Command

apropos
```
apropos "remove files"
```
whatis
```
whatis rm
```
man
```
man man
```
You can use f or b to move forward or backward and use / can be used to search for specific term and press enter to search for next occurrence.
```
/browser
```
q can be used to quit.
```
man 7 man
```
here 7 is the section which you want to look at.
info
```
info git
```
help
```
help if
```

Files

The Linux File Hierarchy Structure or the Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix-like operating systems.

ls

-l: vertical view

-a: show hidden files as well

-R: recursively go inside each directory
file <filename>
stat
which tells the location of the program
touch creates a file with the given file name
pwdprint working directory
cd is used to change the directory
cp is used to copy. cp source destination. Last argument is the destination, so you can copy more than one file.
mv is used to relocate the file/directory. Also used to rename
```
mv myfile myfile2 #rename
mv myfile2 Documents #move
```
rm is to delete a file

mkdir creates a directory(Cannot be space separated)

mkdir my directory #creates two directory
mkdir "my directory" #created a directory
mkdir my\ directory2 #creates a directory

rmdir removes the directory
truncate is used to create files of different sizes
```
truncate -s 1MB file1
```

find

find . -name apple #search if file named apple exists
find . -name "*apple*" #search for files apple in the file name 
find . -size -10M #files with size less than 10M
find . -size +10M #files with size greater than 10M
find . -name "lemon" -type d #returns only directory

f for file, l for link, c for character device, bfor block device.

echo takes text as input and returns it as output
```
echo "krishna"
```
> replaces the contents but >> appends to the file.
| chains the commands. send the output of one command as input to other.
```
ls | wc
```
<< Is for hearing
```
wc << over
> This so something
> over
```

stderr has code 2

ls docs 2> errout.txt #if output is there then it saves in errout.txt

find / -name "home" 2> error.txt 1> output.txt

diff is to find the difference in two files

diff -y file1.txt file2.txt
diff -u file2.txt file2.txt

cmp to compare byte by byte

cmp file1.txt file2.txt #first difference
cmp -l file1.txt file2.txt #all difference

hexdump

glob

* zero or more character
? one character

nano

ctrl + e end of line
ctrl + a start of the line
ctrl + j to justify
M is esc
^ is shift
ctrl + _ to go to a line number
ctrl + k to cut
ctrl + u to paste

Vim

Two modes: command and insert

Command modes takes key strokes as command and insert mode let you type in a document.

i is used to go to insert mode. esc to go back to command mode.

:w to save

:q to quit

In command mode use / and then text to search.

Use ^ to move to the beginning of a line and $ to move to the end.

Links

Hard link and Soft(symbolic) links
Hard link point to inode (address).
Soft link points to the file name(renaming breaks the soft link)

ln -s /home/krishna/users.txt Documents/user-list.txt

ln is for link, -s is for symbolic and then full absolute path and then destination. Relative path will is not used a system will use the relative path from where the sym link is, so it will point to users.txt in Documents

Hard Link:

ln /home/krishna/users.txt Documents/list.txt

Regex

z{2} search for zz
\. search for .

^ beginning of line $ end of line

cat users.txt  | grep -E ".*"
cat users.txt  | grep -E "a"
cat users.txt  | grep -E "[Aa]"
cat users.txt  | grep -E "[A-M][a-g]"

sed: stream editor

cat users.txt | sed 's/e/d/' #s is for substitute (replace the first `e` with `d`)

cat users.txt | sed 's/e/d/g' #`s` is for substitute (replace all `e` with `d`) and `g` is for greedy

cat users.txt | sed 's/[A-Ea-e]/_/g'

cat users.txt | sed 's/\ *//g' #gets the first name

awkis for reformatting

cat users.txt | awk '{print $2}' #print item 2
cat users.txt | awk '{print $2 ", " $1}' #print item 2 , item 1
cat users.txt | awk '{print $2 ", " $1}' | sort

SSH

sudo apt install openssh-server
ip a #to check ip address
ssh user@ip

File transfer

SFTP: SSH File Transfer Protocol

sftp user@ip
get file3 #to download
put fil3 # to upload

SCP: Secure Copy Protocol

scp user@ip:<location_file> . #download
scp file4 user@ip:<location>  #upload

krishnadey30/LinuxBasicCommands.md