The Unix Commandline

var="some value"
export ENVAR="some global value"

command_out=$(some command)
echo $var
echo $ENVAR
echo $command_out

command -l # short form flags
command -la # 2 short form flags
command --long # long form flags
command --input INPUT # example of input

$ pwd

$ cd ~/your/directory
$ pwd
/home/$USER/your/directory
$ cd ~ # cd /home/$USER
$ pwd
/home/jeet
$ cd ..
$ pwd
/home

$ ls
$ ls -a
$ ls -l
$ ls -la

$ touch file

$ touch existing-file -d month/day/year

$ mv SOURCE DESTINATION

$ ls
file1
$ mv file1 file2
$ ls
file2

$ ls
dir1 dir2
$ mv dir1 dir2
mv: cannot move 'dir1' ro 'dir2' : Directory not empy
$ mv dir1 dir2 -f
$ ls
dir2

$ cp SOURCE DEST

$ ls
file1
$ cp file1 file2
$ ls
file1 file2

$ ln source-file dest-file
$ ls -l
-rw-rw-r-- jeet jeet ... source-file
-rw-rw-r-- jeet jeet ... dest-file

$ ln source-file dest-file -s
$ ls -l
-rw-rw-r-- jeet jeet ... source-file
lrwxrwxrwx jeet jeet ... dest-file -> source-file

$ rm file -i
rm: remove regular file 'file'? # your input here
# no more file
$ rm dir
rm: cannot remove 'dir' : Is a directory
$ rm dir -r
# all good

$ mkdir dir

$ mkdir dir1/dir2/dir2
mkdir: cannot create directory 'dir1/dir2/dir3': No such file or directory
$ mkdir dir1/dir2/dir2 -p

$ cat file
# contense of file
$ cat file1 file2
# contense of both files

$ head file -n 20

$ tail file -n 20

$ ls -l
-rw-rw-r-- ... file
$ chmod +x file
$ ls -l
-rwxrwxr-x ... file

$ chmod -rwx file
---------- ... file

echo "a string"
a string
echo -e "this is a \n multiline string "
this is a
 multiline string
echo $VAR
# contense of VAR

$ cat file
fox
fax
fex
$ grep "fox" file
fox
$ grep "f.x" file

$ grep "fox" file -v
fax
fex

$ sed 's/REGEX/REPLACEMENT/FLAG' file

$ cat file
the fox did fox things to say fox you to big fox
$ sed 's/fox/wolf/' file
the wolf did fox things to say fox you to big fox

$ sed 's/fox/wolf/g' file
the wolf did wolf things to say wolf you to big wolf

$ sed 's/fox/wolf/g' file -i

$ cat file
the wolf did wolf things to say wolf you to big wolf

some-pattern { some action }
another-pattern { a different action }
{ all lines match this action }

$0 ~ /some regex/ {
    print $1, $2 # for example
}

$1 == 18 {
    print $2, $1
}

$ cat file
this file is has space seperated words
it has multiple lines
$ awk '{print $0}' file # we just made a slower less convenient cat!
this file is has space seperated words
$ awk '{print $1}' file # print the first element of each line
this
it
$ cat file.csv
name,age,job
Jeet,18,Being a Nerd

$ awk -F, '{printf("name:%s job:%s age:%s", $1, $3, $2)}' file.csv
name:Jeet job:Being a nerd age:18

$ cat file-o-numbers
1
15
2
30
$ sort -n file-o-numbers
1
2
15
30

$ cat file
line 1
line 2
line 3

$ wc file
3 6 24
$ wc -l
3
$ wc -w
6
$ wc -c
24

$ apt update
Permission denied
$ sudo apt update
# works

$ w

 02:26:18 up  8:52,  2 users,  load average: 0.94, 1.29, 1.15
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
jeet     :1       :1               17:34   ?xdm?   4:07m  0.01s /usr/libexec/gdm-x-session --run-script env GNOME_SHELL_SESSION_MODE=pop /usr/bin/gnome-session --session=pop
root     tty4     -                02:24    1:52   0.02s  0.01s -bash

$ pgrep emacs
23878
23878

$ pgrep firefox
6997

$ pkill emacs # waits for emacs to finish

$ pkill 23878 -9 # kills emacs instantly

$ man command

$ cat file1 file2 > file1+file2

$ cat non-existant-file 2> /dev/null # no errors and no output

$ some-output >> some-log-file

$ cat file
Jeet,18
Jen,19
Jeff,20
Jerome,16
Dan,18
Mark,17

$ cat file | grep -i "j." | sort -d | awk -F, '$2 >= 18 {print "name:", $1, "age:", $2}'
name: Jeet age: 18
name: Jen age: 19

gist_list=$(gh gist list)
gist_to_find=$(echo "$gist_list" | awk '{print $2}' | fzf --layout=reverse)

gh gist edit $(echo "$gist_list" | grep $gist_to_find | awk '{print $1}')