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Linux Session #2.md

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Session #2

String Processing :

inside /etc/passwd

  • username:password:UID:GID:comment:homepath:terminalpath

  • [:] Called separator(delimiter)

  • Count characters in a file

wc <FileName>

Eample:

wc /etc/passwd

O/P: 47 82 2783 /etc/passwd

  • Line Word Character respectively
wc -c -l -w /etc/passwd

c : Character
l : Line
w : Word

  • Search for lines that contain a specific text in a file
grep <TextToSearch> <FileName>

Example:

grep ahmed /etc/passwd
  • Search for lines that starts with a specific text in a file Example:
grep ^ahmed /etc/passwd
  • Search for lines that {ends} with a specific text in a file Example:
grep ahmed$ /etc/passwd
  • Search for lines that contain a specific insensitive text in a file Example:
grep -i ahmed /etc/passwd

-i for insensitive, it may be Ahmed or ahmed

  • Search for lines that contain a specific text in all files in Cur. Dir.
grep -r Dir1
  • Search for lines that {Do not} contain a specific text in a file
grep -v ahmed /etc/passwd

Invert (Not Contains)
Work also with [-r] (folders)

  • To sort lines in a files
sort -T: -k -n <FileName>

-T: Means that separator is [:]
-k Number of Field to sort with
-n : treat field as a No. if it is a No. not as string (ASCII) {1 Not '1'}
Example:

sort -T: -k3 -n /etc/passwd

Note: add [-r] flag to reverse sort

sort doesn't change file, it just changes output (how its shown in terminal)

sort -T: -k1 -n /etc/passwd > sortedGroup.txt

Save output of first command as a text in file called sortedGroup.txt

  • Show only first,third (for example) fields for each line
cut -d: -f1,2,3,--- <FileName>

Example:

cut -d: -f 1,3 /etc/group
  • Just Print text in terminal
echo hello world
  • Save text in a file
echo hello world > fil1.txt

Piping & Redirection:

ls /etc | more

ls /etc | grep passwd

[|] pipe character

  • Its like executing first command and save it's output in file instead of showing in in terminal then passing this file to second command after pipe character [|]
sudo ls -R / | grep shadow

-R / Means to show all files/Dir. inside root [/] and also inside each Dir. show its files/Dir and so on, so it outputs all files/Dir. in OS

  • sudo : means {super user do}

  • 'Tee' reads the standard input and writes it to both the standard output (terminal) and to save output in a a file + showing output in terminal (if I want)

  • it shows output + saving output in a file

ls /etc |tee file.txt

Examples:

ls /etc | tee etc_content.txt

cat /etc/passwd | grep ^ahmed |tee file.txt | wc -l

O/P: 1

  • Commands are like water running in a pipe

Redirection :

  • Save text in a file (Override it's content)
echo arafat > fil1.txt
  • Concatenate (append) text in a file {in new line} as str = str+text
echo is my best friend >> file1.txt

cat file1.txt

O/P: arafat
is my best friend {in new line}

  • It only save errors in err_file instead of printing it + printing other paths files (which don't have errors) so, its redirecting error in a file instead of printing them
ls /fooo 2> err_file
  • Concatenate (append) text in a file in new line as str = str+text but for redirecting error
ls /fooo 2>> err_file
  • Will only print errors and save rest in fileX.txt, redirecting standard output (terminal)
ls / > fileX.txt
  • Will redirect both standard errors and standard output
ls > fileX.txt 2>&1

1 : standard output
2 : standard error
& between them

  • Note: > is same as 1>
mail -s "Subject" [email protected] < FileName

[<] called standard input redirection
Means that you take text in FileName as an input for command
FileName contains body of mail (instead of writing body in command)
Install its package first

VI Editor:

  • Only editor available in rescue mode (as safe mode in windows)
vi file.txt

  • hit [i] to enter insert mode

  • hit [esc] to enter command mode

  • In command mode:

    • :q! -> to Quit
    • :wq! -> to Write and then quite
    • :w! -> to Write (save)
    • You can use up/down/right/left buttons in command mode

  • Also In command mode:

    • hit [yy] : to copy a line
    • hit [dd] : to cut a line
    • hit [pp] : to paste a line
    • hit [p OR P] : to paste copied line, small p to put copied line under cur line and big P to put it above
    • hit [D] -> Delete a line (from cursor to end of line) H[e]llo wolrd -> H
    • :n,nd -> Delete Range of lines
    • :%d -> Delete all lines
    • /text -> to highlight text in file
    • hit [enter] then [n]down or [N]up to traverse along highlighted text
    • :set nu -> To show number of lines
1
2
3
  • :set nonu -> to hide number of lines

Environment Variables:

x = arafat
echo $x

O/P: arafat

echo $PWD

O/P: /home/arafat/Document

echo $USER

O/P: arafat

echo $SHELL

O/P: /bin/bash
For type of shell, its bash here
bash : means born against shell 

export CURRENT_USER = arafat

Export is used to create environment variables, best practices is to name it with upper case

echo CURRENT_USER

env USER (X) WRONG

printev current_user -> no need to $ sign

  • Note: this is only for current terminal (not permanent), to solve this save it in initialization files ~/.bashrc
nano ~/.bashrc {save it here}
  • To execute it or just close all terminals (as you are reloading page in web so changes can take place)
source ~/.bashrc
  • To print all environment variables
printenv

OR

env
  • To remove variable
unset <VariableName>

Example:

unset CURRENT_USER

Command Quoting:

echo $PWD

Will print path

echo "$PWD"

Will print path

echo '$PWD'

Will print [$PWD] as text
single quote ignores all meta-character

echo '\$PWD'

Will print [$PWD] as text

echo "\$PWD"

Will print [$PWD] as text
Back slashh ignores next meta-character in Double quotes

echo "Today date is" $(date)

$ rm -r $(ls) = rm -r *

$ history

All commands written by you is saved in ~.bash.history

  • !! -> execute previous command
  • !n -> Example: !350 -> execute command No. 350 in history
  • !-n-> Example: !-2 -> previous previous Command (n Times)
alias q = "ls -a"

Remember not permanent, save it in ~/.bashrc to become permanent

$ q

Execute alias

  • Note: if you name it foe example as ls, t it will execute it not default ls command, to fix it instead of closing terminal use \ls backslash ls