Sci_comp_3_Ignatiev.txt

Exercise: Compare the output of ls vs ls -l and ls -a; describe how they are similar, and how they are different.
ls -l forces the ls to use the long listing format, which makes it display not only the file names, but also the file sizes in kilobytes
and the creation/update dates. Like the regular ls, ls -l ignores the file names that start with a dot.
ls -a makes the program display the files, the names of which begin with a dot. Inside a linux-like system it means, that not only the user-created
files, but also the system-created files and directories will be included in the output. Unlike ls -l, ls -a does not change the format of the output, 
so no additional information gets provided.

Explain pushd and popd; what data structure represents your directory history? Give an example of using them to organize a folder with music
The directory history is represented by a stack. A stack is a data structure which combines several objects into a row, like the linked list does.
The difference from the linked list consists in the fact, that normally it is only the last or the "upper" element of the stack that the user can
either add or pop (get and remove). This structure is useful for organizing sequences, like the directory history.
Since the pushd and popd support setting and getting elements at arbitrary positions in the stack, they can be used to implement a "straightforward" and 
a "shuffled" manner of traversing a music folder (a directory that includes several other directories, like the albums). 
It means that for iterating music directories in a shuffled way the current directory should always
be pushed to a random position in the stack, while always popping the upper directory.

Exercise: Draw a partial tree of your filesystem, starting from the children of your home directory. 
Include ancestors of your home directory, and siblings of those ancestors. Exclude files, just show directories.

|bin/  
|boot/  
|dev/  
|etc/  
|home/
|	|ruthenian8/
|	|	|.cache/
|	|	|.config/
|	|	|.gnupg/
|	|	|.local/
|	|	|projects/
|	|	|.ssh/
|	|	|tests/
|	|root/
|init/
|lib/  
|lib64/  
|lost+found/  
|media/  
|mnt/
|opt/  
|proc/  
|root/  
|run/  
|sbin/  
|srv/  
|sys/  
|tmp/  
|usr/  
|var/

Exercise: Write a shell script that asks the user for their name, and greets them. Sample interaction:
#!/bin/bash
while [ true ];
do
echo "what is your name?"
read -r key
if [ -n "$key" ]
then
echo "hello $key"
break
exit 0
fi
done;

Exercise: Write a shell script that performs "ROT13" (Caesar cipher with shift 13.) For English, encryption and decryption are the same! (Explain why!)
#!/bin/bash
if [ $1 ];
then
string="$1"
var1=( {a..z} )
var2=( {n..z} {a..m} )
output=""
for (( i = 0; i < ${#string}; ++i ));
do
        ch="${string:i:1}"
        for n in "${!var1[@]}"; do
        if [[ "${var1[$n]}" = "${ch}" ]]; then
        var3="${var2[$n]}";
        output="$output$var3"
        fi
        done;
done;
echo "$output";
unset var1 var2 var3 output;
exit 0;
else
echo "no argument given";
exit 0;
fi

Encryption and decryption by 13 are the same in English, since English is a satanic language (no) and since its alphabet contains 26 letters, 
which (yes) makes encryption and decryption symmetrical.

Exercise: Write a shell function that prints "hidden" if the current directory starts with a dot ".", 
or if any parent starts with a dot. (Files and directories that start with dots are considered "hidden" on many UNIX-like systems.)
#!/bin/bash
DIR=`pwd`
echo $DIR
if [[ $DIR == *'/.'* ]];
then
echo "hidden"
exit 0
else
echo "not hidden"
exit 0
fi