Topic 2: Finding Your Way on a Linux System

shell

a program that enables text-based communications between OS and user

Bourne-again shell

another name for the Bash shell

Bash Shell

also known as the Bourne Again Shell, this is the default command line interface in Linux.

csh or tcsh (enhanced)

The C-shell (csh) uses syntax similar to the syntax used in the C programming language.

ksh

The Korn Shell (ksh) was designed to take the best features of the Bourne shell and the C shell and extend them. It has a small but dedicated following among Linux users.

zsh

The Z shell (zsh) takes shell evolution further than the Korn Shell, incorporating features from earlier shells and adding still more.

username@hostname current_directory shell_type

The prompt for an interactive shell

~

When in a prompt, it represents the current user's home directory

$

shell_type meaning regular user

#

shell_type meaning root superuser

command line structure

command [option(s)/parameter(s)...] [argument(s)...]

internal command

A command that is a part of the shell and does not exist as a file on disk. There are about 30 of them. E.g. cd, set, export, ls

external command

a command that is in an individual file as a binary program or script. When run, the shell uses the PATH variable to search for an executable file with the same name as the command. Can be created by user

type

command which shows what type a command is.

$ type echo

echo is a shell builtin

$ type man

man is usr/bin/man

quoting

a feature which encapsulates data. "data", 'data', or escape characters are valid. Usually used for special characters or spaces. Each one acts differently

"data"

Double quotes take the data and treat it as regular characters. Most special characters lose meaning (all except $, \`). Variables, command substitution, and arithmetic functions can be used

$ echo I am $USER

I am tom

$ echo "I am $USER"

I am tom

$ touch new file

$ ls (simplified)

new

file

$ touch "new file"

new file

'data'

Single quotes treat ALL characters as regular characters, no exceptions

$ echo I am $USER

I am tom

$ echo 'I am $USER'

I am $USER

\

escape characters. These remove special meaning from individual characters.

$ echo $USER

carol

$ echo \$USER

$USER

echo

output text on terminal

ls

list the contents of a directory

touch

creates an empty file, or updates an existing file's modification date

hostname

change or show a system's hostname

local variables

variables available to current shell process only

environment variables

variables available both in this session and in its sub processes. Most are in capital letters (PATH, DATE, USER). Default variables are usually this type.

Can be accessed using $variable_name or at the front of a command e.g. ($: TZ=EST date)

$ variable_name=value

creates a local variable

accessing variables

putting a $ in front of variable's name access it's value

$ greeting=hello

$ echo greeting

greeting

$ echo $greeting

hello

unset variable_name

removes a variable

export

turns a local variable into an environment variable. Can be done with or after making a local variable.

$ export greeting=hey

IS THE SAME AS

$ greeting=hey

$ export greeting

PATH

variable holds a list of directories that contain executable programs. new directories can be appended by using a colon

$ PATH=$PATH:new_directory

appends new directory to the PATH variable

$ which command_name

finds where the given command is found

$ env

displays the current environment

/usr/share/doc/

Directory containing additional documentation files, such as readme files and support for third-party software.

$ man command_name

command that provides a manual ("man") page. It displays the command's documentation

--help

an option that, when run with a command, displays brief instructions about the usage of the command

Q

to exit the man page, press this key

man sections

There are 11:

1 NAME

2 SYNOPSIS

3 DESCRIPTIONS

4 OPTIONS

5 ARGUMENTS

6 FILES

7 EXAMPLES

8 SEE ALSO

9 DIAGNOSTICS

10 COPYRIGHT

11 BUGS

NAME

Command name and brief description

SYNOPSIS

Description of the command's syntax

Description

Description of the effects of the command

Options

Available options for the command

Arguments

Available arguments for the command

Files

Auxiliary files of the command

EXAMPLES

a sample of the command line with the command

SEE ALSO

Cross-references of the command

DIAGNOSTICS

warning and Error messages and what they mean

COPYRIGHT

Author(s) of the command

BUGS

any known limitations or bugs of the command

less

a command that displays content in a somewhat interactive format. search with /linux (forwards of cursor) or ?linux (backwards of cursor). Type N to jump to next match. press H for a menu.

$ info command_name

similar to man, but more detailed. hypertext formatting. Navigation is available, and links. ? opens the menu

$ locate string

outputs every name that matches the given string. Supports wildcards and regular expressions.

updatedb

a command that updates the database. Can be run if recent files aren't showing up in locate search

find path_to_search -name name_to_check

instead of maintaining and searching a database, find recursively searches a directory tree. Also supports RegEx and wildcards. Options allow filtering of output

cat

concatenates or views text files

cut

removes sections from a text file

cd

changes directory

cp

copies a file

mv

moves or renames a file

mkdir

makes a new directory

wc

counts the number of words, lines, or bytes in a file

passwd

changes a user's password

rm

deletes a file

rmdir

deletes a directory. Must be empty, unless -r in which case it deletes recursively

more

views text files one screen at a time

whereis

Displays the file path to a specified program and related manual files

head

displays the first few lines of a file

tail

displays the last few lines of a file

sort

orders a file numerically or alphabetically

tr

translates or removes characters of a file

chmod

changes a file's permissions

grep

searches within a file

pwd

print working directory (prints current location)

ls -a

reveals hidden files and directories

..

parent directory

.

current location

tree

display the hierarchical listing of a directory tree

root filesystem

the very very base of the filesystem. anything not found in /home/user is part of this

home directory

Users' "HOME" directory is usually /home/USERNAME

Not Universal

your home directory can be abbreviated using a tilde: ~

$ su - user_name

switch user to user_name. You will be prompted to input a password

cd ~

return to the user's home directory. Can also be done by using cd without arguments

~username

this specifies the home directory of username, as opposed to your own home directory. Requires permissions

ls -l

the '-l' option creates a long list

ls -lh

Long listing with Human readable file sizes

ls -d

lists directories but not contents. ls -d */ shows only subdirectories and no files

ls -t

list sorted by modification time

ls -lrt

r reverses, so this is a long list with oldest modifications first

ls -lX

Combines long list with the option to sort by file eXtension. This will, for example, group all files ending with .txt together, all files ending with .jpg together, and so on.

ls -S

sorts by file size

ls -R

displays a recursive list--includes the contents of all subdirectories

ll

an alias that gives ls -l. Long list

mkdir -p

the parent option allows creation of nested directories

$ echo hello > hello.txt

creates a hello.txt file containing "hello"

>

writes output of command to a file instead of the terminal

mv -i

this checks if you want to overwrite an existing file if the new name you're renaming a file to already exists

rm -ri

remove empty directories, prompt you before removing nonempty directories

cp -r

allows copy to work on directories

globbing

a simple pattern matching language, like regular expressions but simpler

*

matches any number of any characters, including none

?

matches any one character

[class]

matches characters in the brackets