Basic Shell

$PATH is basically a list of locations where my Shell is going to search for programs to run everything I run a command.

pwd = print_working_directory cd = change_directory cd - = quick switch prev directory

ls [OPTION]... [FILE]... = [] is for optional, … is for any number of,

if no values provided the arguement is called a flag, and if value has to provided its called an option.

drwxrwxrwt = d indicates directory, next 3 indicate perms for owner, for group and for everyones else, x means that its executable?

mv = can rename or move, mv old_name new_name cp = copy files, cp file new_file_name

ctrl+l = clear shell

> and < and >> and | = steam modifiers, basically keyboard is default input and console print is default put, you can change the stream of a program using angle brackets. and >> is for appending and | is for making output of one input of another eg) cat < hello.txt > copied_file.txt

tail = outputs the n amount of lines, can be used in a pipe operation grep = for searching, but very useful at the end of the command chain using | eg)

curl --head --silent google.com | grep --ignore-case content-length | cut --delimiter=' ' -f2

xdg-open = opens files with appropriate program

---

Advanced Shell

Shell considers spaces really important so can’t use those freely. foo=bar and foo = bar are not the same. The latter implies foo program is called with arguements.

• $0 - Name of the script
• $1 to $9 - Arguments to the script. $1 is the first argument and so on.
• $@ - All the arguments
• $# - Number of arguments
• $? - Return code of the previous command
• $$ - Process identification number (PID) for the current script
• !! - Entire last command, including arguments. A common pattern is to execute a command only for it to fail due to missing permissions; you can quickly re-execute the command with sudo by doing sudo !!
• $_ - Last argument from the last command. If you are in an interactive shell, you can also quickly get this value by typing Esc followed by . or Alt+.

ls *.sh = finds all the files with .sh extension, maybe you can use regex patterns here? mkdir {abc,bcd} = expands into two seperate mkdir commands with each of them?

? = matches only one character in scripts

#!/user/local/bin/python = shebang, can use it to just call a file directly from the shell as such ./script.py without having to specify python3

grep -R stuff -c 10 /

Finding Directories:

find . -name src -type d = find all directories named src find . -path '*/test/*.py' -type f = find all python files that have a folder named test in their path find . -mtime -1 = find all files modified in the last day find . -size +500k -size -10M -name '*.tar.gz' = = find all zip files with size in range 500k to 10M

Regex:

• . means “any single character” except newline
• * zero or more of the preceding match
• + one or more of the preceding match
• [abc] any one character of a, b, and c
• (RX1|RX2) either something that matches RX1 or RX2
• ^ the start of the line
• $ the end of the line

General Command Line:

CTRL C is SIGINT, a signal to interrupt basically, a signal to the process that is working

CTRL \ is SIGQUIT, signals are an IPC cocnept, used to communicate between processes on a low level

kill is used to send SIGTERM for termination

Ctrl-Z will prompt the shell to send a SIGTSTP signal, short for Terminal Stop (i.e. the terminal’s version of SIGSTOP)

To background an already running program you can do Ctrl-Z followed by bg. Note that backgrounded processes are still children processes of your terminal and will die if you close the terminal (this will send yet another signal, SIGHUP)

Port-forwarding a server to redirect traffic to me:

ssh -R 80:localhost:80 tinyserver

You specify, that a connection made to the port 80 of tinyserver is to be forwarded to port 80 on your local machine. That means if someone connects to the small and slow server with a webbrowser, he gets the response of the webserver running on your local machine. The tinyserver, which has not enough diskspace for the big website, has no webserver running. But people connecting to tinyserver think so.