Unit 02

The pwd command displays the complete path of the directory in which the user is currently working.

Syntax:
pwd

Example:
$ pwd
/home/user

Here, /home/user is the current working directory.

The cd command is used to change the current working directory.

Syntax:
cd directory_name

cd /tmp       # Moves to /tmp directory
cd ..         # Moves one directory level up
cd ~          # Moves to the home directory
cd /          # Moves to the root directory

An absolute path specifies the complete path, such as cd /home/user/Pictures. A relative path specifies a directory relative to the current location, such as cd Downloads.

The mkdir command is used to create a new directory.

Syntax:
mkdir directory_name

Example:
mkdir mydirectory

This creates mydirectory inside the current directory. Multiple directories can also be created at the same time.

The rmdir command is used to remove an empty directory.

Syntax:
rmdir directory_name

Example:
rmdir mydirectory

If a directory contains files or subdirectories, rm -r can be used instead:

rm -r directory_name

The ls command is used to display files and directories present in the current directory.

Syntax:
ls [option]

Important Options:

ls -a    # Displays all files, including hidden files
ls -l    # Displays permissions, owner, group, size, date and filename
ls -R    # Displays files recursively, including subdirectories

Options can also be combined, such as ls -al.

The tree command is used to display files and directories in a tree-like hierarchical structure.

Syntax:
tree [option] [directory]

Example:
tree Documents

It displays the contents of the Documents directory along with its subdirectories in tree form.

The cat command is used to create, display, combine, and copy the contents of text files.

cat > filename          # Creates a new text file
cat filename            # Displays the contents of a file
cat file1 file2 > file3 # Combines file1 and file2 into file3

The rm command is used to delete files from the system.

Syntax:
rm [option] filename

Example:
rm sample.txt

Important Options:

rm -r folder     # Recursively removes a directory and its contents
rm -f file.txt   # Forcefully removes a file
rm -rf folder    # Forcefully removes a directory and its contents

The cp command is used to copy files or directories from one location to another.

Syntax:
cp [OPTION] source destination

Example:
cp original_file new_file

This copies the contents of original_file into new_file while keeping the original file. To copy a directory:

cp -r folder1 copyfolder1

Here, -r recursively copies the directory and its contents.

The mv command is used to move a file from one location to another or rename a file.

mv filename new_location
Example: mv sample.txt /home/user/Documents

mv filename newfilename
Example: mv test.txt newtest.txt

If superuser permission is required, sudo can be used: sudo mv sample.txt /home/user/Documents.

The touch command is used to create a new empty file or update the timestamp of an existing file.

Syntax:
touch [OPTION] FILE

Example:
touch demo.txt

Creates an empty file named demo.txt. Multiple files can also be created with touch file1 file2. If the file already exists, touch updates its timestamp instead of creating a new file.

The chmod (change mode) command is used to change the read, write, and execute permissions of files and directories.

Syntax:
chmod permissions filename

There are two ways to use chmod:

Numeric / Absolute Mode

Permissions are represented using numbers:

0 = No permission          5 = Read + Execute
1 = Execute                 6 = Read + Write
2 = Write                   7 = Read + Write + Execute
4 = Read

Example:
chmod 764 sample.txt

764 means:
7 → Owner  = read + write + execute
6 → Group  = read + write
4 → Others = read only

Symbolic Mode

Symbols are used to add or remove permissions:

chmod u+x file.txt    # Add execute permission to user
chmod g-w file.txt    # Remove write permission from group
chmod o+r file.txt    # Add read permission to others

Here, + adds permission, - removes permission, and = sets/overrides permission.

The chown command is used to change the owner of a file or directory.

Syntax:
chown user filename
Example: chown faizan sample.txt

To change both the user and group:
chown user:group filename
Example: chown faizan:students sample.txt

The chgrp (change group) command changes the group ownership of a file or directory.

Syntax:
chgrp group_name filename

Example:
chgrp students sample.txt

This changes the group owner of sample.txt to students.

The umask command controls the default permissions assigned to newly created files and directories.

Before applying umask, the default file permission is 666 (rw-rw-rw-) and the default directory permission is 777 (rwxrwxrwx). The permissions are calculated as:

Permissions = Default Permissions - umask

umask         # Displays the current umask in numeric form
umask -s      # Displays the umask in symbolic form
umask 0022    # Changes the umask value to 0022

The who command displays information about the users currently logged into the system, such as login name, terminal name, and login time.

Syntax:
who [options] [filename]

who -a     # Displays all available information
who -b     # Displays the time of last system boot
who -q     # Displays logged-in usernames and their count

The whoami command displays the username of the currently logged-in/effective user.

Syntax:
whoami [option]

Example:
$ whoami
faizan

Options: whoami --help displays help information and whoami --version displays version information.

The man command is the built-in manual/help system of Linux. It provides detailed information about commands, including their syntax, description, options, and examples.

Syntax:
man [options] [section_number] command_name

Example:
man ls

This displays the manual page for the ls command. A manual page generally contains sections such as NAME, SYNOPSIS, DESCRIPTION, OPTIONS, EXAMPLES, FILES, and SEE ALSO.

The echo command displays a given string or command result on the standard output.

Syntax:
echo [option] [string]

Example:
echo "Hello, World!"

echo -n "Hello"     # Does not add a new line after output
echo -e "A\nB"      # Enables escape characters
echo -E "A\nB"      # Disables interpretation of escape characters

With -e, escape characters such as \n for a new line and \t for a horizontal tab can be used.

The date command is used to display or set the system date and time. It can also display the date and time in customized formats.

Syntax:
date [option] [+format]

Example:
date

date -d "2 days ago"    # Displays a specified date
date -u                 # Displays date and time in UTC
date -s "..."           # Sets the system date/time

date +%Y     # Year
date +%m     # Month
date +%d     # Day

For example, date +%d/%m/%Y displays the date in day/month/year format.

The clear command is used to clear all previous commands and output from the terminal screen, providing a clean terminal window.

Syntax:
clear

Example:
$ clear

After execution, the terminal screen is cleared.

The head command displays the first lines of a file. By default, it displays the first 10 lines.

Syntax:
head [option] filename

head -n 5 sample.txt    # Displays first 5 lines
head -c 20 sample.txt   # Displays first 20 bytes
head -q file1 file2     # Does not display file-name headers

The tail command displays data from the end of a file. By default, it displays the last 10 lines. It is also useful for monitoring log files.

Syntax:
tail [options] [file]

tail -n 5 sample.txt    # Displays last 5 lines
tail -f sample.txt      # Continuously displays newly added data
tail -q file1 file2     # Removes file-name headers

The cut command is used to extract selected characters, fields, or columns from each line of a file.

Syntax:
cut [option] filename

cut -c 1-5 file.txt      # Extracts characters 1 to 5
cut -f 1 file.txt        # Extracts the first field
cut -d "," -f 2 file.txt # Uses comma as delimiter and extracts field 2

The sort command is used to arrange lines of text in a particular order, such as alphabetical or numerical order.

Syntax:
sort [option] filename

sort -r file.txt       # Sorts in reverse order
sort -n file.txt       # Sorts numerically
sort -u file.txt       # Sorts and removes duplicate lines

The cmp command compares two files byte by byte. It can be used to determine whether two files are identical and locate their first difference.

Syntax:
cmp [OPTION] FILE1 FILE2

cmp -s file1 file2     # Silent comparison
cmp -l file1 file2     # Displays all differing bytes
cmp -n 10 file1 file2  # Compares only first 10 bytes

The tr command is used to translate, replace, or delete characters from text.

Syntax:
tr [option] SET1 [SET2]

tr 'a-z' 'A-Z'    # Converts lowercase to uppercase
tr -d '0-9'       # Deletes digits
tr -s ' '         # Squeezes repeated spaces
tr -c '0-9'       # Complements the specified set

The -d option deletes characters, -s squeezes repeated characters, and -c complements the specified set.

The uniq command is used to detect and remove adjacent duplicate lines from a file.

Syntax:
uniq [OPTIONS] [INPUT_FILE [OUTPUT_FILE]]

uniq -c file.txt       # Displays occurrence count
uniq -d file.txt       # Displays only duplicate lines
uniq -u file.txt       # Displays only unique lines

uniq normally detects duplicates only when the duplicate lines are adjacent to each other.

The wc command is used to count the number of lines, words, characters, or bytes in a file.

Syntax:
wc [option] filename

wc -l file.txt       # Counts lines
wc -w file.txt       # Counts words
wc -c file.txt       # Counts bytes

The tee command reads data from standard input and displays it on the screen while also writing it to a file.

Syntax:
command | tee [option] filename

Example:
ls | tee output.txt

This displays the output of ls on the terminal and also saves it in output.txt.

tee -a file.txt      # Appends instead of overwriting
tee -i file.txt      # Ignores interrupt signals
tee --help           # Displays help information

The ps command displays information about the currently running processes. It is similar to the Task Manager in Windows.

Syntax:
ps [options]

ps -e        # Displays all running processes
ps -f        # Displays full information about processes
ps -u user   # Displays processes of a particular user
ps ux        # Detailed info about processes running under the user
ps 1234      # Checks a particular process using its PID

The bg command is used to resume a stopped process in the background. This allows the user to continue using the terminal while the process runs.

Syntax:
bg [job]

Example:
bg %1

This resumes job number 1 in the background. A command can also be directly started in the background using &: sleep 100 &.

The fg command brings a stopped or background process back to the foreground.

Syntax:
fg jobname

Example:
fg %1

This brings job number 1 to the foreground. A foreground process can be stopped using Ctrl + Z and then resumed using fg or bg.

The top command displays information about all running processes in real time.

Syntax:
top

It displays information such as PID (Process ID), USER (process owner), PR (priority), NI (nice value), %CPU, %MEM, and COMMAND. Press q to exit the top command.

The kill command is used to terminate a running process. The PID of the process is required.

Syntax:
kill [option] PID

Example:
kill 1234

kill -15 1234     # Terminates the process normally
kill -9 1234      # Forcefully terminates the process
kill -1 1234      # Sends hangup/reload signal

The PID of a process can be found using pidof process_name.

The nice command is used to start a process with a particular priority. The niceness value ranges from -20 to 19: a lower nice value means higher priority, and a higher nice value means lower priority. The default nice value is 0.

Syntax:
nice -n value process_name

Example:
nice -n 10 firefox

This starts firefox with a nice value of 10.

The at command is used to schedule a command or job that will execute once at a specified time.

Syntax:
at [option] time

Example:
at 10:30 PM
at> echo "Backup Started" > backup.txt
at> Ctrl+D

This schedules the command to execute at 10:30 PM. Jobs can also be scheduled using relative time: at now + 1 hour.

at -l             # Lists scheduled jobs
at -c 7           # Displays contents of job number 7
at -m 10:30 PM    # Sends mail after job completion

The -M option can be used to avoid sending email notifications.

The batch command schedules a job to execute when the system load becomes low, rather than at an exact specified time.

Syntax:
batch

Example:
batch
at> echo "Job Executed" >> output.txt
at> Ctrl+D

A command can also be passed directly to batch: echo "touch example.txt" | batch. After entering the commands, Ctrl+D is pressed to finish scheduling the job.

Cron is a Linux service used to execute commands or scripts automatically at regular intervals. It is useful for repetitive tasks such as taking backups every day, running scripts every hour, or performing maintenance every week.

Cron jobs are configured using a crontab (cron table). A cron schedule contains five time fields:

Minute  Hour  Day  Month  Weekday  Command
  *      *     *     *       *      command

Example:
30 10 * * * /home/user/backup.sh

This executes backup.sh every day at 10:30 AM.

The crontab command is used to create, view, edit, and remove recurring cron jobs.

Syntax:
crontab [option]

crontab -e     # Edits/adds cron jobs
crontab -l     # Lists existing cron jobs
crontab -r     # Removes all cron jobs

Example:
crontab -e

Then a recurring job can be added:
0 8 * * * /home/user/script.sh

This executes script.sh every day at 8:00 AM.