Shell
Different Linux Shells
- Bourne-again shell (Bash)
- C shell (csh/tcsh)
- Korn Shell (ksh)
- Z shell (zsh)
View your system’s hostname by running the hostname command:
hostname2 types of commands
- internal (~30 built in commands) (builtins)
- External
- reside in individual files
- usually binary programs or scripts
- shellusys PATH variable to find the command (the executable file)
Quoting
Double quotes
- text in between is regular characters
- except $ \ `
variables, command substitution and arithmetic still work
- a space bar loses it’s meaning as an argument seperator
Single Quotes
- revoke special meaning from all characters in quotes
Escape Characters
- takes special meaning out of characters
$ echo \$USER
$ USER
- .\ tells bash to ignore specialness of preceding character
Set a variable
$ $TWOWORDS
2.1
Variables
Local Variable
- available to the current shell process only
- Not accessible to other programs
Environment Variables
- available in shell session and sub processes
- usually in capital letters
- all variables are set when shell is started
Config: Local variables
$ greeting=hello
$ echo $greeting
hello
Remove variable
$ unset greeting
Config: Global Variables
$ greeting=hello
$ export greeting
or
$ export greeting=hello
Use variable in front of a command
$ TZ=EST date
$ TZ=GMT date
Display all environment variables
$ env
The PATH Variable
- stores a list of directories that have executable commands
append new directory to PATH
$ PATH=$PATH:new_directory
append new directory to PATH using another variable
$ mybin=/opt/bin
$ PATH=$PATH:$mybin
which = find out how shell invokes a specific command
$ which nano
Using the command line to get help
/usr/share/doc/
- stores most documentation
Display quick help options
$ bash –help
Man
categories
1 user command
2 system calls
3 functions to the c library
4 drivers and device files
5 configuration files and file formats
6 games
7 misc
8 system administrator commands
9 kernel
- Man page belongs to one category
- Each category can have a man page with the same name
- call a specific page & category
$ man 5 passwd
- otherwise man will select 1st available (passwd(1))
- uses less to display content
- Search man page (while it is displayed)
\ search_word
- searching forward
$ search-word
- searching backward
type N
- jump to next match
type H
- see other features
Info pages
$ info mkdir
- place cursor on hyperlinks & press enter to visit them
/usr/share/doc
- contains directory for most packages installed on the system
- each one contains README or readme.txt
- can also contain changelog, etc.
Locating files
$ locate word
- searches database for matching string
- database is managed by updatedb
can run updatedb to manually update the database
The find command
- searches directory tree recursively
- requires search path
$ find . -name myfile
Using directories and Listing Files
$ sudo apt install tree
$ tree
Shows current directory and sub directories
File and directory names
Navigating the Filesytem
Special relative paths
$ ls -a
- also shows hidden files and directories
. indicates current location
.. indicates parent directory
$ cd ../..
- can be used to navigate up the file tree very quickly
Using directories and Listing Files Part 2
$ tree -L 1 /
- shows only first column of thre root directory tree
FHS (Filesystem Hierarchy Standard)
- Defines standards of standard directory locations
- changes made in root filesystem effect all users
- will also require admin
$ su - michael
- switch user
ls ~michael
- ls the home directory of a user (if you have permissions)
Hidden files
- begin with .
- often used to set user-specific configuration settings
Additional ls Options
l: Long
h: Human readable
d: directories without contents
t: modification time
r: reverse sort
X: file eXtension
S: size
R: Recursive (run ls here and then repeat the command in every subdirectory you find)
Recursion in Bash
- when something is defined in terms of itself
$ du -h
output a list file size of all files, directories, and subdirectories for a certain location
Creating, Moving, and Deleting Files
globbing
* matches any character including no character
? matches any one character
[] matches a class of characters
$ ls file [1-35-6a-c]
can use multiple ranges
$ [1a5]
match certain characters
$ [^a]
match everything but a
Character class
ls file [[:digit:]]
match everything in the digit class
ls file [[:digit:]a]
match digit or a
[:alnum:]
letters and numbers
[:alpha:]
upper or lowercase letters
[:blank:]
spaces and tabs
[:cntrl:]
control characters (backspace, bell, NAK, escape)
[:digit:]
Numerals
[:graph:]
Graphic characters (all characters except ctrl and space)
[:lower:]
lowercase letters
[:print:]
printable characters (alnum, punct, and space)
[:punct:]
Punctuation (!, &,")
[:space:]
whitespace characters (tabs, spaces, newlines]
[:upper:]
Uppercase letters
[:xdigit:]
Hexadecimal numerals