Script Permissions and Running

Before you can run a script directly, it needs the correct permissions. This lesson covers how to make scripts executable and best practices for script permissions.

Understanding Execute Permission

In Linux, files have three types of permissions:

• Read (r): View file contents
• Write (w): Modify file contents
• Execute (x): Run as a program

For scripts, you need execute permission to run them directly.

Checking Current Permissions

Use ls -l to see file permissions:

ls -l script.sh
# -rw-r--r-- 1 user group 45 Jan 10 10:00 script.sh
#  ↑ No execute permission (no 'x')

Making Scripts Executable

Use chmod to add execute permission:

# Add execute for owner
chmod u+x script.sh

# Add execute for everyone
chmod +x script.sh

# Using octal notation
chmod 755 script.sh

Understanding chmod Options

Command	Meaning
chmod +x	Add execute for all
chmod u+x	Add execute for user (owner)
chmod g+x	Add execute for group
chmod o+x	Add execute for others
chmod 755	rwxr-xr-x (common for scripts)
chmod 700	rwx------ (private scripts)

Exercise: Make Script Executable

Create a script and make it executable:

Running Scripts from Different Locations

The way you call a script depends on where it is:

# Script in current directory
./script.sh

# Script with full path
/home/user/scripts/script.sh

# Script in PATH
script.sh  # Only works if directory is in PATH

Why ./ is Required

When you type a command, bash searches the PATH:

echo $PATH
# /usr/local/bin:/usr/bin:/bin

# 'ls' works because it's in /usr/bin (which is in PATH)
ls

# './script.sh' is needed because . is NOT in PATH
./script.sh

Adding Scripts to PATH

To run scripts without ./, add their directory to PATH:

# Temporarily add ~/bin to PATH
export PATH="$HOME/bin:$PATH"

# Now scripts in ~/bin can be run directly
myscript.sh  # Instead of ~/bin/myscript.sh

For permanent changes, add to ~/.bashrc:

# In ~/.bashrc
export PATH="$HOME/bin:$PATH"

Common Permission Patterns

Octal	Symbolic	Use Case
755	rwxr-xr-x	Shared scripts (most common)
700	rwx------	Private scripts
750	rwxr-x---	Scripts for your group only
644	rw-r--r--	Non-executable files

Script Ownership

Sometimes you need to consider ownership:

# Check ownership
ls -l script.sh
# -rwxr-xr-x 1 alice developers 100 Jan 10 script.sh
#              ↑      ↑
#            owner   group

# Change owner (requires sudo)
sudo chown bob script.sh

# Change group
chown :newgroup script.sh

Security Best Practices

Follow these security guidelines:

1. Don't use 777: Never chmod 777 - anyone can modify and run
2. Limit access: Use 700 for sensitive scripts
3. Check scripts before running: Especially downloaded scripts
4. Use appropriate owners: Scripts in /usr/local/bin should be owned by root

# BAD - anyone can modify
chmod 777 script.sh

# GOOD - only owner can modify, others can execute
chmod 755 script.sh

# GOOD - private script
chmod 700 private_script.sh

Troubleshooting Permission Issues

Common errors and solutions:

# Error: Permission denied
$ ./script.sh
bash: ./script.sh: Permission denied
# Solution: chmod +x script.sh

# Error: Command not found
$ script.sh
bash: script.sh: command not found
# Solution: Use ./script.sh or add to PATH

# Error: Bad interpreter
$ ./script.sh
bash: ./script.sh: /bin/bash^M: bad interpreter
# Solution: File has Windows line endings, use dos2unix

Exercise: Check and Set Permissions

View permissions and understand what they mean:

Quick Reference

# Create and run a script - complete workflow
touch myscript.sh                    # Create file
echo '#!/bin/bash' > myscript.sh     # Add shebang
echo 'echo "It works!"' >> myscript.sh  # Add commands
chmod +x myscript.sh                  # Make executable
./myscript.sh                         # Run it

Key Takeaways

• Scripts need execute permission (chmod +x) to run directly
• Use ./script.sh to run scripts in the current directory
• chmod 755 is the standard permission for shared scripts
• Add script directories to PATH for convenient access
• Never use chmod 777 - it's a security risk
• Use chmod 700 for private, sensitive scripts