Appearance
Lecture 1
The Unix Philosophy
- Provide tools that do one thing well
- Let you combine these simple tools into a more powerful Unix commands
- Treat all communication as being via files
- A file is modeled as a character stream, for example
- The keyboard input & screen output
- Input from or output to a file by redirection or pipeline
Introducing Unix commands
Commands consist of
- A name
- Optional flags
- Arguments
For example: Flag -n display files with line numbers.
| Name | Flag | Argument |
|---|---|---|
cat | -n | filename |
For any command, you can easily find the usage with flags and arguments by
sh
man [command name]
Viewing Files
| Command | Description |
|---|---|
cat [filename] | Display a file on screen |
cat -n [filename] | Display a file with line numbers |
less [filename] | Display a file as a book (you can scroll back & forth) |
head -n [filename] | Display the first n lines |
tail -n [filename] | Display the last n lines |
paste [filename] | cat concatenates vertically while paste pastes horizontally |
Useful but less important commands
| Command | Description |
|---|---|
tac [filename] | cat file upside-down (cat->tac) |
rev [filename] | cat file horizontally reversed |
fold [filename] | Fold long lines for finite width output device |
Navigating in Unix
| Command | Description |
|---|---|
ls | list files and directories in a directory |
cd [directory] | change current directory |
pwd | print working directory |
Absolute & Relative addressing
For example
sh
/ (root)
├── bin
├── etc
├── home
│ ├── alice
│ │ ├── document *
│ │ ├── download
│ │ ├── image
│ │ └── video
│ └── bob
│ ├── document
│ ├── download
│ ├── image
│ └── video
├── tmp
└── usr
If we are currently under the directory alice and want to change to the directory the star noted, we can use
| Absolute Path | Relative Path |
|---|---|
cd /home/alice/document | cd document |
Also, if I logged in as Alice, /home/alice equals ~alice and ~. So I can go to alice's document from anywhere by cd ~/document, which is an absolute path.
If we are at the directory alice and want to go to bob's document, we can use cd ../bob/document. Because .. means the parent directory (. means the current one, that why we used ./ to execute executable under the current directory)
Or we can simply just use cd ~bob/document
File Creation and Deletion
| Command | Description |
|---|---|
cp [src] [dst] | copy files |
mv [src] [dst] | move files |
rm | remove files |
mkdir | create directory |
rm -r | remove files recursively (remove directory) |
If you use cp or mv with more than two arguments, the last one must be a directory and rest of the files will be copied into that directory with the name intact
Wildcards
*can replace any strings with arbitrary length?can replace exactly one charator.
For example
| name | effect |
|---|---|
a* | all files starting with a |
*a* | all files with a in their names |
*.cpp | all files with .cpp extension |
????? | all files with 5-charater names |
You can use them with a set or a range as well
| name | effect |
|---|---|
[abc]* | all files with the name contains a, b or c |
[a-c]* | same as above |
[^a-c] | all files with the name not contains a, b or c |
[ab^c] | all files with the name contains a, b, ^ or c |
Remarks
- You might think that the order w.r.t the range is based on ASCII but it's not.
- It depends on the shell. Some shell would allow you to do
a-Bbut notA-b. - The inverse
^only works if it is the first character, otherwise it would be treated as a key.
Exercise
Assume
sh
% ls -l
-rw-r--r-- 1 user group 0 Feb 23 23:24 file1
-rw-r--r-- 1 user group 0 Feb 23 23:24 file2
-rw-r--r-- 1 user group 0 Feb 23 23:24 zfile
Perform
sh
cp *
Problem: What's the effect?
Answer
Error, because what * expands into will be file1 file2 zfile, but zfile is not a directory hence can't perform the copy.
Assume
sh
% ls -l
-rw-r--r-- 1 user group 0 Feb 23 23:38 file1
-rw-r--r-- 1 user group 0 Feb 23 23:38 file2
drwxr-xr-x@ 2 user group 64 Feb 23 23:38 zdir
Perform
sh
cp *
Problem: What's the effect?
Answer
file1 and file2 will be copy into zdir, like below
sh
.
├── file1
├── file2
└── zdir
├── file1
└── file2
Managing Files and Directories
| Command | Description |
|---|---|
ln -s | create a symbolic link |
chmod | change file permissions |
find . -name | search for a file recursively |
diff | compare two files |
fgrep | fixed string search |
ln
Symbolic link:
- Is a neew i-node pointing to the file's data block
- Deleting a symbolic link will not effect the original file
- Deleting or renaming the file a symbolic link points to will leave you with a link that points to nothin, a.k.a a hanging link
- Symbolic links can span across file system(disk partitions)
- Works with files and directories
chmod
Recall
sh
-rw-r--r-- 1 user group 0 Feb 23 23:38 file1
rw-r--r-- among it represents the permission of UGO, user, group and others, respectively. Here, everyone can read, only user can write and nobody can execute it.
To be clear, if all of the permissions are enabled, it will be rwxrwxrwx.
The owner of the file, user here, can use chmod to modify the permissions. chmod can be followed by [identities]+/-[permissions].
To enable all the permissions for everyone, use
sh
chmod ugo+rwx [target]
or don't specify the identities, which is the same as ugo
sh
chmod +rwx [target]
To disable readability from group and others, use
sh
chmod go-r [target]
If you are familiar with the encoding, [1, 2, 4] are for [x, w, r], respectively. So rwx will be 7, rw will be 6, etc.
To enable all the permissions for everyone and to disable readability from group and others, use
sh
chmod 777 [target]
chmod 733 [target]
diff
diff takes two text files and shows their differences. It's super useful when your professor keeps modifying an announced assignment whose instrucions are so long and monotonous. Then diff would be a good friend of yours keeping you from reading the entire instructions over again.
Useful flags
| Flag | Description |
|---|---|
-c | To display changes in context |
-q | Be quiet, just print whether the files have differences |
-y | Side by side comparison |
-W=num | Set the display width, useful with -y |