The “File System”

1. The “File System” • Under UNIX, (almost) everything is a “file”: • Normal files • Directories • Hardware • Sockets • Pipes • Things that are not files: • Users • Groups • Processes

2. File Permissions • Every file has three access levels: • user (the user owner of the file) • group (the group owner of the file) • other (everyone else) • At each level, there are three access types: • read (looking at the contents) • write (altering the contents) • execute (executing the contents)

3. Ownership • Files have two owners • Every file has exactly one user owner • Every file has exactly one group owner • Everyone is a user • Users are in at least one group • Processes have owners, too (known as an “id”) • Every process has exactly one user id • Every process has at least on group id • Users and groups are really just numbers with names • Every username is mapped to a single numeric “uid” • Every groupname is mapped to a single numeric “gid”

4. Who am I? • Commands that tell you who you are: • whoamidisplays your username • iddisplays your username and groups • Commands that tell you who others are: • finger [<name>]displays info for <name> • id [<username>]displays info for <username> • Commands that change who you are: • su <username>“switch user” to <username> • login login as a different user

5. Changing Permissions • The “change mode” command: chmod <level><op><permissions>[,…] <filename> <level> string of: u, g, o, a (user, group, other, all) <op> one of +, -, = (gets, loses, equals) <permissions> string of: r, w, x, s, t, u, g, o (read, write, execute, set-id, text, same as user, same as group, same as other), • Examples: chmod u+rwx,go-w foobar chmod g=u,+t temp/ chmod u=rwx,g=rwxs,o= shared/

6. What is input/output redirection? • Normally, a program’s standard output is displayed on user’s terminal, and its standard input comes from the keyboard. • Redirecting the output of a program means asking the shell to put the program’s output (stdout [C++’s cout]) into a file. • Redirecting the input of a program means asking the shell to feed a file as the program’s standard input (stdin [C++’s cin]). • Note: redirection works with files.

7. How to redirect program’s output? • To redirect just the standard output:<program> > <FILE> • Example: ls –l > root-folders

8. > vs. >> • Both > and >> will create the output file, if it doesn’t already exist • If the file does exist, then: • Using > to redirect output will overwrite the output file: • ls > newlisting • printenv > my_environment • Using >> to redirect output will append to the output file • cat ch1 ch2 ch3 > book • cat ch4 ch5 ch6 >> book

9. Why redirect program’s input? • To run the program repeatedly with the same (or similar input) • Having the program read from standard input may make the program simpler to write.

10. How to redirect program’s input? • Simple!<program> < <FILE> • Examplesort < my_grades.txthead < really_long_book.txt

11. Piping • Piping is connecting programs together by using the output of one program as the input to the next. • Syntax:<program1> | <program2> | … | <programN> • A simple example (view a sorted file-listing a page at a time):ls | sort | less • Note: piping deals with the input/output of programs (that is, stdin, stdout, stderr)

12. Why piping? • Because “the whole is bigger than the sum of its parts. • By combining Unix utilities in a pipeline, you can build tools “on-the-fly” as you need them.

13. Piping examples • How many .c files are in this directory?ls *.c | wc –l • What files were modified most recently?ls –t | head • What processes am I running?ps auxw | grep <mylogin> • Redirection and piping used together • Sort –r < root-folders | more