Basic Unix Commands¶
Working with the file system¶
Overview:
- cd - change directories
- pwd - print working directory
- mkdir - make directory
- rmdir - remove directory
- ls - list directory
- cp - copy files
- mv - move files
- rm - remove files
Changing and Making Directories¶
pwd is a command that prints the current directory. Depending on how your shell is configured, your current directory or part of it is displayed in your prompt (the prompt is the bit in your shell that looks like this iMac:pcfb cliburn$). Typically your shell starts you in your home directory, where you would have permissions to write and create files. To change directories you would use cd, the change directory command.
[jacob@moku ~]$ pwd
/home/jacob
[jacob@moku ~]$ cd /tmp
[jacob@moku /tmp]$ pwd
/tmp
[jacob@moku /tmp]$ cd
[jacob@moku ~]$ pwd
/home/jacob
You can use cd without specifying a directory - this returns you to your home directory. You can also use ~ as an alias for your home directory too. Creating directories uses the mkdir command. If you don’t specify a full path (a path starting with a /) it tries to create one in the current directory.
[jacob@moku ~]$ pwd
/home/jacob
[jacob@moku ~]$ mkdir foo
[jacob@moku ~]$ cd foo
[jacob@moku ~/foo]$ pwd
/home/jacob/foo
[jacob@moku ~/foo]$ mkdir /tmp/bar
[jacob@moku ~/foo]$ cd /tmp/bar
[jacob@moku /tmp/bar]$ pwd
/tmp/bar
If you need to make a deep hierarchy of directories all at once, you can use the -p argument to mkdir to create all the necessary preceding directories.
[jacob@moku ~]$ pwd
/home/jacob
[jacob@moku ~]$ cd foo
foo: No such file or directory.
[jacob@moku ~]$ mkdir -p foo/bar
[jacob@moku ~]$ cd foo
[jacob@moku ~/foo]$ cd bar
[jacob@moku ~/foo/bar]$ pwd
/home/jacob/foo/bar
The rmdir command removes directories. Directories must be empty to be removed. Just like mkdir, if a full path is not specified it tries to remove the directory from the current directory. Similar to mkdir, the -p argument tries to remove all the preceding directories
[jacob@moku /tmp/bar]$ cd
[jacob@moku ~]$ rmdir /tmp/bar
[jacob@moku ~]$ rmdir -p foo/bar
[jacob@moku ~]$ mkdir bar
[jacob@moku ~]$ touch bar/foo
[jacob@moku ~]$ rmdir bar
rmdir: bar: Directory not empty
touch is a command that creates an empty file. We will find out about it when we look at working with files
Examining directories¶
Now that we understand directories, we’d want to look at what files the directories contain. ls will list the files in a directory.
[jacob@moku ~]$ ls
A.txt B.txt C.txt bar
[jacob@moku ~]$ ls bar
foo
Just like mkdir, ls has several useful command line options. ls -l will list out all the extra properties of the directory listed (file permissions, owner, last time modified). ls -a will list hidden files (those files whose name begin with a .). ls -F will append directories names / and (along with other symbols after other special file types).
[jacob@moku ~]$ ls -l
total 5
-rw-r--r-- 1 jacob jacob 32 May 25 08:48 A.txt
-rw-r--r-- 1 jacob jacob 32 May 25 08:49 B.txt
-rw-r--r-- 1 jacob jacob 64 May 25 08:53 C.txt
drwxr-xr-x 2 jacob jacob 3 May 23 15:54 bar
[jacob@moku ~]$ ls -a
. .cshrc .mail_aliases .rhosts A.txt bar
.. .login .mailrc .shrc B.txt
.bash_history .login_conf .profile .ssh C.txt
[jacob@moku ~]$ ls -laF
total 20
drwxr-xr-x 4 jacob jacob 16 May 27 12:11 ./
drwxr-xr-x 4 root wheel 5 May 23 15:12 ../
-rw------- 1 jacob jacob 459 May 25 09:32 .bash_history
-rw-r--r-- 1 jacob jacob 1014 May 23 15:12 .cshrc
-rw-r--r-- 1 jacob jacob 257 May 23 15:12 .login
-rw-r--r-- 1 jacob jacob 167 May 23 15:12 .login_conf
-rw------- 1 jacob jacob 379 May 23 15:12 .mail_aliases
-rw-r--r-- 1 jacob jacob 339 May 23 15:12 .mailrc
-rw-r--r-- 1 jacob jacob 753 May 23 15:12 .profile
-rw------- 1 jacob jacob 284 May 23 15:12 .rhosts
-rw-r--r-- 1 jacob jacob 978 May 23 15:12 .shrc
drwx------ 2 jacob jacob 3 May 23 16:15 .ssh/
-rw-r--r-- 1 jacob jacob 32 May 25 08:48 A.txt
-rw-r--r-- 1 jacob jacob 32 May 25 08:49 B.txt
-rw-r--r-- 1 jacob jacob 64 May 25 08:53 C.txt
drwxr-xr-x 2 jacob jacob 3 May 23 15:54 bar/
Working with files¶
Coping files uses the cp command, copying from the first argument (source) to the last (destination):
[jacob@moku ~]$ cp A.txt bar/A.txt
if the destination is a directory it copes the file into the directory.
[jacob@moku ~]$ cp A.txt bar/
[jacob@moku ~]$ ls bar
A.txt foo
You can also copy multiple files at once. cp will copy all the files listed on the command line into the directory specified in the last argument.
[jacob@moku ~]$ cp A.txt B.txt C.txt bar/
[jacob@moku ~]$ ls bar
A.txt B.txt C.txt foo
Globbing will allow us to use many files at once rather than typing them all out explicitly. Globbing is a form of wildcards.
| Glob | Effect |
|---|---|
| * | any number of any character |
| ? | any single character |
| [abc] | one of a, b, or c |
[jacob@moku ~]$ cp *.txt bar/
[jacob@moku ~]$ ls bar
A.txt B.txt C.txt foo
or
[jacob@moku ~]$ cp ?.txt bar/
[jacob@moku ~]$ ls bar
A.txt B.txt C.txt foo
or even
[jacob@moku ~]$ cp [ABC].txt bar/
[jacob@moku ~]$ ls bar
A.txt B.txt C.txt foo
with the -r command line argument you can recursively copy whole directories
[jacob@moku ~]$ cp -r bar foo
[jacob@moku ~]$ ls foo
A.txt B.txt C.txt foo
Similar to the copy command is the move command mv.
[jacob@moku ~]$ mv A.txt foo
[jacob@moku ~]$ mv [BC].txt foo
[jacob@moku ~]$ mv foo/*.txt bar/
[jacob@moku ~]$ mv foo baz
To remove files, use the rm command. A word of caution, there is no trash can or waste basket. Removed files are gone. It is very easy to accidentally shoot your self in the foot when blindly removing files.
[jacob@moku ~]$ rm bar/A.txt
[jacob@moku ~]$ rm bar/[BC].txt
[jacob@moku ~]$ ls bar
foo
[jacob@moku ~]$ rm -rf bar/
the -r command line flag removes files recursively, while -f attempts to ignore permissions on the file. The combination of -r and -f flags can be useful to remove whole directory tree. BE VERY CAREFUL using -r and -f flags.
Working with file contents¶
- cat - concatenate command
- pipes and redirects - Why concatenate prints to the screen
- globbing - Working with wildcards
- less - a more sensible way to look at the contents of file
- grep - searching for patterns in files
- basics of regular expressions
Examining files¶
The cat command will display files on the screen
[jacob@moku ~]$ cat A.txt
This is file A.
It has 2 lines.
[jacob@moku ~]$ cat B.txt
This is file B.
It has
3 lines.
cat will also concatenate files to print to the screen.
[jacob@moku ~]$ cat A.txt B.txt
This is file A.
It has 2 lines.
This is file B.
It has
3 lines.
Using redirects allows us to save the concatenated file.
[jacob@moku ~]$ cat A.txt B.txt > C.txt
[jacob@moku ~]$ cat C.txt
This is file A.
It has 2 lines.
This is file B.
It has
3 lines.
> is a redirect to create a new file (and delete the old file if it exists). >> is the append redirect, while | (pipe) allow you to send the output of one command as input to a new command.
[jacob@moku ~]$ cat [AB].txt
This is file A.
It has 2 lines.
This is file B.
It has
3 lines.
While cat is useful for displaying small files, longer files would page off the screen quickly. To display longer files, a page aware program will be used, less.
[jacob@moku ~]$ less <file name>
Common useful less keys
| key | effect |
|---|---|
| G | Go to the last line |
| 1G | Go to the first line |
| #G | Go to line number # |
| /foo | Search forward for foo |
| ?foo | Search Backward for foo |
| q | Quit less |
Quitting (or how to escape when you are lost)¶
You may at some point find you self lost, and your prompt doing interesting things you don’t expect. Here are some keys to try and get your prompt back in the state you expect.
- q
- Esc
- ctrl-d (sends an end of file saying there is no more input)
- typing Quit
- typing exit
- ctrl-c (sends a break, telling the program to abruptly halt)
Regexp, grep, and searching in files¶
The grep command allows you to tap into the powerful regular expression language to search the contests of file for complex patterns.
[jacob@moku ~]$ grep 'poor Yorick' hamlet.txt
Ham. Let me see. [Takes the skull.] Alas, poor Yorick! I knew him,
The first argument passed to grep is the pattern to search for, in the above example poor Yorick.
Regular expressions provide the ability to search beyond known text, using wildcards to build complex patterns.
| key | Meaning |
|---|---|
| . | any single character |
| + | one or more of the preceding character |
| * | zero or more of the preceding character |
| ^ | matches the beginning of the line |
| $ | matches the end of the line |
| [abc] | matches a singular character of a, b, or c |
so to find all the lines beginning with the word HAMLET and end withs DEMARK
[jacob@moku ~]$ grep '^HAMLET.*DENMARK$' hamlet.txt
HAMLET, PRINCE OF DENMARK
Ever wonder how many lines in hamlet contain eight l’s in them?
[jacob@moku ~]$ grep 'l.*l.*l.*l.*l.*l.*l.*l' hamlet.txt
Till then sit still, my soul. Foul deeds will rise,
all welcome. We'll e'en to't like French falconers, fly at
married already- all but one- shall live; the rest shall keep as
Clown. I like thy wit well, in good faith. The gallows does well.
Man Pages¶
Unix documentation is typically stored in man pages acces by the man command. Try typing man cat into the console to see the manual page for the cat command. Note, man pages are notoriously terse, technical, and often confusing to new users, so while learning it may be better to ask google instead, but if all you want is to know optional command arguments the man page is the first place to look.
Exercise¶
Make a directory in your home folder named spam containing subfolders eggs, bacon, foo and bar and then remove spam/foo and spam/bar