The Shell

How To Use Shell !

Basic Command

date: return current date

echo: print something

echo "Hello World"

echo Hello\ World: when the string contain a space, use \ to translate

echo $PATH: show all the path value

which + command: return the location of command

e.g:which echo

Absolute Path and Relative Path

pwd: return current directory cd: enter a directory

.: current directory ..: parent directory

e.g:

cd ./temp : cd ../src :

cd / : enter root cd ~ : enter current home cd - : toggle between current directory and last directory

ls : list files in current directory

ls -l : detail of ls

execution of ls -l /:

lrwxrwxrwx   1 root root          7  4月 23 19:24 bin -> usr/bin
drwxr-xr-x   4 root root       4096  5月 19 15:17 boot
drwxrwxr-x   2 root root       4096  4月 23 19:26 cdrom
drwxr-xr-x  20 root root       4140  5月 14 20:42 dev
drwxr-xr-x 143 root root      12288  5月 27 13:31 etc
drwxr-xr-x   3 root root       4096  4月 23 19:27 home
lrwxrwxrwx   1 root root          7  4月 23 19:24 lib -> usr/lib
lrwxrwxrwx   1 root root          9  4月 23 19:24 lib32 -> usr/lib32
lrwxrwxrwx   1 root root          9  4月 23 19:24 lib64 -> usr/lib64
lrwxrwxrwx   1 root root         10  4月 23 19:24 libx32 -> usr/libx32
drwx------   2 root root      16384  4月 23 19:24 lost+found
drwxr-xr-x   2 root root       4096 10月 22  2020 media
drwxr-xr-x   2 root root       4096 10月 22  2020 mnt
drwxr-xr-x   2 root root       4096 10月 22  2020 opt
dr-xr-xr-x 386 root root          0  5月  5 10:14 proc
drwx------   3 root root       4096  4月 25 11:24 root
drwxr-xr-x  32 root root       1040  5月 28 14:04 run
lrwxrwxrwx   1 root root          8  4月 23 19:24 sbin -> usr/sbin
drwxr-xr-x   2 root root       4096  4月 23 19:34 snap
drwxr-xr-x   2 root root       4096 10月 22  2020 srv
-rw-------   1 root root 2147483648  4月 23 19:24 swapfile
dr-xr-xr-x  13 root root          0  5月  5 10:14 sys
drwxrwxrwt  42 root root      12288  5月 28 14:09 tmp
drwxr-xr-x  14 root root       4096 10月 22  2020 usr
drwxr-xr-x  14 root root       4096 10月 22  2020 var

Ls File Format

lrwxrwxrwx 1 root root 7 4月 23 19:24 bin -> usr/bin

First Charater meaning

l: symbolic link file;d: directory; -: normal file; b: block; p: pipe; s: socket; c: character

three character pair

r: read w: write x: execute

PS:- means you don't have the permission

first three pair: permission for user root next three pair: permission for group root last three pairn: permission for everyone else

File execution command

mv foo.bar bar.foo : move(rename) foo.bar to bar.foo cp bar.foo ../ : copy bar.foo into parent directory rm ../bar.foo: remove bar.foo from parent directory mkdir My\ code: make a folder named My Code rmdir My\ code/:remove the folder named My Code

man + command: the manual description of command

ctrl + l: move current prompt to top

Stream

input and output

>: output stream <: input stream

echo hello > hello.txt: write string hello into hello.txt cat hello.txt: show the content of hello.txt cat < hello.txt: cat use hello.txt as input cat < hello.txt > hello2.txt: cat from hello.txt and redirect to hello2.txt cat < hello.txt >> hello2.txt: append hello.txt to hello2.txt echo "something" >> hello2.txt : append something into hello2.txt

Pipe

|: pipe

ls -l / | tail -n1 : return the last line of the ls -l /

curl --head --silent google.com:

HTTP/1.1 301 Moved Permanently
Location: http://www.google.com/
Content-Type: text/html; charset=UTF-8
Date: Fri, 28 May 2021 07:23:47 GMT
Expires: Sun, 27 Jun 2021 07:23:47 GMT
Cache-Control: public, max-age=2592000
Server: gws
Content-Length: 219
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN

curl --head --silent google.com | grep -i Content-Length:

return : Content-Length: 219

curl --head --silent google.com | grep -i Content-Length | cut --delimiter= ' ' -f2:

return: 219

Root User

sudo + command: do command as super user

/sys/class: directory contains a lot of sys config files

echo 500 > /sys/class/backlight/intel_backlight/brightness : permission denied sudo !! : still permission denied why? because sudo is used on echo, while > doesn't care about the previous command.

$: means you're not running as root #: means you're running as root

sudo su: turn into root shell

tee: write something to a file

echo 1060 | sudo tee brightness: now brightness is 1060

xdg-open file: open a file with correct application

Exercises

All classes in this course are accompanied by a series of exercises. Some give you a specific task to do, while others are open-ended, like “try using X and Y programs”. We highly encourage you to try them out.

We have not written solutions for the exercises. If you are stuck on anything in particular, feel free to send us an email describing what you’ve tried so far, and we will try to help you out.

1.For this course, you need to be using a Unix shell like Bash or ZSH. If you are on Linux or macOS, you don’t have to do anything special. If you are on Windows, you need to make sure you are not running cmd.exe or PowerShell; you can use Windows Subsystem for Linux or a Linux virtual machine to use Unix-style command-line tools. To make sure you’re running an appropriate shell, you can try the command echo $SHELL. If it says something like /bin/bash or /usr/bin/zsh, that means you’re running the right program.

2.Create a new directory called missing under /tmp.

3.Look up the touch program. The man program is your friend.

4.Use touch to create a new file called semester in missing.

5.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.

# bobby @ BobbydeMBP in /tmp/missing [21:14:21] 
$ touch semester

# bobby @ BobbydeMBP in /tmp/missing [21:14:32] 
$ echo "#!/bin/sh" > semester 
zsh: event not found: /bin/sh

# bobby @ BobbydeMBP in /tmp/missing [21:14:47] C:1
$ cat semester 

# bobby @ BobbydeMBP in /tmp/missing [21:15:01] 
$ echo '#!/bin/sh' > semester 

# bobby @ BobbydeMBP in /tmp/missing [21:15:20] 
$ cat semester 
#!/bin/sh

# bobby @ BobbydeMBP in /tmp/missing [21:15:24] 
$ echo "curl --head --silent https://missing.csail.mit.edu" >> semester 

# bobby @ BobbydeMBP in /tmp/missing [21:16:03] 
$ cat semester 
#!/bin/sh
curl --head --silent https://missing.csail.mit.edu

6.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).

# bobby @ BobbydeMBP in /tmp/missing [21:16:17] 
$ ./semester 
zsh: permission denied: ./semester

# bobby @ BobbydeMBP in /tmp/missing [21:16:56] C:126
$ ls -l semester 
-rw-r--r--  1 bobby  wheel  61 10  5 21:16 semester

7.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?

• when we use sh command, we take semester as input, what we really did was executing sh command, and sh is executable, so it worked.

# bobby @ BobbydeMBP in /tmp/missing [21:17:04] 
$ sh semester 
HTTP/2 200 
server: GitHub.com
content-type: text/html; charset=utf-8
last-modified: Sun, 29 Aug 2021 15:44:11 GMT
access-control-allow-origin: *
etag: "612bab4b-1f31"
expires: Tue, 05 Oct 2021 13:27:51 GMT
cache-control: max-age=600
x-proxy-cache: MISS
x-github-request-id: 4C3C:7D12:6B9300:749FA1:615C507F
accept-ranges: bytes
date: Tue, 05 Oct 2021 13:17:51 GMT
via: 1.1 varnish
age: 0
x-served-by: cache-hnd18720-HND
x-cache: MISS
x-cache-hits: 0
x-timer: S1633439872.565752,VS0,VE149
vary: Accept-Encoding
x-fastly-request-id: fc0471f99b294b1f9bc9a71654c37e578904967f
content-length: 7985

8.Look up the chmod program (e.g. use man chmod).

9.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.

# bobby @ BobbydeMBP in /tmp/missing [21:18:50] 
$ ls -l semester 
-rwxr-xr-x  1 bobby  wheel  61 10  5 21:16 semester

# bobby @ BobbydeMBP in /tmp/missing [21:18:54] 
$ ./semester 
HTTP/2 200 
server: GitHub.com
content-type: text/html; charset=utf-8
last-modified: Sun, 29 Aug 2021 15:44:11 GMT
access-control-allow-origin: *
etag: "612bab4b-1f31"
expires: Tue, 05 Oct 2021 13:27:51 GMT
cache-control: max-age=600
x-proxy-cache: MISS
x-github-request-id: 4C3C:7D12:6B9300:749FA1:615C507F
accept-ranges: bytes
date: Tue, 05 Oct 2021 13:18:58 GMT
via: 1.1 varnish
age: 67
x-served-by: cache-hnd18732-HND
x-cache: HIT
x-cache-hits: 1
x-timer: S1633439939.541585,VS0,VE1
vary: Accept-Encoding
x-fastly-request-id: 199d79a9ffc3c879c1367fc960d99ffd126b1b2b
content-length: 7985

10.Use | and > to write the “last modified” date output by semester into a file called last-modified.txt in your home directory.

# bobby @ BobbydeMBP in /tmp/missing [21:18:59] 
$ ./semester | grep -i last-modified > ~/last-modified.txt

# bobby @ BobbydeMBP in /tmp/missing [21:19:58] 
$ cat ~/last-modified.txt 
last-modified: Sun, 29 Aug 2021 15:44:11 GMT

11.Write a command that reads out your laptop battery’s power level or your desktop machine’s CPU temperature from /sys. Note: if you’re a macOS user, your OS doesn’t have sysfs, so you can skip this exercise.

• I'm using MacBook Pro, so, no can do ):