By Geoff Richards
http://training.gbdirect.co.uk/courses/linux/
Talk in Portable Document Format (PDF)
1. The Unix and Linux Command Line
- Typed commands were first used for necessity
- Only interface to the computer was a simple and slow terminal
- But the command line is still a powerful and efficient interface for many types of work
2. Pros and Cons
- The command line still has its place:
- Typing commands can be quicker than getting to the same functionality with a mouse
- Commands can be very powerful, in ways GUIs rarely are
- Newer shells have many features to make commands easier to type
- But graphical interfaces are also useful:
- The command line makes no sense for some things, like editing graphics
- Easier to get into — you can see what options are open to you
- Both command line and graphical interfaces have a place in modern Unix and Linux
3. What’s Involved
- The command line relies on three things:
- The terminal, which displays the output of commands and echos what you type
- The shell, which reads commands and works out what to do about them
- The commands themselves — Unix and Linux have a vast array of small programs which can be run from a shell
4. The Terminal
- Lets you ‘get at’ the shell to work with it
- Usually a program which opens a window in a graphical environment, like xterm, konsole, gnome-terminal, …
- A program displays text on the terminal, and receives key-presses from it
- Communication done with a simple protocol, using escape sequences for special things
- The shell you are using might be connected over a network
- Things like rsh and (much better) ssh make this work
- So the command line is just as powerful when used on a remote machine
5. The Shell
- The shell is your interface to the machine
- On Linux the most commonly used shell is Bash
- The ‘Bourne again shell’ — a reincarnation of the original Unix shell by Steve Bourne
- Compatible with the original, but has many new features
- The shell does three things every time you enter a command:
- Reads lines of text from the user
- Interprets commands and works out what to do with them
- For most commands, runs one or more programs to do the work
6. Some Examples of Commands
$represents the prompt — don’t type it- Ask a philosophical question:
$ whoami geoffr
- Copy a file:
$ cp report.txt report-backup.txt
- View a PostScript file:
$ gv unix_command_line.ps
- Output a message:
$ echo hello, world hello, world
7. Filename Completion
- Commands are often used to manipulate or examine files
- So there’s a lot of tedious filenames to type…
- The shell can do a lot of the typing for you:
- Type the start of a filename
- Type TAB
- The shell will type the rest of the name, if there’s only one file whose name starts like that
- For example, if the only file starting with rep is called report.txt:
$ less rep<TAB>
- If there are several files which Bash thinks you might mean, press TAB again to get a list of possible completions
8. Other Completion Goodies
- Completion also works for commands
- Type the start of a program name and see what it completes to
- Handy for finding programs when you were only guessing they existed
- As the shell completes filenames, it escapes special characters automatically
- Makes it much easier to type filenames containing spaces and such
- After a
$symbol, the shell completes variable names
9. History
- Another handy feature of modern shells is history
- The shell keeps a record of each command you type
- Pressing Up and Down keys scrolls through the history, retrieving each command
- Emacs users may prefer to use Ctrl+P and Ctrl+N
- After going back to a command it can be edited and rerun
- It is possible to search back through the history
- Type Ctrl+R, and then any part of the original command line
- The shell can save the history between logins
10. Globbing
- The shell can collect filenames for a command to use
- Write a glob pattern, and the shell will find files whose names match it
- For example, to delete all files in the current directory:
$ rm *
- To count words in all the files whose names end in .txt:
$ wc *.txt
*means “anything is allowed hereâ€- Other symbols can be used too:
?means “any single character is allowed hereâ€[a-z]means “any lowercase letter can go hereâ€
11. Redirection
- A command can be ‘connected‘ to input and output files
- The shell does this, setting it up before running the appropriate program
- Done with the
<and>symbols
- For example, to write a copy of this year’s calendar to a file:
$ cal 2003 >2003.txt
<is the same, but reads from the file
12. Piping
- Similar to redirection — but connects a program to another program
- Use the
|symbol — often called the ‘pipe character’ - For example, pipe the output of
echointo the programrev(which reverses each line of its input):$ echo Happy Birthday | rev yadhtriB yppaH
- The output of the
echocommand is fed into the input
ofrev:
13. Shell Scripting
- Put a load of command lines into a file, and it’s called a shell script
- Same idea as a Windows ‘batch file’, but much more powerful
- Shell scripts can use looping, variables and other programming features
- But these can also be used interactively
- Useful for running the same command on many files, or creating a whole set of users in one go, or…
- If you end up typing the same sequence of commands over and over again, turn them into a shell script
- But if you want to write a ‘full size’ program, learn Perl
14. So…
- Unix and Linux shells can be complicated and tricky, and are certainly feared by the uninitiated
- But those who are practiced at them find them powerful and efficient
- Not as much typing is involved as you might think
- Completion saves your fingers
- History lets you go back and try again
- The shell is not for everyone, but well worth learning for anyone serious about Linux