LINUX COMMANDS

The kernel operates in a single address space. That is, one module or driver can corrupt the entire kernel.
Ctrl + Alt + F(1-6) switches between virtual consoles. ttyN - where N is the console number?
Ctrl + Alt + F7 switches to the graphical console
startx _launches the X Windows System (switches to graphical mode)
ho -r or runlevel _displays the OS runlevel !!!
init n (n from 0 to 6) starts one of the OS runlevels, for example, 3. That is, it launches the rc3.d file, which contains symbolic links to files in init.d.
The link name can start with K-stop or S-start, followed by the startup priority number.
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mount /dev/hda4 mnt/hda4 -t data=<journaling mode> _writeback, ordered, journal?
Creating a new ext3 file system
1. Create a partition using parted or fdisk
2. Format the partition with mkfs
3. Assign a label to the partition using e2label
4. Create a mount point with mkdir
5. Add the partition to the /etc/fstab file
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GRUB bootloader
Press E to enter the record editor interface. Esc to discard changes and return to the menu.
Press B (after editing) to execute commands and boot the OS
Press C to enter the command-line interface, enter commands, and press Enter to execute
help all - displays all commands
Select the kernel and press A. At the end of the line, add the runlevel number separated by a space.
less /boot/grub/grub.conf _bootloader configuration file
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2.1 Superblocks - important disk information?
dumpe2fs /dev/sda1 | grep superblock _superblock information
Using a backup superblock: all?
1. Boot from a Live CD or CentOS installation disk
2. fsck -y -b 8193 /dev/sda1 _Check disk; 8193 is the number of the first backup superblock
3. Mount the file system
3.1 In the usual way
3.2 If unsuccessful:
mount sb=8193 /dev/sda1 /mnt _backup superblock number /device /mount point
4. Add the sb option (sb=8193) to our file system in /etc/fstab
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File access permissions
123 456 789
s t rwx rwx rwx
s _SUID bit - 4 (000) The file acquires the rights of the file owner, not the user who launched it (the passwd utility must have execute permissions). ***In the shell, files with the SUID bit are highlighted in red.
t _Sticky bit - 1 (000)
1/2/3 _read/write/execute for the owner
4/5/6 _read/write/execute for the owner's group
7/8/9 _read/write/execute for others
Read permission is denoted by the number 4, write by 2, and execute by 1.
If all permissions are allowed, the permission number set for a specific group on the file is 7.

chmod [options] [mode] file - change access permissions. -v - outputs debug information, -R - recursive mode, changes permissions for all subdirectories and files
chmod 4755 test.txt _4-adds the suid bit, 7-full permissions for the owner, 5 and 5 - read and execute for group and others - this command on a file in the root home directory (folder) issued the message: You have new mail in /var/spool/mail/root ?
-rwsr-xr-x root root 1 _should be read as rws (s instead of x) rx rx, root - owner, root - owner's group, 1 - number of links
Reverse: chmod 0755 test.txt or chmod 755 test.txt - 0 before 7 is substituted by default.
chmod 750 test.txt - 0 zeroes the permission value
chmod -R u=rwx,g=rx,o=x myfiles - 2nd method of changing permissions for a directory or file.
chmod -R u-x,g-x,o-x myfiles - 3rd method of changing permissions for a directory or file separately for each user group, where a - owner, g - owner's group, o - all others.
chmod -R a-x myfiles - 4th method of changing permissions for a directory or file for all users. In 3 and 4 you can use -/+
chmod +x script.sh - add execute permission for all groups. 5th method of changing permissions.

sgid-bit - 2 (000) - owner group permissions are inherited. If the sgid bit is set for a directory, then files created in it, when run, will take the directory's group identifier, not the group to which the file owner belongs.
Running a program with owner group permissions? How to do it correctly?
sgid-bit for a file: chmod 2755 test.txt

sticky-bit - 1 (000) - if set on a directory, then only the owner of this directory, the file owner, or root can delete and rename files in this directory. Having only write permission to this directory is not enough.
chmod 1755 test.txt

chown [options] [owner]:[group] test.txt - change file owner and group. -v - outputs debug information, -R - recursive mode.
[owner]:[group] - this is the new owner and new group.
chown -R user1:user1 * - change owner and group for all files and subdirectories of the current directory. Cannot change the owner of a subdirectory. Who has the right to change the owner of a file or directory?
chgrp group1 file1 – change group.

umask XXXX - change (set) access mode, sets access permissions for newly created directories and files!!!
The umask value is subtracted from 777 for directories and from 666 for files.
umask - find out what umask is used in the system, mine: 0022.
New directories: 0777 - 0022 = 0755. New files 0666 - 0022 = 0644

umask 0 1 2 3 4 5 6 7
Access mode mask r w x r w - r - x r - - - w x - w - - - x - - -

mount [options] [device] [mount_point] - mount a filesystem
umount [options] [device] OR [mount_point] -f force unmount
mount - list mounted filesystems
When mounting, you can specify permissions. You can mount the entire filesystem as read-only. This will override previously assigned permissions.
Mount a CD: mount -t iso9660 /dev/cdrom /mnt _specify filesystem type (not mandatory for CDs)
Mount an image: mount -o loop -t iso9660 image.iso /mnt/iso didn't work?
iso9660 for mounting Unix systems and CDs/DVDs; mount -t ext3 /dev/sda4 /disk mount does not create partitions.
nfs for mounting network filesystems
vfat, ntfs for mounting Windows partitions
ufs for Solaris
The file /etc/fstab contains a list of filesystems to be automatically mounted at OS startup.
[device] [mount_point] [filesystem_type] [options] [dump_flag _obsolete] [fsck_flag (0-do not check, 1-check, 2-check after 1 has been checked)]
cat /etc/fstab –
Device names:
hdc – hard disks
scsi -

cat /etc/mtab (mount) _ view information about mounted filesystems. Not manually editable.
/etc/filesystems _ list of possible filesystems. Used by the mount command for auto-completion if the -t flag with the filesystem type is not specified.

The proc filesystem exists only in RAM. It is used as an interface to kernel data structures. This is a virtual filesystem, loaded into memory at system startup, and does not occupy space on the hard drive. Many files in it can be modified, thereby changing kernel parameters in real time.
cat /proc/cpuinfo, meminfo, mounts, devices, filesystems, modules, version, cmdline - list of parameters passed to the kernel at boot. A great source of system information!!!

Working with files:
Create a text file using a text editor like vi/vim, or by redirecting the output of the echo or cat command.
vim editor:
i - enter insert mode. Esc - after entering text, return to command mode.
: - to enter commands for file operations, editing settings, and other commands.
Now you can enter: w - to write/save the file, and q to exit the editor. q! – exit without saving.
/ - search
touch test.txt – update the access time of an existing file (create a new one in the current directory). By default, a file is created with permissions 644. Usually changed to 775/ 755
touch 's probelom'.txt – create a file with a space in the name.

cp [options] [source] [destination] - copy a file or directory.
cp test.txt test1.txt - create test1 as a copy of test.txt, cp /etc/test.txt ~ - copy test.txt to the home directory.
cp -r /etc/ /root/dok - copy the contents of the /etc/ directory into the newly created folder /root/dok. You cannot copy a file while simultaneously creating a folder, only the contents of the directory.

mv [options] [source] [destination] - move files
mv file.txt /tmp/ - move file.txt to /tmp/ without renaming
mv /tmp/file.txt ~/file2.txt - move file.txt, renaming it to file2.txt

A soft link points to a file, a hard link points to the file's inode.
ln [options] [target] [link_name] - create links between files.
ln -s file.txt lnk - create a symbolic (soft) link lnk (via -s) - instead of the file's path name, you can specify an alias, which points to the file.
ln file2.txt file.txt - create a hard link - points to the file's inode
unlink lnk - remove a soft link

cat – view, less – page-by-page/line-by-line output, q – exit, Shift + PgUp/PgDn – navigation.
cat test.txt test1.txt – display the contents of 2 files on the screen.
cat test.txt test1.txt > test2.txt – merge files into 1.
cat test1.txt > test2.txt – replace the contents of file test2 with the contents of file test1.
cat test1.txt >> test2.txt – append the contents of file test1 to the contents of file test2.
cat - >> test1.txt – append data to file test1 directly from the shell. Ctrl + C – save changes.

head -n 20 test.txt – display the first 20 lines of the file (10 lines by default).
tail -n 20 test.txt – display the last 20 lines of the file (10 lines by default).
nl test.txt – display the file with line numbers.

df [options] [file] – filesystem usage statistics. You can use df, df -h, or df -h /dev/sda1.
du [options] [file] – disk space statistics. du -hs /usr/share – display the size of the /usr/share directory.
-h – provides statistics in a human-readable format (e.g., MB, etc.). -s – summarizes the results of subdirectories and files within them.

echo [options] [string] – display a string of text. echo -n 'blablabla' – displays blablabla on the same line.
echo 'blablabla' – displays blablabla on a new line.
echo 'blablabla' > file.txt – replace the contents of the file; echo 'blablabla' >> file.txt – append to the file contents.

wc [options] [file] – file statistics !!!
-c – number of bytes in the file, -m – number of characters in the file, -l – number of lines in the file, -L – length of the longest line.
-w – number of words in the file.

find [search_path] [search_option] [value] [action_option] – search for files or directories.
-name – name of the file or directory to find.
-type – object type: f – regular file, d – directory, b – block device, c – character device, p – named pipe, l – symbolic link, s – socket.
-size – use file size as a search criterion.
-mtime – modification date.
-ctime – time of the last change to file attributes. What is the difference between mtime and ctime?
-atime – time of the last access to the file touch?
-perm 777 – with access rights 777.
-print – simply print the full path of the file.
-exec – execute a specific command for the found objects.
find / -name pab -type d
find /tmp/ -name core -type f -print | xargs /bin/rm -f – search for a file, print it to the screen, and using xargs, pass it as an argument to the rm command.
find /etc/ -perm 777 -type f – search for files with permissions 777 in the /etc directory.
find /etc/ -name pablish -exec rm -rf {} \; – execute the rm -rf command for the found objects. The found items will be substituted in place of {} as arguments.

F4 - open file
Working with directories:
mkdir - create, ls - view, mv - rename/move, rm -rf - delete occupied directories, rmdir - delete empty directories !!!
mkdir –p ~/test/cda/blabla _100% create subdirectories in the home directory
mkdir -p /root/dok/dir2{dir1,dir2,dir3} – create directory structure dok/dir2 with 3 sections dir1-3
«.» - current directory, «..» - parent directory, for root «.» = «..»
mkdir -m 700 dir2 - explicitly set permissions for the new directory
rm -rf* - delete all contents of the current partition,
where -r - recursive mode, -f - disable confirmation prompts
rmdir ~/test _delete empty subdirectory of the home directory
mknod _create byte-oriented devices - printers, modems rm _delet?
mknod _create block-oriented devices - hard disk?
mknod _create named pipes rm _delet?
socket can be deleted with rm

ls - display directory contents.
ls -lhR /etc/ | less - redirect output through a pipe to the less command
-a - all including hidden files (starting with a dot), -h - file sizes, -l - full info, -i - inodes (index descriptors), -R - info for all subdirectories -t – sort by time, /*.txt – search by mask
Man ls – help.
q – exit, Pg Up, Pg Dn – navigation. / - enter search, n - down N - up
ls ../.. _view two levels up, ls ../man _find directory man one level up
ls -l /dev/sda = file /dev/sda, ls –l /root /tmp /usr …- display multiple files
ls –l /etc/*/*.conf – search by extension, "skipping" 1 level
ls -l /etc/[^a-w]*.conf - searches for directories not starting with a-w with conf extension.
ls –ld – permissions for the current directory.
cd – change directory – change directory
cd /usr/share/doc - navigate using absolute path
cd - _to previous directory, cd .. _one level up, cd ~ _to home directory,
cd ~/myfiles _navigate from any location to a newly created subdirectory of the home directory.

grep [options] pattern [file path to search] - Pattern search.
rpm -qa | grep chk - search for chk in the output of all packages installed in the system.

/dev/null - the so-called black hole, used to absorb unnecessary program output
command > /dev/null 2>&1 - redirect all errors to /dev/null.
/dev/zero - can be used to create a file of a specific size
dd if=/dev/zero of=file.iso bs=1024 count=1024 - create a 1 Mb iso file.
/dev/random, /dev/urandom - used by scripts to generate random numbers.

BASH:
# - command line with administrator privileges
$ - command line of a regular user - can work in the home directory without affecting the system as a whole

Ctrl + C - interrupt current process (SIGINT signal)
Ctrl + D - exit command interpreter
Ctrl + L - clear screen
Ctrl + Z - move process to background mode
Alt + D - delete all characters from current position to end of word
Alt + R - clear entire line
Double TAB - all available commands
Esc + D - delete from cursor to end of word

echo $BASH_VERSION - BASH version. $ - variable sign, i.e. output of BASH_VERSION variable.
echo $BASH - BASH location

Command format: command_name [flags/options] [arguments]
./name_script.sh - run a script from the home directory
BASH Configuration Files
1. Local - located in the home directories of system users and take precedence over Global files.
.bash_logout - script executed upon system logout
.bash_history - file containing command history, which is saved to the file after the user logs out. Can be configured to save immediately?
.bash_profile - script executed when you connect to the system (in some systems, bashlogin is used)
.bashrc - script executed every time a new shell is opened. It is executed, along with all commands listed within it.
2. Global:
/etc/profile
/etc/bashrc

history - view command history. The number of lines is set by the variable echo $HISTSIZE, defaulting to 1000 lines.
history N - display the last N lines.
history -d N - delete line N.
!! - execute the last entered command
!N - execute command N
!string - the most recent command starting with the string "string"

Input/Output Redirection.
File descriptor - a non-negative integer assigned by the OS to a file opened by a process. There are always 3 file descriptors open in the system.

Number Name Description Comment
0 stdin Standard input - keyboard input is handled by the keyboard
1 stdout Standard output - monitor display is handled by the monitor
2 stderr Used for outputting error messages

cat file1 file3 2>err_file - error outputting non-existent file3 is redirected to the newly created file err_file. 2 represents stderr.
cat < /etc/resolv.conf - redirection from a file.
cat > file, then press Enter, and type text on a new line. End input with Ctrl + C. The entered text is added to the newly created file.
date > date.txt - when redirecting to a file, existing data is overwritten.
date >> date.txt - when redirecting to a file, append to existing data.
vi <(ls /etc) - redirect the contents of the etc directory into a text editor. Under what name should the result be saved?
echo -e "'ls -l /etc/[^a-w]*.conf'\n" - searches for directories not starting with a-w with the .conf extension. Does not work??? '- ?
Possible redirections:

From Where
stdout File
stdout Stderr
stdout and stderr Stdout
stdout and stderr Stderr
stdout and stderr File
stderr File
stderr Stdout
Pipes connect the standard output of one command to the input of another and are denoted by: |
ls /usr/share | wc -l - count the number of lines in the directory

Built-in commands are commands built into bash. enable (enable -a) - view all (outputs a list of commands)
rpm -qa | grep hist - searches for "hist" in system packages; this does not exist because the history command is built into bash.
When running, built-in commands are executed first, and only then is a search performed in the $PATH variable.
enable -n pwd - disables the built-in pwd command. After this, the system command pwd located in /bin/pwd will be executed.
enable -n - list of disabled built-in commands

Variables
Parent process > child process > child process, etc.
Processes can sequentially pass variable information to a lower level from the parent process to child processes, but not the other way around. A child process cannot pass anything to the parent process.
Value=10 – creates a variable and assigns its value. The variable will be available only in the current session.
Export Value=10 – now the variable can be used from other programs.
Unset value – deletes the variable.
env – view environment variables (exported ones). Here we will see them only after executing the command: Export Value.
typeset – view environment variables, including those created by the user.
Vim script.sh, the script itself:
#!/bin/bash
Echo $value
./script.sh – executes the script in the root user's home directory.
The Shell is the parent process for the script. We could allow it to use the variable by exporting it, or deny it by not exporting it.
Common variables: view echo $
$HOSTNAME – hostname
$Shell – name of the command interpreter
$USER – name of the user logged into the system
$PATH – list of OS directories where the system searches for a command when typed in the console. The search proceeds sequentially through the directories until the first match is found.
$PWD – current working directory
$HOME – current user's home directory
$UID – numeric identifier of the current user
$# – number of passed arguments
echo $PATH – a variable storing the list of directories in the OS

Local variables.
$0 – contains the name of the executed file
$1 – first argument
$2 – second argument
$PS1 – command line prompt

Vim script.sh, the script itself:
#!/bin/bash
Echo “first arg” - $0
Echo “second arg” - $1
…
Echo “10 arg” - ${10} – starting from the 10th argument, they must be specified in curly braces.
./script.sh first_arg second_arg – passing the 1st and 2nd arguments to the script

Useful commands:
cat /etc/passwd – displays the list of users
pwd – view the working directory. pwd [-LP]
ps aux | grep mingetty – find all system processes and output only those containing the word mingetty
whoami – returns the current user account under which the user is working.
netstat -nr – for network analysis
su user1 – switch to another user. Question: in what case, after switching to a regular user, is it meaningful to run the command: cd ~?
exit – return to the root command line
useradd – add a user – command not found when entered before startx?
Question: sudo useradd PPetrov – error occurs after entering the requested password?
#passwd Uliashov – change the password for any user from under root
$passwd Ivanov – change the password for a user according to security criteria.
date – view system date and time
env – view environment variables (exported ones)
typeset – view environment variables, including those created by the user.
df –h – disk space usage
rm mod pcspkr – unload the kernel driver from RAM
modprobe pcspkr
lsmod – list of modules
modinfo pcspkr – display information about the module

Lecture #4

Each directory contains 2 links. Adding each subdirectory adds +1 link.
Some disable swap to avoid constant disk thrashing.
fdisk -l – disk configuration
free – RAM statistics
vnc viewer 10.95.4.229
DOS system allows creating 3 partitions
Primary - standard
Extended – extended, within which you can create a logical partition
Partitions 1 to 4 are Primary or Extended. From 5 onwards – logical
swapon -a / swapoff -a – enable/disable swap
etoolabel ???
partprobe?
Download VNC viewer
mkswap – ???
mkfs hdc1(2-6)
If you mount a partition into a new folder, files created in that folder will actually reside on the mounted partition, not in the folder itself.

1 free
2 cat /proc/swaps
3 swapoff -a /dev/mapper/VolGroup00-LogVol01 - path from step 2
4 mkfs /dev/mapper/VolGroup00-LogVol01
5 mkdir /novyi
6 mount /dev/mapper/VolGroup00-LogVol01 /novyi
7 create folders and files in the mount point folder
8 umount
9 mkswap /dev/mapper/VolGroup00-LogVol01 – formatted the device
10 swapon -a

alias 'fuck' = 'init 0' – do not do this
Only root can mount filesystems (mount).
You can mount a remote filesystem, for example, temporarily for backup purposes.
In /etc/fstab, you can specify user permissions to mount a specific folder.

Lecture #6

LVM – Logical Volume Manager – see description online.
LV > VG – group > PV – hd1, ... physical disks are combined into 1 group. Or conversely, 1 disk is split into several volumes???. Possible performance loss ~ 5%.
vgs, pvs, lvs
Task: obtain a logical volume. ??????????
pvcreate /dev/hdc5
PV
VG
vgcreate VG1 /dev/hdc5 - create group
lvcreate -L 100M -n DataBase VG1 - create logical volume
ls /dev/VG1/DataBase - check.
Do:
1 free
2 cat /proc/swaps
3 swapoff -a /dev/mapper/VolGroup00-LogVol01 - path from step 2 - disabled
4 lvremove /dev/mapper/VolGroup00-LogVol01 - deleted
5 cat /proc/swaps
6 vgs – shows free space
7 lvcreate -L 100M -n MYVOL1 VolGroup00
8 mkfs /dev/VolGroup00/MYVOL1 – formatting
8 mount /dev/mapper/VolGroup00-MYVOL2 /novyi or mount /dev/VolGroup00/MYVOL1 /novyi
ls -l /Dev/VolGroup00 – link to dev/mapper/Vol…
You can make an instant snapshot of the partition state:
lvcreate -s /dev//VolGroup00/MYVOL1 -n SNAP_MYVOL1 -L 100M - a snapshot is also a new partition. It can be mounted as a partition to a mount point, but if you create a file in it, it will not appear in the snapshot
when creating files in the mount point, lvs will show changes in the snapshot partition
Before changing data, their history is written to the snapshot
Snapshots are created in the same volume group.
Create a read-only snapshot:
lvcreate -p r -s /dev//VolGroup00/MYVOL1 -n SNAP_MYVOL1 -L 100M – prevents writing to the mount point.
Commands:
Ctrl + r + enter search text
Ctrl + u – clear entire output (error when entering password)
Ctrl + W – clear by words
Ctrl + L – clear screen ???
Alt + . – output last parameter
Ctrl + S – lock screen. Ctrl + Q - unlock

article taken from here