Working with bash shell
bash has several features that make command-line usage easier
and more effective. You do not have to always type the whole command-line
character by character. When you are typing the command, you can go
backwards and forward by using the left and right arrow
keys. You can delete the character left of the cursor with Backspace
key. Pressing Ctrl-d or the Delete key deletes the character under
the cursor. In addition to these basic command-line editing functions,
you can use the keyboard commands listed in the table below.
| Command | Function |
|---|---|
Ctrl-a |
Move the cursor to the beginning of the command-line |
Ctrl-e |
Move the cursor to the end of the command-line |
Ctrl-k |
Cuts all the characters right of the cursor |
Ctrl-y |
Paste the characters to the command-line (cut with Ctrl-k) |
In bash the executed commands are stored in user's home directory in a
file called .bash_history. To see the full list of recently executed
commands, give command history. On the command-line, you can browse
the list of previous commands with the up and down arrow keys.
In cases where you need to give similar commands several times, it is
often handy to get one of the previous commands to the command-line with
the arrow key. Then you can edit just the modifications needed to the
old command and execute the modified command by pressing Enter.
It's possible to search through the command history. Press Ctrl-r, then
type what you want to search for. You do not need to type a complete
search as the search results will update with each character pressed.
To move between the search results, use the up and down arrow keys.
To run the command, press the Return key. If you
wish to edit the search result before running the command, use the
left arrow key, then edit the command.
Automatic Tab completion
If you press the Tab key, the bash shell tries to complete the command or
argument you are writing. The completion is done as far as possible
using the list of available commands and files. Using auto-completion
is very recommended as it saves the user from typing all command or
argument characters and also takes care that commands don't get mistyped.
For example, let's say we are in a directory where we have two files:
final_research_report_old.pdf and final_research_report_new.pdf.
Tip
This will save you a lot of typing and typos!
To open the latter of the files with Evince program, we would need to type the command:
This command would require you to type 36 characters. However, by using Tab completion you need to type only 5 characters. First type
and then press the Tab key. Normally at CSC, Evince is the only available program or command that starts with letters evi, so when the Tab completion is executed, it knows to complete the rest of the command:
Then, to define the file name, you can type just the first letter of the file name
When you now press Tab, the completion process checks which files starting with f are available. In this case there are two of them and as the beginning of the two file names is the same string, the command can now be completed to:
Now you just need to type one n to the end of file name to distinguish the file from the old version,
When the Tab key is now pressed again there is only one option that matches the beginning of the argument that has been typed, and thus the command is completed and ready to be executed:
Stopping programs and running programs in background
In Linux, graphical interfaces and commands that are not interactive once they have started, can be executed as background processes. When the command is executed as a background process, the command shell does not wait until the command is finished. Instead, it remains active and allows the user to submit new commands while the background command gets executed. However, note that in the computing clusters of CSC, heavy computing tasks should not be executed as background processes, but they should be submitted to the batch queue system.
In normal interactive usage you can launch the command to be executed in
the background by adding & character to the end of the command. For
example, the command
would open the Eye of GNOME image viewing program (requires X-term
connection) to the background so that the command shell could
still be used even though the eog command is still running. A background
process can be changed to normal, foreground process with command
fg. When a command and program is running interactively, i.e. the
command shell is waiting that the execution finishes, you can terminate
the execution by pressing Ctrl-c. Another possibility is to
halt the program by pressing Ctrl-z. When a command (or process) is
halted, it can be continued with fg command or changed to be executed
as a background process with command bg.
A command that is still running can be terminated with kill
command. To be able to use kill, you need to know the process
identification number (PID) of the command you want to terminate. You
can check your active processes, meaning the commands that you are
currently executing, with command ps. For example:
$ ps
PID TTY TIME CMD
385 pts/12 00:00:00 tcsh
2001 pts/12 00:00:00 eog
2003 pts/12 00:00:00 gconfd-2
2203 pts/12 00:00:00 ps
By default, the ps command shows only those processes that have been
launched from the command shell you are currently using. To see all your
processes in the server you have logged in to, give command:
Once you have identified the correct process number, you can kill the process with command:
For example, the command
would kill the eog process, listed in the output of the previous ps
example. You can only kill processes that are owned by your account.
Sometimes when you want to kill a process that is malfunctioning, the
normal kill command may not able to terminate the process. In those
cases you can try to terminate the process by adding option -9 to
the kill command: