UNIX In 90 Minutes

By Brandon Hutchinson (brandonhutchinson@hotmail.com)

Modified by Tivon Luker to more suit our environment.

Quick overview of essential UNIX commands:

File and Directory Permissions Demystified

When doing a long listing (“ls –l”) of the directory /home/hutchib, we see the following entry:

drwxr-xr-x 2 hutchib nsm 512 Apr 20 11:09 bin

This is a directory named “bin” that is owned by user “hutchib” and group “nsm.” Let’s explore the first 10 characters in detail.

The first character is a “d,” meaning that we are looking at a directory. Other possibilities for the first character include an “-” for an ordinary file and an “l” for a symbolic link (like a “shortcut” in Windows).

The next three characters (characters 2 through 4) correspond to user permissions, or permissions corresponding to the owner of the directory. They are “rwx,” meaning that the directory is readable, writable, and executable by the user—the owner of the directory (hutchib).

Characters 5 through 7 correspond to group permissions. If you are not the owner of the directory, but are a member of the group “nsm,” these permissions apply to you. They are “r-x,” meaning that the directory is readable and executable by any user belonging to group “nsm.”

Characters 8 through 10 correspond to “other” permissions. If you are not the owner of the directory, nor a member of the group, the “other” permissions are applicable. They are “r-x,” meaning that the directory is readable and executable by “everyone else.”

What do read, write, and execute permissions mean on a directory?

• Read – Able to list contents of the directory, and display contents of files

• Write – Able to create, modify, or delete files in the directory

• Execute – Able to change into the directory (even if you had read and write privileges on a directory, if you do not have execute privileges on the directory, you would not be able to “cd” into the directory)

One important thing to remember is that if you have write privileges to a directory, you may delete any file in that directory, even if it is not writable, and is owned by somebody else.

The following is an example of a long listing of a file:

-rwxr-xr-x 1 hutchib nsm 36 Jun 4 10:13 long_cmd.sh

The first character is a “-,” meaning that we are looking at an ordinary file (i.e. a textfile, binary, etc.). The file is readable and executable by user, group, and other, and is user-writable—the owner of the file, hutchib, can write to the file.

What do read, write, and execute permissions mean on a file?

• Read – Able to list the contents of a file

• Write – Able to modify or delete the file (if you have permission to write to a file, you may modify it, but may not delete it, if the directory is not writable)

• Execute – Able to execute the file (needed if the file is a shell script or executable)

How do I change file/directory permissions?

You may change file and directory permissions with the “chmod” command.

The syntax is:

chmod octal-mode file_or_directory

When looking at the following table, remember these values when setting file or directory permissions:

Read = 4

Write = 2

Execute = 1

You may recursively change permissions with the “chmod –R” command.

Useful “vi” commands

The “vi” editor has two modes: “Input” and “Command” mode. In “Input” mode, any characters you type will be entered into the text file. In “Command” mode, characters you type are used to move around the screen, and otherwise modify the file. If you are unsure which mode you are in, press “Esc,” and you will be put into “Command” mode.

File extension primer

You may also use the UNIX command “file” to attempt to determine the file type.

Example:

$ file /home/hutchib/long_cmd.sh

/home/hutchib/long_cmd.sh: executable shell script

$ file /usr/bin/ls

/usr/bin/ls: ELF 32-bit MSB executable SPARC Version 1, dynamically linked, stripped

How do I redirect command output?

Generally, output of your UNIX commands is sent to standard output (STDOUT). You may redirect command output to a file:

command > file

If “file” does not exist, it will be created. If file already exists, it will be overwritten. You could instead append to the end of “file”:

command >> file

In addition to standard output, UNIX has a mechanism called standard error (STDERR). It is sometimes useful to discard standard error when performing commands. For example, when executing a “find” command on a UNIX system, you normally don’t want to see “Permission denied” messages when “find” attempts to read directories you don’t have access to. The following command would redirect standard error to /dev/null, effectively discarding the output:

command 2>/dev/null

If you absolutely do not want the command to generate any output (e.g. a crontab entry), use the following command to send both standard output and standard error to /dev/null:

command >/dev/null 2>&1

How do I put a process in the background?

$ long_cmd.sh

Z[1] + Stopped (SIGTSTP) long_cmd.sh

$ bg %1

[1] long_cmd.sh&

$ jobs

[1] + Running long_cmd.sh

$ fg %1

long_cmd.sh

In the above example, I executed a shell script called “long_cmd.sh” that would take several minutes to complete. I cannot interact with the terminal while this script is executing, so I pressed “Ctrl-Z” to suspend the script. After suspending the script, I put it in the background with the “bg %1” command. I list the background processes with the “jobs” command. The background process would have continued to execute and would have completed; the shell will notify you when the job has completed. For this example, I put the job back in the foreground with the “fg %1” command.