Running an executable from an rpm package
rpm -ivh /usr/src/redhat/RPMS/`arch`/package.rpm	Installing a built package from an rpm source code
rpmbuild --rebuild package_name.src.rpm	Building an rpm package from an rpm source code

YUM Package Upgrader

command code	Explanation of notes
yum install package_name	Download and install an rpm package
yum localinstall package_name.rpm	An rpm package will be installed, using your own repositories to resolve all dependencies for you!
yum update package_name.rpm	Update all installed rpm packages on the current system
yum update package_name	Updating an rpm package
yum remove package_name	Removing an rpm package
yum list	List all packages installed on the current system
yum search package_name	Search for packages in the rpm repository
yum clean packages	Clear rpm cache to remove downloaded packages
yum clean headers	Remove all header files
yum clean all	Remove all cached packages and headers

DEB package

command code	Explanation of notes
dpkg -i package.deb	Installing/updating a deb package
dpkg -r package_name	Remove a deb package from the system
dpkg -l	Show all installed deb packages on your system
dpkg -l \| grep httpd	Show all deb packages that have "httpd" in their name.
dpkg -s package_name	Get information about a particular package already installed on the system
dpkg -L package_name	Displays a list of files provided by a deb package already installed on the system
dpkg --contents package.deb	Displays a list of files provided by a package that has not yet been installed
dpkg -S /bin/ping	Confirm which deb package provided the given file.

APT software tools

command code	Explanation of notes
apt-get install package_name	Installing/updating a deb package
apt-cdrom install package_name	Installing/updating a deb package from a CD-ROM
apt-get update	Upgrade packages in the list
apt-get upgrade	Upgrade all installed software
apt-get remove package_name	Remove a deb package from the system
apt-get check	Verify that the dependent repositories are correct
apt-get clean	Clear cache from downloaded packages
apt-cache search searched-package	Returns the name of the package containing the string to be searched for

View File Contents

command code	Explanation of notes
cat file1	View the contents of the file in a forward direction starting from the first byte
tac file1	Reverse the contents of a file starting from the last line
more file1	Viewing the contents of a long file
less file1	Similar to the 'more' command, but it allows the same reverse operations as forward operations in a file
head -2 file1	View the first two lines of a file
tail -2 file1	Viewing the last two lines of a file
tail -f /var/log/messages	Real-time view of what is being added to a file

text processing

command code	Explanation of notes
cat file1 \| command( sed, grep, awk, grep, etc...) > result.txt	Merge the text of a document's detailed description and write the introduction to a new file
cat file1 \| command( sed, grep, awk, grep, etc...) >> result.txt	Merge the text of a document's detailed description and write the introduction to an existing file
grep Aug /var/log/messages	Look for the keyword "Aug" in the file '/var/log/messages'
grep ^Aug /var/log/messages	Look for words starting with "Aug" in the file '/var/log/messages'.
grep [0-9] /var/log/messages	Select all lines containing numbers in the '/var/log/messages' file
grep Aug -R /var/log/*	Search for the string "Aug" in the directory '/var/log' and subsequent directories.
sed 's/stringa1/stringa2/g' example.txt	Replace "string1" with "string2" in the example.txt file.
sed '/^$/d' example.txt	Remove all blank lines from the example.txt file
sed '/ *#/d; /^$/d' example.txt 从example.txt	Remove all comments and blank lines from the file
echo 'esempio' \| tr '[:lower:]' '[:upper:]'	Merge the contents of upper and lower cells
sed -e '1d' result.txt	Exclude the first line from the file example.txt
sed -n '/stringa1/p'	View rows containing only the word "string1
sed -e 's/ *$//' example.txt	Remove the last blank character of each line
sed -e 's/stringa1//g' example.txt	Remove only the word "string1" from the document and keep all the rest.
sed -n '1,5p;5q' example.txt	View from line 1 to line 5
sed -n '5p;5q' example.txt	View row 5
sed -e 's/00*/0/g' example.txt	Replacing multiple zeros with a single zero
cat -n file1	Number of lines in the labeled document
cat example.txt \| awk 'NR%2==1'	Delete all even-numbered lines in the example.txt file
echo a b c \| awk '{print $1}'	View the first column of a row
echo a b c \| awk '{print $1,$3}'	View the first and third columns of a row
paste file1 file2	Merge the contents of two files or two columns
paste -d '+' file1 file2	Merge the contents of two documents or two columns, distinguished by a "+" in the center
sort file1 file2	Sorting the contents of the two files
sort file1 file2 \| uniq	Take out the concatenation of two files (keep only one copy of duplicate lines)
sort file1 file2 \| uniq -u	Delete the intersection, leaving the other rows
sort file1 file2 \| uniq -d	Take out the intersection of two files (leaving only the files that exist in both files)
comm -1 file1 file2	Comparing the contents of two files removes only the contents of 'file1'.
comm -2 file1 file2	Comparing the contents of two files removes only the contents of 'file2'.
comm -3 file1 file2	Comparing the contents of two files removes only the parts that are common to both files

Character Setting/File Format Conversion

command code	Explanation of notes
dos2unix filedos.txt fileunix.txt	Converting the format of a text file from MSDOS to UNIX
unix2dos fileunix.txt filedos.txt	Convert the format of a text file from UNIX to MSDOS
recode ..HTML < page.txt > page.html	Convert a text file to html
recode -l \| more	Show all allowed conversion formats

File system analysis

command code	Explanation of notes
badblocks -v /dev/hda1	Checking for bad blocks on disk hda1
fsck /dev/hda1	Repair/check integrity of linux filesystem on hda1 disk
fsck.ext2 /dev/hda1	Repair/check integrity of ext2 filesystem on hda1 disk
e2fsck /dev/hda1	Repair/check integrity of ext2 filesystem on hda1 disk
e2fsck -j /dev/hda1	Repair/check integrity of ext3 filesystem on hda1 disk
fsck.ext3 /dev/hda1	Repair/check integrity of ext3 filesystem on hda1 disk
fsck.vfat /dev/hda1	Repair/check integrity of fat file system on hda1 disk
fsck.msdos /dev/hda1	Repair/check integrity of dos file system on hda1 disk
dosfsck /dev/hda1	Repair/check integrity of dos file system on hda1 disk

Initialize the file system

command code	Explanation of notes
mkfs /dev/hda1	Create a file system on the hda1 partition
mke2fs /dev/hda1	Create a linux ext2 filesystem on the hda1 partition
mke2fs -j /dev/hda1	Create a linux ext3 (log type) filesystem on the hda1 partition
mkfs -t vfat 32 -F /dev/hda1	Creating a FAT32 file system
fdformat -n /dev/fd0	Formatting a floppy disk
mkswap /dev/hda3	Create a swap file system

SWAP file system

command code	Explanation of notes
mkswap /dev/hda3	Create a swap file system
swapon /dev/hda3	Enable a new swap filesystem
swapon /dev/hda2 /dev/hdb3	Enable two swap partitions

backing up

command code	Explanation of notes
dump -0aj -f /tmp/home0.bak /home	Make a full backup of the '/home' directory
dump -1aj -f /tmp/home0.bak /home	Make an interactive backup of the '/home' directory
restore -if /tmp/home0.bak	Restore an interactive backup
rsync -rogpav --delete /home /tmp	Synchronize both directories
rsync -rogpav -e ssh --delete /home ip_address:/tmp	rsync over SSH channel
rsync -az -e ssh --delete ip_addr:/home/public /home/local	Synchronizing a remote directory to a local directory via ssh and compression
rsync -az -e ssh --delete /home/local ip_addr:/home/public	Synchronize local directories to remote directories via ssh and compression
dd bs=1M if=/dev/hda \| gzip \| ssh user@ip_addr 'dd of=hda.gz'	Perform a single backup of the local disk on the remote host via ssh
dd if=/dev/sda of=/tmp/file1	Backup disk contents to a file
tar -Puf backup.tar /home/user	Perform an interactive backup of the '/home/user' directory
( cd /tmp/local/ && tar c . ) \| ssh -C user@ip_addr 'cd /home/share/ && tar x -p'	Copying the contents of a directory in a remote directory via ssh
( tar c /home ) \| ssh -C user@ip_addr 'cd /home/backup-home && tar x -p'	Copying a local directory in a remote directory via ssh
tar cf - . \| (cd /tmp/backup ; tar xf - )	Locally copy a directory to another location, retaining original permissions and links
find /home/user1 -name '*.txt' \| xargs cp -av --target-directory=/home/backup/ --parents	Find and copy all files ending in '.txt' from one directory to another directory
find /var/log -name '*.log' \| tar cv --files-from=- \| bzip2 > log.tar.bz2	Find all files ending with '.log' and make a bzip package
dd if=/dev/hda of=/dev/fd0 bs=512 count=1	Do a copy of the MBR (Master Boot Record) contents to a floppy diskette
dd if=/dev/fd0 of=/dev/hda bs=512 count=1	Recovering MBR contents from a backup that has been saved to a floppy diskette

CD-ROM

command code	Explanation of notes
cdrecord -v gracetime=2 dev=/dev/cdrom -eject blank=fast -force	Emptying the contents of a rewritable CD-ROM
mkisofs /dev/cdrom > cd.iso	Create an iso image file of the CD-ROM on the disk
mkisofs /dev/cdrom \| gzip > cd_iso.gz	Creating a compressed CD-ROM iso image file on disk
mkisofs -J -allow-leading-dots -R -V "Label CD" -iso-level 4 -o ./cd.iso data_cd	Create a directory of iso image files
cdrecord -v dev=/dev/cdrom cd.iso	Burn an ISO image file
gzip -dc cd_iso.gz \| cdrecord dev=/dev/cdrom -	Burning a compressed ISO image file
mount -o loop cd.iso /mnt/iso	Mount an ISO image file
cd-paranoia -B	Transcribing audio tracks from a CD-ROM to a wav file
cd-paranoia -- "-3"	Transcribing audio tracks from a CD-ROM to a wav file (parameter-3)
cdrecord --scanbus	Scanning the bus to identify scsi channels
dd if=/dev/hdc \| md5sum	Verifies the md5sum encoding of a device, e.g. a CD.

reticulation

command code	Explanation of notes
dhclient eth0	Enable 'eth0' network device in dhcp mode
ethtool eth0	Show traffic statistics for NIC 'eth0'
host www.example.com	Finding hostnames to resolve names with IP addresses and mirrors
hostname	Show hostname
ifconfig eth0	Displays the configuration of an Ethernet card
ifconfig eth0 192.168.1.1 netmask 255.255.255.0	Control IP address
ifconfig eth0 promisc	Set 'eth0' to promiscuous mode to sniff packets (sniffing)
ifdown eth0	Disable an 'eth0' network device
ifup eth0	Enabling an 'eth0' network device
ip link show	Shows the connection status of all network devices
iwconfig eth1	Displaying the configuration of a wireless card
iwlist scan	Show wireless network
mii-tool eth0	Display the connection status of 'eth0'.
netstat -tup	Shows all enabled network connections and their PIDs
netstat -tup1	Displays all listening network services on the system and their PIDs
netstat -rn	Display the routing table, similar to the "route -n" command.
nslookup www.example.com	Finding hostnames to resolve names with IP addresses and mirrors
route -n	Display routing table
route add -net 0/0 gw IP Gateway	Control of preset gateways
route add -net 192.168.0.0 netmask 255.255.0.0 gw 192.168.1.1	Control static routes to network '192.168.0.0/16'
route del 0/0 gw IP gateway	Remove static routes
echo “1”> /proc/sys/net/ipv4/ip_foward	Activate IP Forwarding
tcpdump tcp port 80	Show all HTTP loops
whois www.example.com	Lookup in Whois database

Microsoft Windows Networks

command code	Explanation of notes
mount -t smbfs -o username=user,password=pass //WinClient/share/mnt/share	Mount a windows network share
nbtscan ip addr	Netbios name resolution
nmblookup -A ip addr	Netbios name resolution
smbclient -L ip addr/hostname	Show remote shares for a windows host
smbget -Rr smb://ip addr/share	Like wget can download files from a windows host via smb.

IPTABLES(firewall)

command code	Explanation of notes
iptables -t filter -L	Display all links in the filter table
iptables -t nae -L	Display all links in the nat table
iptables -t filter -F	Clean up all rules based on the filter table
iptables -t nat -F	Clean up all rules based on nat table
iptables -t filter -X	Delete all links created by the user
iptables -t filter -A INPUT -p tcp --dport telnet -j ACCEPT	Allow telnet access
iptables -t filter -A OUTPUT -p tcp --dport telnet -j DROP	Blocking telnet access
iptables -t filter -A FORWARD -p tcp --dport pop3 -j ACCEPT	Allow forwarding of POP3 connections on the link
iptables -t filter -A INPUT -j LOG --log-prefix	Logging of seized packets on all links
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE	Set up a PAT (Port Address Translation) to send out packets in the eth0 mask
iptables -t nat -A POSTROUTING -d 192.168.0.1 -p tcp -m tcp --dport 22-j DNAT --to-destination 10.0.0.2:22	Steering packets destined for one host address to other hosts

Monitoring and debugging

command code	Explanation of notes
free -m	List RAM status in megabytes
kill -9 process id	Force close the process and end it
kill -1 process id	Force a process to reload its configuration
last reboot	Show restart history
lsmod	Kernel modules for listing states
lsof -p process id	List a list of files opened by a process
lsof /home/user1	List the opened files in the given system paths
ps -eafw	Listing linux tasks
ps -e -o pid,args --forest	List linux tasks in a hierarchical manner
pstress	Displaying the program in a tree diagram
smartctl -A /dev/hda	Monitor Hard Disk Device Reliability by Enabling SMART
smartctl -i /dev/hda	Checking whether SMART is enabled for a hard disk device
strace -c ls >/dev/null	List system calls made and receive them in one process.
strace -f -e open ls >/dev/null	Listing library calls
tail /var/log/dmesg	Display internal events during kernel boot
tail /val/log/messages	Displaying system events
top	List the linux tasks that use the most CPU resources
watch -nl ‘cat /proc/interrupts’	Listing of real-time interruptions

Other common commands

command code	Explanation of notes
alias hh='history'	Set an alias for the command history
apropos ...keyword	Listing a list of commands that includes the keywords of the program is especially useful when you only know what the program does and don't remember the commands
chsh	Changing shell commands
chsh --list-shells	Nice command for finding out if you have to connect remotely to another machine
gpg -c filel	Encrypting a file with GNU Privacy Guard
gpg filel.gpg	Decrypting a file with GNU Privacy Guard