Linux commands

Connecting to a Unix/Linux system
 Open up a terminal:

Connecting to a Unix/Linux system
 Open up a terminal:
The “prompt”
The current directory (“path”)
The host

What exactly is a “shell”?
 After logging in, Linux/Unix starts another
program called the shell
 The shell interprets commands the user types
and manages their execution
The shell communicates with the internal part of the
operating system called the kernel
The most popular shells are: tcsh, csh, korn, and bash
The differences are most times subtle
For this tutorial, we are using bash
 Shell commands are CASE SENSITIVE!

Help!
Whenever you need help with a command
type “man” and the command name

Unix/Linux File System
/home/john/portfolio/
/home/mary/
The Path
NOTE: Unix file names
are CASE SENSITIVE!

Command: pwd
To find your current path use “pwd”

Command: cd
To change to a specific directory use “cd”

Command: cd
 “~” is the location of your home directory

Command: cd
“..” is the location of the directory below
current one

Command: ls
 To list the files in the current directory use “ls”

Command: ls
 ls has many options
 -l long list (displays lots of info)
 -t sort by modification time
 -S sort by size
 -h list file sizes in human readable format
 -r reverse the order
“man ls” for more options
Options can be combined: “ls -ltr”

Command: ls -ltr
 List files by time in reverse order with long listing

General Syntax: *
“*” can be used as a wildcard in unix/linux

Command: mkdir
To create a new directory use “mkdir”

Command: rmdir
To remove and empty directory use “rmdir”

Displaying a file
Various ways to display a file in Unix
 cat
 less
 head
 tail

Command: cat
 Dumps an entire file to standard output
Good for displaying short, simple files

Command: less
 “less” displays a file, allowing
forward/backward movement within it
return scrolls forward one line, space one page
y scrolls back one line, b one page
 use “/” to search for a string
Press q to quit

Command: head
“head” displays the top part of a file
 By default it shows the first 10 lines
 -n option allows you to change that
 “head -n50 file.txt” displays the first 50
lines of file.txt

Command: head
Here’s an example of using “head”:

Command: tail
Same as head, but shows the last lines

File Commands
Copying a file: cp
Move or rename a file: mv
Remove a file: rm

Command: cp
To copy a file use “cp”

Command: mv
 To move a file to a different location use “mv”

Command: mv
mv can also be used to rename a file

Command: rm
To remove a file use “rm”

Command: rm
To remove a file “recursively”: rm –r
Used to remove all files and directories
Be very careful, deletions are permanent
in Unix/Linux

File permissions
Each file in Unix/Linux has an associated
permission level
This allows the user to prevent others from
reading/writing/executing their files or
directories
Use “ls -l filename” to find the permission
level of that file

Permission levels
“r” means “read only” permission
“w” means “write” permission
“x” means “execute” permission
In case of directory, “x” grants permission to list
directory contents

File Permissions
“The World”

Command: chmod
 If you own the file, you can change it’s permissions with
“chmod”
 Syntax: chmod [user/group/others/all]+[permission] [file(s)]
 Below we grant execute permission to all:

Command: ps
To view the processes that you’re running:

Command: top
To view the CPU usage of all processes:

Command: kill
To terminate a process use “kill”

Input/Output Redirection (“piping”)
 Programs can output to other programs
 Called “piping”
 “program_a | program_b”
 program_a’s output becomes program_b’s input
 “program_a > file.txt”
 program_a’s output is written to a file called “file.txt”
 “program_a < input.txt”
 program_a gets its input from a file called “input.txt”

Command: wc
To count the characters, words, and lines
in a file use “wc”
The first column in the output is lines, the
second is words, and the last is characters

Command: grep
To search files in a directory for a specific
string use “grep”

Command: diff
To compare to files for differences use
“diff”
Try: diff /dev/null hello.txt
/dev/null is a special address -- it is always
empty, and anything moved there is deleted

Linux commands

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