Lecture 1 - Course overview + the shell

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Exercises

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.

As mentioned ealier I am using zsh in Ubuntu-20.04 using WSL2 on a Windows 10 machine. echo $SHELL gives me this output /usr/bin/zsh. If you are using bash you’ll get the following output /usr/bin/bash.
Create a new directory called missing under /tmp.
```
mkdir /tmp/missing
```
Look up the touch program. The man program is your friend.
```
man touch 
```
Use touch to create a new file called semester in missing.
```
touch /tmp/missing/semester
```
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.
```
echo '#!/bin/sh' > semester ; echo 'curl --head --silent https://missing.csail.mit.edu' >> semester
```
echo '#!/bin/sh' > semester writes the first line as a literal string to semester as we are using single quotes. Double quotes are special in bash/zsh. They are used for variable expansion and other things.
; is used to string together multiple commands such that they are executed sequentially from left to right.
echo 'curl --head --silent https://missing.csail.mit.edu' >> semester appends the 2nd line to the 1st one in the file. Notice the >> operator.
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).

ls -l will show all the files in the current directory along with permission details for the user, owing group and others. The semester file created by us only has read/write permissions. We don’t have execute permissions.
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?

The explanation from a similar question on Stackoverflow is as follows:

Running it as ./script.sh will make the kernel read the first line (the shebang), and then invoke bash to interpret the script. Running it as sh script.sh uses whatever shell your system defaults sh to (on Ubuntu this is bash, which is sh-compatible, but doesn’t support some of the extra features of Bash).

Check out the link for more details.
Look up the chmod program (e.g. use man chmod).
```
 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.
```
chmod  +x semester
```
This gives execute permissions to the user, group and others. If you want to restrict execute permissions to say only to the user/owner of the file use chmod u+x. Alternatively, you can replace u with g (group) or o (other) depending on your use-case.

For the second part of the question the wikipedia link mentioned above says the following:

When a text file with a shebang is used as if it is an executable in a Unix-like operating system, the program loader mechanism parses the rest of the file’s initial line as an interpreter directive.
Use | and > to write the “last modified” date output by semester into a file called last-modified.txt in your home directory.
```
 ./semester | grep "last-modified" > ~/last-modified.txt
```
The STDOUT we get with ./semester is piped through grep which tries to match the "last modified" substring. Once it gets the match the shell takes that line and writes it to the last-modified.txt file located in the home directory.
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.

Functionality not available with WSL.