Unix Files and Operations Barry Britt System Support

Unix Files and Operations Barry Britt, System Support Group Department of Computer Science Iowa State University

Statement: In UNIX (Linux), everything is a ____. FILE

File Operations mv cp mkdir rm → Move or Rename a file → Copy file to a new location → Create a new folder → Remove a folder → Remove files and folders Others: cat → concatenate files stat → display file/directory information ln → Create hard/soft links

mv Usage 1 – Rename files/directories: mv <file> <new-filename> mv <directory> <new-directoryname> Renames file to new-filename or directory to newdirectoryname. This ONLY changes the name, not the file or directory contents.

mv, cont Usage 2 - move files/directories: mv <filename> <directory> mv <directory> <new-subdirectory> Moves <filename> into <directory> or directory into new-subdirectory Can rename the file while moving

mv - concerns You CAN completely overwrite files using the mv command. You can prevent this by using some commandline switches: -b → make a backup of the destination file before moving -i → interactive. Prompt me before moving a file

cp cp <file> <new-filename> Creates <new-filename> and populates with all data from <file>. This makes an entirely new file cp <filename> <directory> Create new file with path <directory>/<filename> Can rename the file while moving

cp - concerns You can STILL completely overwrite files using the cp command. You can prevent this by using some commandline switches: -b → make a backup of the destination file before copying -i → interactive. Prompt me before copying a file Copy directories? Use 'cp -R <dir> <new-dir>'

mkdir / rmdir mkdir <directory> → Make a new directory. if the parent directory doesn't exist, mkdir will NOT create the directory rmdir <directory> → Remove an empty directory. If the directory is not empty, rmdir will NOT remove the directory

rm rm <file> Remove a file rm -r <list-of-files-and/or-directories> Recursively remove files and directories By default, rm does NOT remove directories. You have to use 'rm -r' to remove directories.

rm - concerns rm will not prompt you if you want to delete a file unless you use the '-i' switch (interactive mode) rm will not prompt you at all if you use the '-f' switch (force) What do you think the following command does: rm -rf /

rm – concerns, cont Answer: It depends. . On later versions of Linux, rm will NOT remove '/' On earlier versions of UNIX/Linux, rm doesn't care what directories you remove. Be sure to check the man page for the default rm options. . . (--preserve-root or --no-preserve-root)

cat <file> Print contents of <file> cat <file 1> <file 2> <file 3> Print contents of <file 1> followed by contents of <file 2> followed by contents of <file 3> Very useful for seeing what is in a small file. Can also use shell redirects.

cat, cont. cat file 1 file 2 > file 3 Print contents of file 1, followed by contents of file 2 and put them into file 3. So, logically, file 3 is just a joining of file 1 followed by file 2

Unix Files (Most) files exist on hard drive. File system keeps information on where to find all files in what is called an 'inode'. The file name is completely separate from the 'inode'. We call of this “accounting information”

stat <filename> Display file information Size Disk Blocks # Links Inode number Privileges/permissions (coming soon. . . ) Access/Modify/Change times

stat, cont. Has GREAT information about the file. All of the file information (except file name) is contained in the inode. What to remember? Links Concept of an 'inode' → pointer to a location on the hard drive

ln Ln [-s] <source> <link-name> Create a new link (file or directory): Hard link -or. Soft link

Soft Links An indirect pointer to a file's contents. A soft link does NOT copy data. A soft link will work anywhere on the hard drive on any partition. Most common type of link. Think of it as an alias to a file or directory.

Soft Links, cont. If a soft link is deleted, nothing happens to the file. If the link source is deleted, the soft link becomes invalid!!!!! You CAN use soft links for directories.

Hard Links A direct pointer to a file's contents. A hard link does NOT copy data. A hard link will NOT work across different disk partitions. Think of this as a direct reference to the file's inode.

Hard Links, cont. If a hard link is deleted, nothing happens to the file. If the link source (original file) is deleted, the hard link DOES NOT become invalid. You can NOT use a hard link for a directory.

Links So, what if: 1) Create a file 2) Create a hard link to the file Will the hard link be valid?

Links, cont. So, what if: 1) Create a file 2) Create a hard link to the file 3) Create a soft link to the original file Then, what if we “rm file”? Will the soft link be valid?

Links, cont. Questions. . . When is a file “deleted”? What does “deleting” a file mean?

Permissions Unix is a true multiuser Operating System, so. . . - How do you keep people from accessing files that aren't theirs? - How do you only allow certain people to access files? - If everything in UNIX is a file, how do you give permission to EVERYONE to access a file? Concept of permissions. . .

Permissions, cont. EVERY file has a set of permissions. EVERY user has an identification number (UID) EVERY group has an identification number (GID) EVERY user is a member of at least one group.

Permissions, cont. Let's take the output of an 'ls -l': drwxr-xr-x -rw-r—r-- 2 bbritt. . 1 bbritt. . stat file What about the first 10 characters of these lines?

File/Device Bits - → ordinary file d → subdirectory l → symbolic link s → socket (CS 352) b → block device (disk drive) c → character device (tty, keyboard, mouse)

User mode bits r → read bit w → write bit x → execute bit Special: s → Set. UID bit is in place of the X (execute) bit. It allows the program to be run as that user. Example → /usr/bin/passwd (has to run as root) Mode is '-rwsr-xr-x'

Group Mode Bits r → read bit w → write bit x → execute bit Special: s → Set. GID bit is in place of the X (execute) bit. On a directory, it ensures that all files/directories created as subdirectories inherit the group (but not necessarily the permissions).

Other Mode Bits Note: Other, meaning everyone else. . . that is, you are not the owner of the file or in the file's group. r → read bit w → write bit x → execute bit Special: t → Sticky bit. Takes the place of the execute bit. When used, tells the OS that only the owner can delete the file or directory.

ls example, again. . . drwxr-xr-x 2 bbritt. . Red: File/Device bit Green: User Mode bits Blue: Group Mode bits Yellow: Other Mode bits Purple: Owner Turquoise: Group stat

Read Bit If user has read permission, they can view the contents of a file or directory. Does not affect whether you can read files in a directory.

Write Bit If not set on a file, you cannot write to the file. If not set on a directory, you - cannot create or delete files or directories within a directory. - cannot rename files within or move files within the directory. - cannot link files in the directory Basically, you can't do anything that would involve changing the contents of that directory.

Execute Bit If the x bit is not set on a file, you cannot directly execute that file. If the execute bit is not set on a directory, you cannot access files within that directory.

Permission Functions, cont. Some interesting combinations: -rwx--x--x : Group and Other can cd into a directory, but not 'ls' the contents. Group and Other cannot modify the directory Where would these be useful?

chmod: change the mode (permissions) on a file or directory. You can use this executable in two ways.

chmod, usage 1 Format 1, symbolic mode: <mode> → 'r', 'w', 'x' for read, write, execute <who> → 'a', 'u', 'g', 'o' for all, user, group, other Usage: chmod <who>±<mode> Example using (-rwxr-x---): 'chmod o+rwx' Sets perms to (-rwxr-xrwx) 'chmod g-rx' Sets perms to (-rwx------)

chmod, usage 2 Format 2, octal mode: That is, (rwx) = (22 + 21 + 20): - or, for those non-mathematically inclined r = 4 = 22 w = 21 x = 1 = 20 So, (rwx) = 7 (rw-) = 6 just add them up. . . (r—) = 4 (---) = 0

chmod, usage 2, cont. Example using (-rwxr-x---): 'chmod 757' Sets perms to (-rwxr-xrwx) 'chmod 700' Sets perms to (-rwx------) 'chmod 772' Sets perms to (-rwxrwx-w-)

chmod with special bits Adding sticky bit: chmod 1777 <dir> will set mode (-rwxrwxrwt) chmod a+rwxt <dir> does the same thing chmod 2755 <file> will set mode (-rwxr-sr-x) chmod u+rws, go+rx <file> is the same chmod 4777 <file> will set mode (-rwsr-xr-x) chmod a+rwx, u+s <file> is the same

Examples User: neo Group: hackers Listing: drwx--x--x -rwxr-x--- 2 trinity hackers (size/date) redpill/ 2 trinity (size/date) smith Can neo 'cd' into the 'redpill/' directory? Can neo get a directory listing of the files in 'redpill/'? Can neo read the 'smith' file?

More Examples Take the UNIX '/tmp' directory: Needed by all users for program execution: X windows Temporary files so. . . directory should be mode (drwxrwxrwx). Are there any drawbacks to this? ?

A few more programs chown → change owner chgrp → change group Only 'root' can 'chown' files to a different user. Only the file owner and root can 'chgrp'. If file owner, the owner must be in the group you want to 'chgrp' to.

chown <user>[: <group>] <file|directory> -R → recursive, operates on all files and directories (group is optional, but allowed) chgrp <group> <file|directory> -R → recursive, operates on all files and directories