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Linux Intermediate Part2

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Linux Intermediate Part2
A
Asit Kumar Mohanty
Part of seriesLinux Fundamentals

which: This command will locate the executable files associated with a given command by searching through the directories listed in the PATH environment variable. Below example is to locate the gcc compiler. The output of that show that the gcc compiler is located in usr/bin/gcc location.

labex:project/ $ ls
labex:project/ $ which gcc
/usr/bin/gcc
labex:project/ $ which python
/usr/local/bin/python

Note: The which command searches through the directories listed in your PATH environment variable and returns the first match it finds.

which -a: This is to find all location for python. The below example shows there are multiple python installations in the system.

labex:project/ $ which -a python
/usr/local/bin/python
/usr/bin/python
/bin/python
/usr/local/bin/python
/usr/local/bin/python

## The PATH variable
## The order matters: directories listed earlier have higher priority. 
## This means if there are two executables with the same name in different directories, the one in the directory that appears first in PATH will be used.
labex:project/ $ echo $PATH
/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/home/labex/.local/bin:/usr/local/go/bin:/home/labex/golang/bin:/usr/share/maven/bin:/usr/local/bin:/usr/sbin/nodejs/bin:/usr/sbin/yarn/bin:/home/labex/.local/bin:/usr/local/go/bin:/home/labex/golang/bin:/usr/share/maven/bin:/usr/local/bin:/usr/sbin/nodejs/bin:/usr/sbin/yarn/bin

##Example
labex:project/ $ echo '#!/bin/bash
echo "Hello from TechCorp!"' > ~/hello
chmod +x ~/hello
labex:project/ $ hello
zsh: command not found: hello
labex:project/ $ 
labex:project/ $ export PATH=$PATH:$HOME
labex:project/ $ hello
Hello from TechCorp!
labex:project/ $ which hello
/home/labex/hello
labex:project/ $

whereis: This command will help us to locate binary, source and manual page files for various commands in Linux system. See the below example and the output.

  1. /usr/bin/ls: This is the location of the ls binary executable. When you type ls in the terminal, this is the actual program that runs.

  2. /usr/share/man/man1/ls.1.gz: This is the location of the manual page for ls. Manual pages contain detailed information about how to use commands.

labex:project/ $ whereis ls
ls: /usr/bin/ls /usr/share/man/man1/ls.1.gz

  • /usr/bin is a common location for user commands.

  • /usr/share/man is where manual pages are typically stored.

  • man1 indicates this is a user command manual (as opposed to system calls or library functions).

  • The .gz extension means the file is compressed to save space.

## whereis -b is to check if the executable file exists.
labex:project/ $ whereis -b grep
grep: /usr/bin/grep
labex:project/ $ whereis -b nonexistent
nonexistent:

## whereis -m is to locate the manual page
labex:project/ $ whereis -m ssh
ssh: /usr/share/man/man1/ssh.1.gz

## To comine both binary and manual pages to show the output
labex:project/ $ whereis -bm python3
python3: /usr/bin/python3 /usr/lib/python3 /etc/python3 /usr/share/python3 /usr/share/man/man1/python3.1.gz

whereis -s: This is to seach the source file.

labex:project/ $ whereis -s bash
bash:
labex:project/ $ whereis bash
bash: /usr/bin/bash /usr/share/man/man1/bash.1.gz

find: As the name suggest, it’s used for finding a file. See the below example.

labex:project/ $ find . -name "clue.txt"
./evidence/clue.txt

##Example of finding all .txt and all .log files
labex:project/ $ find . -name "*.txt" -o -name "*.log"
./evidence/clue.txt
./evidence/new_lead.txt
./logs/system.log
./logs/recent_activity.log
./notes/interview.txt

  • find is the command we're using

  • . tells find to start searching from the current directory

  • -name "clue.txt" tells find to look for a file exactly named "clue.txt"

## finding all files greater than 1 MB in size.
labex:project/ $ find . -type f -size +1M
./evidence/large_file.dat

  • find . is our familiar starting point

  • -type f tells find to look only for regular files (not directories or other special types)

  • -size +1M specifies that we want files larger than 1 megabyte

## -mtime -1 is new. It tells find to look for files modified less than 1 day ago.
labex:project/ $ find . -type f -mtime -1
./evidence/clue.txt
./evidence/large_file.dat
./evidence/new_lead.txt
./logs/system.log
./logs/recent_activity.log
./notes/interview.txt

labex:project/ $ find . -name "*.txt" -exec cat {} \;
The suspect was last seen wearing a red hat.
New lead: Check the security camera footage from the back alley.
Witness reported hearing a loud noise at approximately 10 PM.

  • find . -name "*.txt" we've seen before - it finds all .txt files

  • -exec cat {} \; is new. It tells find to execute the cat command on each file it finds

    • cat is a command that displays the contents of a file

    • {} is a placeholder that find replaces with each filename it finds

    • \; marks the end of the -exec command

  • Additional find command options not covered in this lab include:

    • -user: Find files owned by a specific user

    • -group: Find files belonging to a specific group

    • -perm: Find files with specific permissions

    • -maxdepth: Limit the depth of directory traversal

    • -mindepth: Start searching from a minimum depth

    • -empty: Find empty files or directories

    • -newer: Find files newer than a specified file

grep: This command is use to search for specific pattern in the file. See the below example to search for ‘ERROR’ in the server.log file. grep is case sensitive.

Note: grep stands for ‘Global Regular Expression Print’.

labex:project/ $ grep "ERROR" logs/server.log 
[2023-11-23 00:00:00] ERROR: Database connection failed
[2023-10-23 00:00:00] ERROR: Database connection failed
[2023-03-04 00:00:00] ERROR: Database connection failed
[2023-08-19 00:00:00] ERROR: Database connection failed

## Get the count of the pattern
labex:project/ $ grep -c "ERROR" logs/server.log 
146

## to make the search pattern case insensitive, use -i
labex:project/ $ grep -i "error" logs/server.log 
[2023-11-23 00:00:00] ERROR: Database connection failed
[2023-01-18 00:00:00] error: File not found
[2023-12-09 00:00:00] error: File not found
[2023-01-20 00:00:00] error: File not found

## Searching multiple files
labex:project/ $ grep "database connection failed" logs/*
logs/error.log:[2023-07-02 11:25:40] WARNING: database connection failed, retrying...
logs/server.log:[2023-07-01 10:20:30] ERROR: database connection failed

## Use regular expression in grep search
grep "2023-[0-9][0-9]-[0-9][0-9]" logs/server.log

grep -v: This is used for not showing result with that pattern. -v mean ‘Not this‘

labex:project/ $ grep -v "ERROR" logs/server.log
[2023-01-18 00:00:00] error: File not found
[2023-12-09 00:00:00] error: File not found
[2023-01-20 00:00:00] error: File not found
[2023-02-22 00:00:00] WARNING: High CPU usage detected

wc: This is used for counting the words. wc stands for word count.

labex:project/ $ ls
project_overview.md  README.md  requirements.txt  src  technical_specs.txt
## -l is for line. The below command will show number of lines.
labex:project/ $ wc -l requirements.txt 
51 requirements.txt

labex:project/ $ wc -l src/main.py
801 src/main.py

## Let's count the number of words using -w
labex:project/ $ wc -w project_overview.md 
2305 project_overview.md
labex:project/ $ wc -w technical_specs.txt
461 technical_specs.txt

##count the character using -m
labex:project/ $ wc -m src/utils.py
10103 src/utils.py
labex:project/ $ wc -m src/database.py
10106 src/database.py

##combining multiple commands
labex:project/ $ wc -l -w -m README.md
 121  278 8388 README.md

cut: This is to extract and analyse the data from a text file. See the below example for customer.txt file where the data is is comma separated format.

labex:project/ $ cat customers.txt            
ID,Name,Age,Email
1,John Doe,25,john.doe@email.com
2,Jane Smith,35,jane.smith@email.com
3,Lily Chen,30,lily.chen@email.com
4,Andy Brown,22,andy.brown@email.com
labex:project/ $ cut -d ',' -f 2 customers.txt
Name
John Doe
Jane Smith
Lily Chen
Andy Brown

## This to starting from the 2nd line, basically to skip header.
labex:project/ $ cut -d ',' -f 2 /home/labex/project/customers.txt | tail -n +2
John Doe
Jane Smith
Lily Chen
Andy Brown

  • cut: The name of the command we're using

  • -d ',': Specifies that we're using a comma as the delimiter between fields

  • -f 2: Tells cut to extract the second field

  • /home/labex/project/customers.txt: The path to our input file

## Examples
## The below example is to show the 2nd and 3rd column
labex:project/ $ cut -d ',' -f 2,3 /home/labex/project/customers.txt
Name,Age
John Doe,25
Jane Smith,35
Lily Chen,30
Andy Brown,22

## Extracting a range of columns. In the below example it is from 1st to 3rd col.
## We can also combine 1-3,5 to exatract 1st,2nd, 3rd and 5th col.
labex:project/ $ cut -d ',' -f 1-3 /home/labex/project/customers.txt
ID,Name,Age
1,John Doe,25
2,Jane Smith,35
3,Lily Chen,30
4,Andy Brown,22

## -c 11-25: This tells cut to extract characters 11 through 25 from each line
labex:project/ $ cat inventory.txt 
ISBN     Title          Quantity
1234567890The Great Adv      100
2345678901Mystery in th       75
3456789012Cooking Basi       50
4567890123Science Exp        125
labex:project/ $ cut -c 11-25 inventory.txt
itle          Q
The Great Adv  
Mystery in th  
Cooking Basi   
Science Exp  

## Mix -grep with cut
labex:project/ $ cat books.txt 
ISBN,Title,Author,Price
978-1234567890,The Great Adventure,Alice Writer,19.99
978-2345678901,Mystery in the Woods,Bob Author,24.99
978-3456789012,Cooking Basics,Carol Chef,15.99
978-4567890123,Science Explained,David Scientist,29.99
labex:project/ $ grep -E ',[2-9][0-9]\.[0-9]{2}$' /home/labex/project/books.txt | cut -d ',' -f 2
Mystery in the Woods
Science Explained

sort: This is for arranging and organizing the data. See the below example

labex:project/ $ cat students.txt 
David Lee
Alice Johnson
Charlie Brown
Bob Smith
Eve Wilson
labex:project/ $ sort students.txt 
Alice Johnson
Bob Smith
Charlie Brown
David Lee
Eve Wilson

labex:project/ $ cat student_ages.txt 
David Lee:21
Alice Johnson:18
Charlie Brown:19
Bob Smith:20
Eve Wilson:18
labex:project/ $ sort -n -t: -k2 student_ages.txt 
Alice Johnson:18
Eve Wilson:18
Charlie Brown:19
Bob Smith:20
David Lee:21

## -nr is for reverser sort for number
labex:project/ $ sort -nr -t: -k2 student_grades.txt  
Charlie Brown:95
Alice Johnson:92
Eve Wilson:91
Bob Smith:88
David Lee:87

## -u is to remove duplicate line in the list. 
## See in the below example, It sorted the lines and remove the duplicate one
## which is 'Alice Johnson:Chess Club'
labex:project/ $ cat student_clubs.txt 
Alice Johnson:Chess Club
Bob Smith:Debate Team
Charlie Brown:Chess Club
David Lee:Science Club
Eve Wilson:Debate Team
Alice Johnson:Chess Club
Bob Smith:Science Club
labex:project/ $ sort -u student_clubs.txt 
Alice Johnson:Chess Club
Bob Smith:Debate Team
Bob Smith:Science Club
Charlie Brown:Chess Club
David Lee:Science Club
Eve Wilson:Debate Team

  • -n: This option tells sort to perform a numeric sort.

  • -t:: This specifies that fields are separated by colons.

  • -k2: This tells sort to use the second field (the age) as the sorting key.

uniq: This is to identifying and filtering duplicate lines.

labex:project/ $ sort customer_purchases.txt > sorted_purchase.txt
labex:project/ $ cat sorted_purchase.txt 
Alice,Electronics
Alice,Electronics
Alice,Electronics
Bob,Books
Bob,Books
Charlie,Clothing
Charlie,Clothing
David,Home Goods
Eve,Toys
Frank,Sports
labex:project/ $ uniq sorted_purchase.txt unique_purchases.txt
labex:project/ $ cat unique_purchases.txt 
Alice,Electronics
Bob,Books
Charlie,Clothing
David,Home Goods
Eve,Toys
Frank,Sports

## Some more example with -c for count
labex:project/ $ uniq -c sorted_purchase.txt purchase_counts.txt 
labex:project/ $ cat purchase_counts.txt 
      3 Alice,Electronics
      2 Bob,Books
      2 Charlie,Clothing
      1 David,Home Goods
      1 Eve,Toys
      1 Frank,Sports
labex:project/ $ uniq -d sorted_purchase.txt repeat_customers.txt
labex:project/ $ cat repeat_customers.txt 
Alice,Electronics
Bob,Books
Charlie,Clothing
#linux#linux-commands

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Linux Fundamentals

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