missing_class

Table of Contents

• Paths and permissions
• Long listing
• Move, copy, create
• Streams and Redirection
• root user
• Exercises
• Solutions

bash

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

Bourne again shell

• man baz: give manual page for baz
• -h, --help, Windows: /?: giving these as arguments typically gives some text on usage and options
• foo\ baz: the backslash is used to escape single characters
  • Quoting reference
  • use of 'foo' inhibits interpretation of a sequence of characters
  • use of "foo" inhibits most of the interpretation of a sequence of characters
• ctrl+l: clear terminal and go to the top
• ./: execute file

Paths and permissions

• echo $PATH lists the path environment variable, a list of paths in which the shell looks for a binary you are trying to execute
• which foo indicates the absolute path of foo that would be executed if you try to run it
• pwd gives you the path
• . the current directory
• .. the parent directory
• ls lists files in current directory
• ~ expands to the home directory
• cd - changes to the previous directory you were in

Long listing

• ls -l: long listing of files in directory

As an example:

sinkers@DESKTOP-HQ8VENU:~$ ls -l
total 16
-rw-r--r-- 1 root    root      65 Feb  5 17:27 hello_world.c
-rwxrwxrwx 1 sinkers sinkers 8304 Feb  5 17:28 hello_world.out

• column 1: file type and permissions
  • first letter: file type
    • d: directory
    • l: symlink/soft link; pointer to another location in the file system
    • -: file
  • following letters gives user permissions:
    • first 3: owner
    • next 3: group
    • last 3: anyone else
  • permission values on a file
    • r: read from the file
    • w: write to the file
    • x: execute the file
    • -: do not have permission
  • permission values on a directory
    • r: list contents of directory
    • w: rename, create, remove files from directory
    • x: “search”; are you allowed to enter this directory? to enter a directory you need to have execute permission on all parent directories and directory itself
• column 2: owner
• column 3: group
• column 4: file size [/bytes]
• column 5: last modified date-time
• column 6: file name

Move, copy, create

• mv old_path new_path: move file at old_path to new_path
• cp: copy
• rm: remove
  • rm -r: recursive remove
  • rmdir: remove directory if empty
• mkdir: make directory

Streams and Redirection

(Simplifying) Programs have two main streams:

• input: this is fed to the program; by default it is what you type in the terminal
• output: this is what comes out of the program; by default it is printed to the terminal

Shell gives you ways to redirect those streams:

• < foo: rewire input to preceding program to be contents of foo

• > baz: rewire output of preceding program to baz

• cat: prints contents of file

• cat < hello.txt > hello2.txt: the effect is to copy hello.txt to hello2.txt. But cat is unaware of the redirection, the shell handles the redirect.

• >>: append
• |: pipe; take output of program to left as input of program to right
• tail: prints last n lines of input
  • tail -n1: print last line
• $ ls -l / | tail -n1: prints last line of long file listing; programs again unaware of redirection

root user

• superuser, gets to do whatever they want
• sudo: do as su; do as superuser
• #: pound means “run this as root”
• $ means you are not root!
• sudo su: gets you a su shell

In /sys is a bunch of parameters for devices on the computer. Say we were in backlight directory:

$ echo 500 > brightness
bash: brightness: Permission denied

But would if we use `sudo

$ sudo echo 500 > brightness
bash: brightness: Permission denied

Here sudo applies to the echo command, so when it gets redirected brightness is not executed with the shell running as root

• tee: takes input, and writes it to a file, but also to the standard output

  $ echo 500 | sudo tee brightness
  500
  

  Now this works: here output from echo gets piped as input for tee, which is run with sudo, so it has permission to write to brightness file

Exercises

1. Create a new directory called missing under /tmp.
2. Look up the touch program. The man program is your friend.
3. Use touch to create a new file called semester in missing.
4. 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.

5. Try to execute the file. Investigate why it doesn’t work with ls.
6. Look up the chmod program.
7. Use chmod to make it possible to run the command ./semester.
8. Use | and > to write the “last modified” date output by semester into a file called last-modified.txt in your home directory.
9. 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.

Solutions

1. $ mkdir /tmp/missing
2. $ man touch. Changes file timestamps to current time
3. touch semester

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

5.  $ ./semester
    -bash: ./semester: Permission denied
    $ ls -l
    total 0
    -rw-rw-rw- 1 sinkers sinkers 96 Feb  7 01:06 semester
   

   There are no execute permissions set for anyone.

6. $ man chmod: chmod changes file mode bits chmod calculator

7.  $ chmod u+x semester
    $ ls -l
    total 0
    -rwxrw-rw- 1 sinkers sinkers 96 Feb  7 01:06 semester
   

   Now semester can be executed

8.  $ ./semester | grep Date > ~/last-modified.txt
   

9.  $ cat /sys/class/power_supply/battery/capacity
    98