Go back to Main Index . . .
Go back to Linux User's Help Page
1 What is bash, anyway?
bash is a shell, that means a program that sits in between you and Linux.
bash lets you invoke and control processes on the system.
2 How do I start bash?
When you log in, a session of bash is automatically opened for you, to enable you to type in
commands.
A shell script will start its own bash session, for usually it does not know which shell you are using.
With Xwindows, you can have as many bash sessions working for you as you like, though you
really do need just one.
3 Options and parameters
Actually, bash is used to start other processes and handling options and parameters to them.
A process is invoked by its name, followed by
- options,
- parameters,
- and maybe some piping.
This sounds thrilling and complicated, but it's not. I shall discuss this by the example
ls -l *
You know what this does: it gives a long list of the contents of the current directory.
Let's steep a little deeper:
- it invokes the process ls,
- handles option l on it,
- and puts parameter * on it.
It is a really easy syntax, istn't it?
4 Piping input and output
Let us pipe something, temporarily:
ls -l * > ~/myfiles.txt
The > sign takes the output not onto standard output, but into the file myfiles.txt. You should
you less this file now to confirm yourself.
Linux processes tend to read from standard input (the keyboard) and to write processed data
to standard output (the monitor).
So, of course, the input and output has to be redirected ("piped") by you, using ">" for output
files and "<" for input files.
Most programs do without a "<". If you give them a file to process just by its name, most times
they will do.
povray forms an exception. TeX also does:
tex thesis
will seek file thesis.tex.
If it finds it, it will automatically write to thesis.dvi.
If not found, TeX will process file thesis instead. Most likely, this is no TeX file, so TeX
will complain.
Starting
tex
without a filename, it will read from standard input, that is the keyboard, and process and complain
until you have this ended with \bye.
5 Foregrounding, Backgrounding and Killing of processes
- yes
- starts process yes, which throws and endless stream of y's to standard output, that is: the monitor.
- C-z
[1] 456
- stops process yes. yes was process no. 1 in this session of bash, and process no. 456 on the
system.
- fg %1
- starts process no. %1 again, in the foreground.
- C-z
[1] 456
- stops process yes again. Numbers have not changed, this is the same process.
- bg %1
- starts yes in the background.
- jobs
1 yes running
- yes is still running, in the background.
- kill 456
process 456 yes terminated
- process yes is dead.
- jobs
process 456 yes terminated
- process yes is in fact dead.
- yes &
- starts another process yes in the background.
- jobs
[2] 890 yes running
- yes, it's running.
- killall
[2] 890 yes terminated
- another dead process.
killing by [session number], fg and bg are built-in bash commands, not available for other shells.
6 Writing shell scripts
As a very example, create file ll with vi:
#!/bin/bash # this script is for bash.
# a new bash session is started.
ls -l a* > ~/all.a.files # create a list of all files starting with a,
# and store it in all.a.files
Now make ll executable, and run it.
There is syntax including if .. then, repeat .. until, while .. wend et cetera available.
Get your own Free Home Page