Just because you can read a file doesn’t mean that you can change it.
by Linda Mui
WYNTK: UNIX System Admininistrator
- WYNTK: UNIX System Admininistrator
- What you need to know about this book
- This book is not for administrators.
- We do assume some things about the reader.
- This book has seven chapters, each ending in a quick-reference section.
- This series puts technology in context.
- This book is designed to be browsable.
- Contact us to suggest improvements, or for sales information.
- I’d like to thank some people...
- 1. What you need to know about solving problems
- There’s no magic involved.
- For user problems, administrators are more like detectives than like magicians.
- Administrators have many responsibilities; user support is only one of them.
- Educated users get better help.
- Put yourself in your administrator’s shoes.
- Gather the facts before you alert your administrator.
- Recreate the problem.
- Look at the error message.
- Try the most obvious answers .
- If it used to work, what changed?
- Try to identify (and eliminate) as many factors as you can.
- Tell the administrator what you tried.
- Quick Reference: contacting your administrator
- 2. What you need to know about logging in
- If you can’t log in, you can’t do anything else.
- Logging in is a series of steps; each must be successful.
- If the terminal doesn’t seem to be on, check the power supply.
- If the terminal is running, but there is no login prompt, check the cables and brightness control.
- If everything is plugged in and connected, reboot and try again.
- If you still can’t get the terminal to respond, let your system administrator know.
- The computer checks your login name and password.
- If your login isn’t accepted, it’s most likely a typing error. Try again.
- If you still can’t log in, try another terminal to rule out equipment problems.
- Passwords are stored in a database.
- Login accounts may be disabled by the system administrator.
- You can ask another user to check if your account was disabled in the password database.
- If you’ve forgotten your password, you’ll need to ask your system administrator for help.
- Quick Reference of login troubleshooting
- 3. What you need to know about running programs
- You run programs in UNIX by typing them on the command line.
- The error “Command not found” means that the command isn’t in your search path.
- Check if the command was misspelled.
- Most programs reside in bin directories.
- You can run a program using its full pathname.
- You can run a program without its full pathname because of your command “search path.”
- You can find the directory where a program is installed with the whereis or which command.
- Add directories to your search path.
- The command might run, but not work correctly.
- There may be more than one version of the same command on your system.
- When there are two versions of a program, the one executed is the one earliest in your search path.
- You can have “aliases” for commands.
- The which command tells you what version of a program you are using or whether you have an alias.
- If a program isn’t working properly, you could be running a different version of it.
- There might also be permission problems.
- The TERM environment variable tells programs what type of terminal you use.
- If TERM is not set, programs may not run.
- If TERM is set wrong, you may get gibberish.
- Use the man command to learn how to use a command.
- Try the apropos command(or man -k) to find the name of a command.
- Quick Reference of troubleshooting techniques
- 4. What you need to know about sharing files
- On a UNIX system, your files can be shared with other users.
- Security measures make sure that people aren’t reading or editing files that they shouldn’t be.
- Sometimes file security gets in the way of what you want to do.
- Each UNIX file has its own distinct set of permissions to define who can read, write, or execute the file.
- Each file is owned by a particular user.
- Each file is assigned to a particular “group” of users.
- Use ls -lg to find out to what group a file belongs.
- You can find out to what groups a user belongs.
- Each file has three separate sets of permissions: for the owner of the file, the group, and everyone else.
- If you don’t have read permission, you won’t be able to see the contents of the file.
- Just because you can read a file doesn’t mean that you can change it.
- Directory permissions refer to the ability to list the files or create new files.
- Use ls -l to learn the permissions on a file.
- Use ls -ld to see the permissions for a directory.
- If you need permissions changed, you can ask the owner of the file or the system administrator.
- The owner of a file can extend read and write permission to all users.
- The owner of a file can extend read and write permission to just the group.
- The owner of a file can change the group that the file belongs to.
- You can also remove permissions.
- You can change permissions for all files in a directory.
- You can “become” another user to gain their privileges.
- Only the administrator can legitimately change the ownership of a file.
- If you can read a file but can’t write it, you can copy the file and make your changes in the copy.
- If you have already made edits when you learn the file is read-only, you can still save your changes.
- Sometimes several users have to work on several sets of files at once.
- It’s dangerous if two people edit a file at the same time.
- There are systems for making sure that two people aren’t editing the same file.
- Quick Reference to file sharing
- 5. What you need to know about printing
- Printing is a multistep process for the computer.
- Print jobs are sent to the printer by a printer daemon.
- Printing works differently depending on whether you use System V or BSD UNIX.
- Files need to be translated into a format that printers recognize.
- On UNIX, most printers use the PostScript standard.
- A PostScript file has a unique header.
- The lpr or lp command prints PostScript files. Other commands are needed to translate and print other kinds of files.
- If a file doesn’t print, the problem might be the PostScript file.
- Although most of a PostScript file is incomprehensible, you can sometimes read page counts.
- You can control what printer a file goes to, cancel print jobs, or monitor the print queue.
- You can use the lpc status or lpstat -t command to identify printers.
- On BSD-derived UNIX, printers are defined in /etc/printcap.
- Each user is assigned a default printer.
- Change the printer you use on the command line or with the PRINTER environment variable.
- Check the printer queue to see the status of your print job.
- If the printer queue isn’t moving, a large file could be processing.
- You can cancel your print job with the lprm or cancel command.
- Rebooting will fix a “hung” printer.
- Fonts can look wrong if the fonts requested aren’t loaded on a printer.
- Quick Reference to printing
- 6. What you need to know about space and time
- You share space and time with other users.
- Even if your system resources are adequate today, they may not be tomorrow.
- Files are stored on disks.
- On UNIX, disks are mounted as directories, or filesystems.
- Disk space is shared on UNIX systems.
- Some filesystems are mounted from remote machines.
- You can look at disk names and the space available on each disk.
- You can look at how much disk space your files take up.
- You can look at the size of your directories and subdirectories.
- Commands may create temporary files that also take up disk space.
- Clean up your directories occasionally (or when you get a warning).
- Don’t forget to check saved mail messages.
- Always remove a “core” file.
- Look for temporary files that weren’t removed after they were created.
- Remove files that you can easily recreate as needed.
- If you save files, you can compress them to take up less room.
- When you get a message that a disk is full, first find out which filesystem is affected.
- You may be able to retrieve deleted files from backups.
- UNIX systems multitask by working on each process a little at a time.
- When the system gets slow, take an inventory of what you’re doing.
- You can use the ps command to see what processes you’re running.
- On a UNIX machine, you can run many programs simultaneously.
- X users commonly have many programs running at once.
- Some programs are more resource intensive than others.
- Commands with dynamic displays eat up more time than “snapshot” displays.
- X programs are much more resource intensive than character-based programs.
- Some single commands actually spawn multiple processes.
- Look for unwanted processes.
- You can stop processes with the kill command.
- Job control makes killing programs a little easier.
- Be aware of your resources.
- Quick Reference to space and time questions
- 7. What you need to know about everyone else
- UNIX is a multiuser system; you can find out what other users are doing.
- You can find out who is logged in.
- Use who with the more or grep command to find out if a particular user is logged in.
- You can find out what other users are doing with ps.
- Use grep with ps to see what a single person is doing.
- The finger command shows you how long terminals have been idle.
- Finger shows when other users last read their email.
- You can use the finger command to find out what people are doing on other systems.
- You can use finger to find out someone’s login name.
- Don’t believe finger when it tells you when someone last logged in.
- Some users have a .plan file to tell others what they’re up to.
- You can find out who is on a mail alias.
- Quick Reference to finding out about other people
- Index
- Colophon
- What you need to know about this book
If you do have read permission for a file, you may still not be able to edit it. For example, the /etc/passwd file on your system is one that all users can read, but only your system administrator can edit it.
When you have read but not write permission for a file, the file is opened “read-only .” You can open the file in an editor and even make changes within the editor; but if you try to save those changes to the file, you won’t be able to write the file.
-
No Comment
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.