Missing Semester MIT 1

MIT the missing semester

这门课主要会教授一些课堂上不会讲的东西, 比如debug, shell, vim. 刚好国庆假期将近刷题刷烦了, 来学一下, 毕竟vim和gdb还不太熟练.
课程链接: https://missing.csail.mit.edu/

Okay, let's get started!!!.

environment variables

when type echo hello (in any path), you can see hello in your terminal, but how does the system get the echo program working? when ls in current location, we cannt find a program named echo. And the answer behind this magic is environment variables.

when type:
echo $PATH, the following will show up:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

these are the locations that are registered in the environment variable, when we type echo hello, the system will go to these locations to find whether there is a program named echo, if true, will call it.
eg:
type: which echo

/bin/echo

which means we are using the echo program under directory /bin/

some commands

1 cd change directory:

`cd ~` change dir to user space;
`cd -` change dir to the last location, this is super useful;
`cd ..` go to upper directory;
`cd /` go to root;

2 ls list

`ls` list stuff under cur directory
`ls -l` list stuff using a long listing format including permission, date, owner, group...
`ls -a` list stuff (including hidding file)
`ll` list all stuff from root to cur directory

3 cat show the content of a file, more can do the same thing;
4 df show free disk;
5 free show free memory;
6 top show the usage of computer resources;
7 sudo do something with root previlege;
8 sudo su enter root mode, exit with exit;
9 pwd print working directory;
10 cp copy file to dest;
11 mv move file to dest, if dest in the same dir, this will simply change file name;
12 touch create a file;
13 rm delete file, directory;
14 mkdir create a directory;

permissions

in the pic above, the first letter d shows whether this is a directory, then followed by 9 letters, these are permissions, the first three letters are for owner; the second are for group, the third are for all users.r means read, weighted 4, w means write, weighted 2, x means execute, weighted 1. if we dont have x and try to execute the file, we ll get a permission denied. Fortunately, we can use chmod to change permission, as mentioned above,
chmod 777 means open all permissions(read, write, execute) to all types of users;
chmod 755 means only owner can write this file;

redirection

< input
> output
>> append
| pipe: make the output of left program be the input of right program;

EXERCISES

Create a new directory called missing under /tmp.
Look up the touch program. The man program is your friend.
Use touch to create a new file called semester in missing.
Write the following into that file, one line at a time:
```
#!/bin/sh
curl --head --silent https://missing.csail.mit.edu 
```
The first line might be tricky to get working. It’s helpful to know that # starts a comment in Bash, and ! has a special meaning even within double-quoted (") strings. Bash treats single-quoted strings (') differently: they will do the trick in this case. See the Bash quoting manual page for more information.
Try to execute the file, i.e. type the path to the script (./semester) into your shell and press enter. Understand why it doesn’t work by consulting the output of ls (hint: look at the permission bits of the file).
Run the command by explicitly starting the sh interpreter, and giving it the file semester as the first argument, i.e. sh semester. Why does this work, while ./semester didn’t?
Look up the chmod program (e.g. use man chmod).
Use chmod to make it possible to run the command ./semester rather than having to type sh semester. How does your shell know that the file is supposed to be interpreted using sh? See this page on the shebang) line for more information.
Use | and > to write the “last modified” date output by semester into a file called last-modified.txt in your home directory.

Ans:

1.cd /tmp; mkdir missing

touch semester
echo '#!/bin/sh' > semester; echo curl --head --silent https://missing.csail.mit.edu > semester or echo \#\!/bin/sh
I guess sh have top previlege
sudo chmod 777 ./semester
./semester | grep -i last-modified > ~/last-modified.txt