Crontab Basics

• crontab -e → edit crontab
• crontab -l → list crontab entries
• crontab -r → remove crontab entries
• systemctl status crond → check crond service status

AT: One-time Scheduled Tasks

• Used to schedule jobs only once.

Scheduling Examples:

• at HH:MM PM → schedule a job

• Example :

• at 2:45PM 01/30/25

• at 4PM + 4 days

• at now + 5 hours

• at 8:00AM Sun

• at 10:00AM next month

Managing AT Jobs

• atq → list the scheduled at jobs
• atrm # → remove a specific at job (use number from atq)
• systemctl status atd → check atd daemon status

ps Command

• ps → Current shell process status
• ps -e → All running processes
• ps aux → All in BSD style with details
• ps -ef → Running process with details
• ps -u root → Processes of specific user
• ps -ef | grep ? → Find process with specific id

top Command

• top -u username → Show only that user's processes
• top then c → Absolute path
• top then k → Kill by PID
• top then m or then p → Sort by memory or process usage

kill Commands

• kill -l → List all signals
• kill PID → Default terminate
• kill -1 → Restart
• kill -2 → Equivalent to Ctrl+C
• kill -9 → Force kill
• kill -15 → Graceful kill
• killall → Kill all processes by name
• pkill → Kill by process name

• touch one two three → Create 3 files

• mkdir folder → Make directory

• mv -R / cp -R → Move or copy folder including subfolders

• find . -name "george" → Search from current directory

• locate → Faster, but relies on a database

• updatedb → Update the locate database

Wildcards

File Display Commands

cat → Show entire content
cat -A → Show non-printable characters

more → Paginate output
less → Same as more but allows navigation with arrow keys

head → Show top lines (default: 10)
tail → Show bottom lines (default: 10)

head -2 myfile
tail -2 myfile

Filters / Text Processing Input

• cut → Cut input
• awk → List by columns
• grep & egrep → Search by keyword
• sort → Sort in alphabetical order
• uniq → Remove duplicate lines
• wc → Word count (including lines)

cut Command Examples

• cut -c1 filename → First letter of each line
• cut -c1,2,4 filename → Characters 1, 2, and 4
• cut -c1-3 filename → Range: characters 1 to 3
• cut -c1-3,6-8 filename → Ranges: characters 1-3 and 6-8
• cut -b1-3 filename → Byte range 1 to 3
• cut -d: -f6 /etc/passwd → Field 6 using : delimiter
• cut -d: -f6-7 → Fields 6 and 7
• ls -l | cut -c2-4 → Extract character range 2 to 4 from ls output

awk Command Examples

Extract fields from a file or an output

• awk '{print $1}' file → Print 1st column of file
• ls -l | awk '{print $1, $3}' → Print 1st and 3rd columns from ls -l
• ls -l | awk '{print $NF}' → Print last column
• awk '/jerry/ {print}' file → Search for "jerry" and print matching lines
• awk -F: '{print $1}' /etc/passwd → Use : as delimiter, print 1st field
• echo "hello tom" | awk '{$2="adam"; print}' → Replace column 2 with "adam"
• awk 'length($0) > 15' file → Print lines longer than 15 characters
• `awk '{print NF}' → Number of colum

grep Command Examples

• grep keyword file → Search for keyword
• grep -c keyword file → Count matching lines
• grep -i keyword file → Ignore case sensitive
• grep -n keyword file → Show line numbers
• grep -v keyword file → Invert match (everything except keyword)
• grep keyword file | awk '{print $1}' → Pipe grep to awk to extract 1st column
• ls -l | grep keyword → Filter ls -l output by keyword
• egrep -i "keyword1|keyword2" file → Search for multiple keywords (case-insensitive)

sort Command Examples

• sort file → Sort alphabetically
• sort -r file → Sort in reverse order
• sort -u file → Sort and remove duplicates
• sort file | uniq → Same as above with explicit uniq
• sort file | uniq -d → Show only duplicate lines
• sort file | uniq -c → Count duplicates
• sort -k4 -n file → Sort by 4th field (numeric)
• ls -l | sort -k4 -n → Sort by size from ls -l

wc Command Examples

• wc -c filename → Byte count
• wc -w filename → Word count
• wc -l filename → Line count
• ls -l | wc -l → Count lines from ls -l
• wc -l filename → Count number of lines in file

cmp / diff

• diff → Compare files line by line
• cmp → Compare files byte by byte

tar / gzip

• tar cvf export.tar somedir → Compress directory
• tar xvf export.tar → Extract tar archive
• gzip export.tar → Compress with gzip
• gzip -d export.tar.gz → Decompress gzip archive

truncate Command

• truncate -s10 filename → Chop file to 10 bytes
• truncate -s60 filename → Extend file to 60 bytes

Combining & Splitting Files

• cat file1 file2 file3 > fileN → Combine multiple files into one

File Ownership

• chown → Change ownership
• chgrp → Change group

Access Control List (ACL)

• setfacl → Set file ACL
• getfacl → Get file ACL

Add Permission to User

setfacl -m u:user:rwx /path/to/file
setfacl -m g:group:rw /path/to/file

Recursive Inheritance from Folder

setfacl -R -m entry /path/to/dir

Remove Specific ACL Entry

setfacl -x u:user /path/to/file

Remove All ACL Entries

setfacl -b /path/to/file

Check ACL

ls -ltr

Example output:

-rw-rw-r--+

Help Commands

• whatis command
• command --help
• man command

Add Text to a File

echo "my text" > myfile    # Overwrite
cat myfile                 # Read the text
echo "hello" >> myfile     # Append

Input and Output Redirects
3 Redirect Types

• stdin → 0 → < or << → Feeding file content to a command
• stdout → 1 → > or >>
• stderr → 2 → 2> or 2>> → Write error output to a file

tee Command

• tee → Output + Save

echo "test" | tee myfile
tee -a file  # Append

Links

Soft Link (Symbolic Link)

• ln -s
• Link will be removed if the original file is removed or renamed.

Hard Link

• ln
• Deleting, renaming, or moving the original file will not affect the hard link.

Diagram

       INODE ← MYFILE.TXT → MY-SOFT-LINK
        ↑
MY HARD LINK 

Command to check links:

ls -ltri

File Permissions

Permission Types

Each Permission (R, W, X) Can Be Controlled at Three Levels

• U → User
• G → Group
• O → Others (Everyone)

Permission Strings

Example:

-rwxrwxrwx
 |   |  |
 |   |  └── Others
 |   └───── Group
 └───────── User

To remove write permission from all:

chmod a-w filename

To add read and write permission for user:

chmod u+rw filename

Directory Permissions

Example:

drwxrwxrwx
^   ^  ^
|   |  └── Others
|   └───── Group
└───────── User

Numeric Permission Mapping

Example to set permissions:

chmod 764 file

Breakdown:

• User (7) → rwx
• Group (6) → rw-
• Others (4) → r--

• ls -l → List
• pwd → Print working directory

dr-xr-xr-x
|
directories

-rw-r--r--
|
file

Linux File Types

Inode = Pointer or number of a file on a hard disk.