모든 DevOps 엔지니어가 알아야 할 🔐 Linux 권한

If you’re learning Linux for DevOps, sooner or later you’ll encounter errors like:

Permission denied

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or

Could not read private key

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or

Operation not permitted

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At first, these errors can feel confusing. But once you understand Linux permissions, ownership, file searching, and command chaining, troubleshooting becomes much easier.

In this article, I’ll cover some of the most practical Linux concepts that DevOps engineers use every day:

• ✅ Linux permissions
• ✅ chmod
• ✅ chown
• ✅ sudo
• ✅ cat, head, and tail
• ✅ grep and find
• ✅ Piping and redirection

Let’s dive in.

Why Permissions Are a Big Deal in DevOps

Linux was designed as a multi-user operating system. Multiple users and processes can run on the same machine at the same time.

Permissions exist to answer one important question:

Is this user or process allowed to read, write, or execute this file?

As a DevOps engineer, permissions appear everywhere:

• SSH won’t allow login if your private key permissions are too open.
• Nginx needs permission to read your application’s files.
• Docker containers often run as specific users, causing permission mismatches.
• CI/CD pipelines frequently fail because of permission issues.

Understanding permissions can save hours of debugging.

Understanding Linux Permissions

Run the following command:

ls -l

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Example output:

-rwxr-xr-- 1 ana developers 220 Jul 27 10:00 deploy.sh

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The first section contains the permission information:

-rwxr-xr--

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Let’s break it down.

File Types

The first character indicates the file type.

Symbol Meaning - Regular file d Directory l Symbolic link

Permission Groups

The next nine characters are divided into three groups.

rwx     r-x     r--
---     ---     ---
Owner   Group   Others

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Each group represents permissions for a different category of users.

Permission Meanings

For Files

Permission Meaning r Read the file w Modify the file x Execute the file

For Directories

Permission Meaning r List directory contents w Create or delete files x Enter the directory using cd

Example

Consider:

-rwxr-xr--

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This means:

Owner (rwx)

• Read
• Write
• Execute

Group (r-x)

• Read
• Execute
• Cannot modify

Others (r--)

• Read only

Understanding the Full Output

-rwxr-xr-- 1 ana developers 220 Jul 27 10:00 deploy.sh

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Value Meaning 1 Number of hard links ana Owner developers Group 220 File size Jul 27 10:00 Last modified deploy.sh File name

chmod — Changing Permissions

The chmod command is used to modify permissions.

There are two common approaches.

Method 1: Symbolic Notation

Add execute permission for the owner:

chmod u+x deploy.sh

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Remove write permission from the group:

chmod g-w deploy.sh

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Set others to read-only:

chmod o=r deploy.sh

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Add read permission for everyone:

chmod a+r deploy.sh

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Symbol Meanings

Symbol Meaning u User (owner) g Group o Others a All

Method 2: Numeric (Octal) Notation

This is the method you’ll see most often in DevOps.

Each permission has a value:

Permission Value Read 4 Write 2 Execute 1 None 0

Common Permission Values

Number Permission 7 rwx 6 rw- 5 r-x 4 r– 0 —

Example

chmod 754 deploy.sh

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This translates to:

Owner  = 7 = rwx
Group  = 5 = r-x
Others = 4 = r--

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Common chmod Values Used in DevOps

SSH Private Keys

chmod 600 id_rsa

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Only the owner can read and write the file.

SSH often refuses to use private keys with weaker permissions.

Configuration Files

chmod 644 config.yaml

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Owner can modify the file.

Everyone else can read it.

Scripts and Executables

chmod 755 deploy.sh

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Everyone can run the script.

Only the owner can modify it.

Sensitive Files

chmod 400 secrets.txt

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Owner can read the file.

Nobody else has access.

chown — Changing Ownership

While chmod controls permissions, chown controls ownership.

Change Owner

chown ana deploy.sh

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Changes the file owner to ana.

Change Owner and Group

chown ana:developers deploy.sh

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Changes both owner and group.

Recursive Ownership Change

chown -R ana:developers app/

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Applies ownership changes to all files and directories inside the folder.

Real-World Example

Suppose a web application runs under:

www-data

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If the files are owned by another user, the application may fail with:

Permission denied

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A common fix is:

sudo chown -R www-data:www-data /var/www/myapp

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sudo — Temporary Administrator Access

sudo allows a user to run commands with administrator privileges.

Update Packages

sudo apt update

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Open a Root Shell

sudo -i

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Use carefully.

Change Ownership

sudo chown root:root file.txt

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Why Use sudo?

Accountability

Every sudo command is logged.

Security

You only elevate privileges when necessary.

Control

Administrators can grant limited access without giving full root permissions.

Reading Files in Linux

When troubleshooting systems, reading logs and configuration files is a daily task.

cat

Display the entire file.

cat app.log

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head

Display the first few lines.

head app.log

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Display the first 20 lines:

head -n 20 app.log

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tail

Display the last few lines.

tail app.log

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Display the last 50 lines:

tail -n 50 app.log

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tail -f

Monitor a file in real time.

tail -f /var/log/nginx/access.log

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This is one of the most frequently used commands in DevOps.

It’s perfect for watching logs while debugging applications or deployments.

grep — Searching Inside Files

The grep command searches file contents for a specific pattern.

Find Errors

grep "ERROR" app.log

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Case-Insensitive Search

grep -i "error" app.log

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Search Recursively

grep -r "TODO" ./src

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Count Matches

grep -c "ERROR" app.log

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Show Line Numbers

grep -n "ERROR" app.log

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Exclude Matches

grep -v "DEBUG" app.log

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find — Searching for Files

Unlike grep, the find command searches for files and directories.

Find Log Files

find . -name "*.log"

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Files Modified Within One Day

find /var/log -type f -mtime -1

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Find Specific Directories

find . -type d -name "node_modules"

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Find Large Files

find . -size +100M

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grep vs find

This is a common beginner confusion.

grep

Searches inside file contents.

grep "ERROR" app.log

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find

Searches for files and directories.

find . -name "*.log"

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Piping Commands

One of Linux’s greatest strengths is combining commands together.

The pipe operator (|) sends the output of one command as the input to another.

Find Error Messages

cat app.log | grep "ERROR"

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Count Error Messages

cat app.log | grep "ERROR" | wc -l

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Find Running Nginx Processes

ps aux | grep nginx

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Search Command History

history | grep docker

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Output Redirection

Redirection allows you to save command output into files.

Overwrite Output

echo "Deployment started" > deploy.log

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Creates a new file or replaces existing content.

Append Output

echo "Step completed" >> deploy.log

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Adds content to the end of the file.

More Examples

Save directory contents:

ls -la > files.txt

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Save only error messages:

grep "ERROR" app.log > errors.log

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Important Warning

Be careful when using:

>

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It overwrites the file completely.

If you want to preserve existing content, use:

>>

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Many engineers have accidentally overwritten important files by using the wrong operator.

Quick Command Reference

Command Purpose chmod Change permissions chown Change ownership sudo Run commands as administrator cat Display file contents head View beginning of a file tail View end of a file tail -f Follow logs in real time grep Search inside files find Search for files and directories ` ` > Overwrite file output >> Append file output

Final Thoughts

Linux permissions, ownership, file searching, and command chaining are fundamental skills for every DevOps engineer.

The better you understand these concepts, the easier it becomes to troubleshoot servers, debug deployments, manage containers, and automate infrastructure.

I’m currently documenting my DevOps learning journey and will continue sharing beginner-friendly guides on Linux, Docker, Git, Kubernetes, Jenkins, Terraform, AWS, and more.

If you have any tips or favorite Linux commands, feel free to share them in the comments.

Happy Learning! 🐧🚀