Week2.md

Week 2 Notes

• Multiple uses of / is as good as one
  • ie : cd usr//////bin will take you to usr/bin
• The root folder / is its own parent
  • ie : if you do cd .. within the root directory you stay in the same directory.
• Options / Flags can be written in multiple combinations
  • ls -l level1 -di
  • ls -d level1 -il
  • ls level1 -ldi
  • ls -ldi level1
• long formats for options are also available

• ls -a is equivalent to ls --all

Commands

• ls
  • R flag lists all subdirectories recursively
  • Passing directory name to ls shows what is within that directory. ie : ls -l level1
  • d flag displays details of a folder without traversing inside it. it : ls -ld level1
• ll
  • a shortcut for the ls -la command
• which
  • which command will show the location of the command
  • which less will show usr/bin/less
• whatis
  • gives a brief description of the command
• alias
  • give a nickname to a frequently used command
  • usage : alias ll = 'ls -l'
  • Just typing alias will show a list of aliases
  • alias date = 'date -R'
  • If the command is executed by typing the whole path eg : /usr/bin/date the alias is not invoked. (cd /usr/bin and ./date)
  • An alias can be escaped by prefixing a \ ie: \date
• unalias
  • used to remove an alias
• rmdir
  • removes an empty directory
• ps
  • displays current processes
  • ps --forest - which process has launched which child process.
  • ps -f - displays parent process id
  • ps -ef - all the processes running in the operating system now
  • PID is the process ID , PPID is the parent process ID.
  • PID 1 is /sbin/init
• bc - bench calculator
  • exit using Ctrl+D

Commands to know contents of a text file

• less
  • displays the content in one screen
  • ls -l /usr/bin/less shows that the command takes 180KB
• wc
  • prints newline,word and byte counts for the file
  • the -l flag shows just the number of lines
• head
  • head profile displays the first ten lines
  • use -n flag to specify the number of lines
• tail
  • tail profile displays the last ten lines
  • use -n flag to specify number of lines to be displayed
• cat
  • in /etc , cat profile would just dump contents on the screen without any further prompts.
  • disadvantages : cant move back and forth to view page by page, can't come out half way through.
  • if the file is very long cat is not the best way to look at the content.
• more
  • similar to less. Allows page by page viewing
  • ls -l /usr/bin/more shows that the command takes 43KB

Knowing more commands

• man
• which
• apropos
  • For a keyword it shows you all the commands which have that keyword in the description
  • Used to discover new commands
  • If you type ls -l /usr/bin/apropos you see that it is a symbolic link to whatis, but the outputs are different : Why?
  • Reason : In Linux every executable will know in what name it has been invoked - can have different behaviour depending on the name that invoked it.
  • It also has the same output as man -k : Searching for a keyword
• info
  • Allows browsing through commands using the cursor
  • Can go back using < or 'shift'+','
• whatis
• help
  • displays keywords reserved for the shell being run
• type
  • displays what type of command it is
  • type type shows that it is a 'shell built in' being offered from the shell and not the os
  • type ls shows that it is aliased with some option. which ls shows that it is coming from os because there is an executable available.

Multiple Arguments

• Recap : Arguments and Options

• Options are enhanced features of the command
• Arguments are specific names of files or directories

• Second arrument behaviour and interpratation of last argument should be seen in the man pages
• Recursion is assumed for mv and not cp

• recursion is assumed for some commands and should be explicitly stated in others
• For copy command recurssion is not assumed
• cp dir1 dir2 need not work. dir1 has 2 files in it.
• cp -r dir1 dir2 works - recurssion is specified explicitly.
• mv dir1 dir3 works - it just renames the directory.

• touch file1 file2 file3 creates all 3 files in one go with identical timestamp.

Links (Hard Links and Soft Links)

• Can determine whether a link is HL or SL by looking at the Inode numbers
  • Hard links will have the same inode numbers
  • Soft Link will have different inode numbers
  • If you delete a certain file using the rm command (rm unlinks the file from the filesystem. the data is still at the memory location. shred for permanant deletion)
    • Its hard link will still give you access to the original file data.
    • Its soft link will not work
• ln -s source destination to create symbolic link. ln -s file1 file2
  • file2 is a separate inode entry but it is just a shortcut to file1
  • file2 has only 1 hardlink.
• ln source destionation to create a hard link . ln file1 file3
  • file1 and file 3 have the same inode number - They are basically the same file.
  • file1 and file3 have 2 hard links when we do ls -li
• You can create a Soft Link ln -s ../dir/filex fileSL but creating a hard link using ln ../dir/filex fileHL will not work.
  • the first/source-file parameter is interpreted in the case of hard link creation and not in soft link creation
  • In the above example, assume that ../dir/filex does not exist.
  • soft links useful in version control systems

File Sizes

• ls -s
  • file size appears in the first column
• stat
  • in /usr/bin we look at stat znew
  • Gives information about the size, how many blocks are being occupied
  • Here the size is little more than 4kb
  • stat zmore shows that it takes less than one block
• du
  • in /usr/bin we look at du znew or du -h znew
  • Gives information about the size
  • Here the size is displayed as 8.0KB since there is a block overflow.
  • This means that files that are smaller than the block size will actually take up a whole block
  • du -h zmore shows that it occupies one block - around 4.0K
• Role of block size
  • explained in stat and du

In-Memory File Systems

• /proc
  • Is an older system
  • ls -l will display several zero-size files, even though we can read content from them.
  • These are only a representation and not real files on the HDD.
  • less cpuinfo - information about the cpu
  • cat version - information about the OS. Also accessible using uname -a
  • cat meminfo - information about the memory - also free -h
  • cat partitions - information about the partitions - also df -h
  • The kcore file appears to take huge space - Shows maximum virtual memory that the current linux os is able to handle. 2^47 or 140 TB
• /sys
  • Used from Kernel v2.6 onwards, however information about various processes that are running are still stored in the /proc directory itself.
  • Much more well organised than /proc
  • eg : sys/bus/usb/devices/1-1 points to a specific usb device.
• These are directories that are visible in the root folder. They are not on the disk but only in the memory.
• Important system information can be viewed from these directories in a read-only manner.

Shell Variables

• Makes it possible to communicate between 2 processes very efficiently. Need not write and read the filesystem.
• Security Concern : Some information that you write to the filesystem may be visible to other processes.
• Shell variables are available only within the shell or its child processes.
• echo prints strings to screen
  • uses space as a delimiter so multiple spaces between words are ignored. For multiple spaces, enclose the string in quotes.
  • can print a multi-line string by using double quotes and not closing it
  • ** Difference between ' and " **
  • echo $USERNAME and echo "$USERNAME" give the same result but echo '$USERNAME' is not interpreted to give the value of the shell variable.
  • ** Escaping to prevent interpretation **
  • echo "username is $USERNAME and host name is \$HOSTNAME"
  • Escaping is usefule when you want to pass on the information to a child shell, without it being interpreted by the shell launching it.
• echo $HOME prints values of variables
  • By convention every shell variable starts with a Dollar
• Commonly used shell variables
  • $USERNAME eg : echo "User logged into system now is : $USERNAME"
  • $HOME
  • $HOSTNAME
  • $PWD
  • $PATH - variable contains a list of directories which will be searched when you type a command. When ever you type a command the system scans these paths from left to right to see if the command is in the directory.
• Commands like printenv , env , set to see variables that are already defined
  • printenv displays all the shell variables defined in the shell that you are running.
  • env gives the same output
  • set displays some functions defined to interpret what you are typing on the command line.
• Special Shell Variables
  • $0 : name of the shell eg bash or ksh
  • $$ : process ID of the shell
  • $? : return code of previously run program
  • $- : flags set in the bash shell . The man page for bash shows the meaning of the flags.
• Process Control echo $$
  • use of & to run a job in the background
  • fg - bring process to foreground
  • coproc - run a command while also being able to use the shell
  • jobs - list programs running in the background
  • top - See programs that are hogging the CPU or memory (refreshed every second)
  • kill - kill process owned by you
• Program Exit Codes echo $?
  • exit code always has a value between 0 and 255
  • 0 : Success
  • 1 : Failure
  • 2 : Misuse (insufficient permissions)
  • 126 : command cannot be executed (usually due to insufficient permissions to execute a file)
  • 127 : command not found (usually due to command typos)
  • 130 : processes killed using control+c
  • 137 : processes killed using kill -9 <pid>
  • If the exit code is more than 256 then the exitcode%256 will be reported as the exit code
  • exit 0 or exit 1 or exit <n> exits with exit code n
  • Used when there are command dependencies (ie: run second command only if first command completes successfully)
• Flags set in bash echo $-
  • h : locate hash commands
  • B : braceexpansion enabled
  • i : interactive mode
  • m : job control enabled (can be taken to bg or fg)
  • H : !style history substitution enabled
  • s : commands are read from stdin
  • c : commands are read from arguments

Linux Process Management

• sleep command to create processes
  • usage : sleep 3 for 3 seconds
• If you have a command running in the Foreground for a long time but you need to write something else on the command line :
  • kill the process
  • suspend the process
  • run it in the background coproc sleep 10 - When complete it gives a message.
• coproc is a shell keyword. No manual entry for it.
  • To learn more about a shell key word use help coproc
  • a running background process can be killed by process id (use : ps --forest to find PID and kill -9 <pid>)
• A command followed by an & means that it is being assigned to the background
  • Executing the command fg will bring it back to foreground
• jobs is a shell builtin - it lists active jobs in the current shell
• top shows processes taking up maximum cpu and memory. Exit gracefully by pressing Q
• Ctrl+z suspends a process.
  • Suspended processes can be seen with jobs
  • Can be brought back to foreground using fg command
• Ctrl+c kills a process
• fg is a shell builtin
• bash -c "echo \$-" creates a child shell, gets the value of echo $-, gives the output to the parent shell
  • bash -c "echo \$-; ps --forest;" - multiple commands separated by ;
  • bash -c "echo \$$ ; ps --forest ; exit 300" : custom error code mod 256 = 44
• history displays a list of commands that have been run on that computer
  • !n executes command line no n displayed by history
  • useful for repeating long commands
  • The H flag in bash means the history is being recorded
• Brace expansion option B
  • if you type echo {a..z} character in the ASCII sequence will be expanded.
  • In combination echo {a..d}{a..d} will display all possible combinations of the 2 alphabets.
  • * exapnds to all the files in the current directory
  • echo D* lists all the files begining with D.
  • Examples :
    • mkdir {1..12}{A..E} or rmdir {1..12}{A..E} or touch {1..12}{A..E}/{1..40}
• ; acts as a separator between individual commands eg : echo hello ; ls

REPLIT CODE WITH US

Link to Replit

• date -d "2024-04-01" +%A - Day of the week for given date
• file --mime-type somefile - mime type of a given file
• mkdir {1..12}{A..E}
• rmdir {1..12}{A..E}
• touch {1..12}{A..E}/{1..40}
• lscpu | grep -i "model name"| cut -d ":" -f "2"