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Lecture 1

https://missing.csail.mit.edu/2020/course-shell/

Exercises

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

cd /tmp
mkdir missing

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

man touch

The touch utility sets the modification and access times of files. If any file does not exist, it is created with default permissions.


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

cd missing
touch semester

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.

echo '#!/bin/sh' > semester
echo "curl --head --silent https://missing.csail.mit.edu" >> semester

From the bash documentation:

Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !.

NOTE: Use single quotes if there are $, `, \, or ! characters inside of the string.


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).

./semester
# permission denied: ./semester

ls -l semester
# -rw-r--r--  1 henrikh  wheel  61 Jan 28 12:53 semester

The script is not executable, as the x (execute) bits are missing.


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?

  • with sh, you're running a program that will interpret the lines in your script just as if you would have typed them on the interactive prompt of the terminal. The script doesn't need to be executable.
  • with ./ you're making a shortcut assuming that the script is just right here in the current directory you're sitting in AND it will be executable.

Source: https://askubuntu.com/a/22948


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

The chmod utility modifies the file mode bits of the listed files as specified by the mode operand. It may also be used to modify the Access Control Lists (ACLs) associated with the listed files.


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.

chmod +x semester

ls -l semester
# -rwxr-xr-x  1 henrikh  wheel  61 Jan 28 12:53 semester

#!/bin/sh (shebang line) tells the shell to interpret the script using sh.


10. 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" | cut -d ' ' -f 2- > ~/last-modified.txt

Get the request header, pluck the "last-modified" line from it, split it on space, take all fields starting from the second one, and write the output of the previous command to ~/last-modified.txt.