Lecture 3

File permissions:

• ls -al lists long form of listing alongside of hidden files, of all files in current directory
• ls -l shows: permissions owner group size (bytes) datelastmodified filename

For permissions: it is a 10 digit string

 -rw-r----- # first first char, then it is user group other

It goes read, write, execute

The first character in permissions is a “type” can be - (file) and a d means (directory)

Permissions are three groups of 3 bits and when all set, are:

(-|d)rwxrwxrwx

1. user bit applies to files owner
2. group bit applies to files group
3. other bit applies to everyone else

r: read
w: write
x: execute

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For DIRECTORIES:

• r: means we can ls the directory, use globbing, and tab completion
• w: we can add or rm files from the directory
• x: means the directory can be navigated (can cd into it). If a directory’s executable bit is not set, regardless of the other read or write bits, access to the that directory, nor any file within it, are set

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• • for FILES:**
• r: can be read
• w: contents can be modified
• x: file’s contents can be executed as a program

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How can we change permissions? We have a cmd for that:

chmod Mode file

The mode consists of 3 parts:

• User types:
  • u = user
  • g = group
  • o = other
  • a = all
• operator:
  • • add permissions
  • • subtract permissions
  • = set permissions exactly (the equal signs not in quotes are simply relay the definitions of the symbols, will lose other permissions that prev. existed
• permissions
  • r = read
  • w = write
  • x -executable

\#e.g. say we ls -l and get this:
 
ls -l
 
-rwx------ 1 a7yin cs246  77 May  9 13:15 testing.txt

\#running:
chmod u=rwx,go+rw testing.txt

This will give group and other read and write permissions. The resulting permissions looks like:

 -rwxrw-rw- 1 a7yin cs246  77 May  9 13:15 testing.txt

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Shell Scripts

Files containing a sequence of shell commands executed as a program

Example: Print the date, current user, and current directory

 #!/bin/bash -> shebang line

This line tells OS #!/bin/bash is the location of the program you should use to interpret this file.

Give the file executable permissions:

• cd.. means go back one directory
• cd../ means go back to the parent directory of the cwd

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Bash can have variables

x=1 #(no spaces here)
echo $x #(dollar means fetch the value of)

So $ with no brackets is fetch value, with bracket is subshell

NOTES:

• Use $ when fetching the value of variable
• No $ when setting a variable
• Good practice:
• ${x} also means fetch the value of x
• This is good practice because if I want to print “1st” echo $xst wont work, need echo ${x}st

Ofc putting the above substitution inside of ” will not change it

---

LAST NOTE:

All bash variables are strings, e.g. x contains the string “1”

$expansion

Occurs in the double quotes, but not in single quotes

• *Special was: **

$1 $2,..,etc - cmd line arguments

Example: check whether a word is in the dictionary or not

In our file, we would write:

 #!/bin/bash
 egrep "^${1}$" user/share/dict/words

So, we egrep any line in the dictionary (every line is just the words found in a dictionary) that starts and ends with the first command line argument.

e.g.

./isitaword hello

This prints nothing if word not found, otherwise, prints the word

Example: A good password should not be in the dictionary

 egrep "^${1}$" user/share/dict/words > /dev/null
 #(output where we dont care about)
# redirecting stout to /dev/null supresses stdout.
# Every program returns a status code to the OS when finished
# $? - the satus of the most recently executed command
 
# how to interpet this return alue? we can use ifs
 
 if [ $? -eq 0]; then #(THE SPACES ARE VERY IMPORTANT)
    echo Bad password
 else
    echo Maybe not a terrible password
 fi

• • WE NEED TO END IF STATEMENTS WITH FI**

As seen above, a return value of 0 is success and anything other than a 0, is a fail. So in the above code snippet, as the passed password command line argument is not in the dictionary, the return value is not 0.

 -eq is for arithematic equality

• *Example: ** Verify correct # of args, print how to use the program otherwise

#!/bin/bash
 
egrep "^{1}$" /user/share/dict/words ? /dev/null

(1) usage: write a function. good idea to make usage of a fn, in case you need # to print it multiple place

$0 is current running program

---

 #!/bin/bash
 usage() {
            echo "usage: ${0} password"
        }
 
 if [ $# -ne 1]; then
    usage
    exit 1
 fi

$# is a special variable in bash, that expands to the number of arguments (positional parameters) e.g $1, $2 ... passed to the script in question or the shell in case of argument directly passed to the shell

---

if .... elif .... elif ....else

For all available comparisons and other conditions, see the reference sheet given on Learn.

LOOPS:

Print numnbers from 1 to $!

 #!/bin/bash
 
 x=1
while [ $x -le $1]
do
  echo $x
  x=$((x + 1))       #$((...)) sytnax for arithematic
done
 
# Looping over a list
 
 for x in a b c; do echo $x; done
 
# (prints a b c)
 
# for loops
 
\#version 1
for x in a b c; do
  echo $x; done
# prints out a (new line) b (new line) c
 
# we need the ; when we write this function on one line, else, it
# could look like this.
 
\#version 2
for x in a b c; do
  echo $x
done
# personally, I like the second option better, more spaced out
 
for x in "a b c"; do echo $x; done
# prints out a b c (the "" make it an entire string)
 
for x in $(ls); do echo "File:$x"; done
# prints out File: and the name of every file in the current dir.

---

The loop below will rename all .cpp files to .cc

for name in .cpp; do
   mv $name ${name%cpp}
   # ${name%cpp}  menas give me everything in name without the cpp
done

To run a file in bash, we need to give ourselves executable permissions to the file through chmod. Then, we need ./ in front of the executable file since we need the path to the file to run it.

END OF LECTURE 3