Week 9.docx

**Week 9** **Shell Variables** 1. **Shell Variables (\$foo and \${foo}):** - Variables in shell scripts store text or numeric values. - Assign values using = without spaces around it. - Examples: - Use \$ to expand variables in commands or scripts. 2. **Quoting to Preven...

**Week 9** **Shell Variables** 1. **Shell Variables (\$foo and \${foo}):** - Variables in shell scripts store text or numeric values. - Assign values using = without spaces around it. - Examples: - Use \$ to expand variables in commands or scripts. 2. **Quoting to Prevent Variable Expansion:** - Use single quotes or backslashes to prevent immediate variable expansion. - Example: 3. **Double-Quote All Uses of Variables:** - Always double-quote variables to prevent word-splitting and GLOB expansion. - Example: 4. **Unrecognized Variables Are Empty:** - Undefined variables expand to nothing unless nounset option is set. - Example: 5. **Appending to Variables:** - Append to a variable using \$VARNAME=\$VARNAME:newtext. - Example: 6. **Local vs Environment (Global) Variables:** - Local variables are specific to the current shell. - Exported (environment) variables are inherited by child processes. - Example: 7. **Pre-set Environment Variables:** - Examples of common environment variables: \$PATH, \$USER, \$HOME, \$SHELL, \$TERM, \$\$. 8. **Variable Best Practices:** - Use descriptive names. - Always quote variable expansions unless you specifically want word-splitting or GLOB expansion. These practices ensure correct handling of variables in shell scripting, preventing errors and unintended behavior. **\ ** **Search Path** 1. **Command Identification**: - The shell identifies commands by the first token/word after processing I/O redirection. - Built-in commands (e.g., echo, cd) are executed directly by the shell without searching in PATH. 2. **Searching in PATH**: - If a command name (without slashes) isn\'t a built-in, the shell looks for an executable file in directories listed in the PATH environment variable. - PATH is a colon-separated list of directories (/usr/local/bin:/bin:/usr/bin). - The shell searches these directories from left to right and executes the first match found. 3. **Exact Executable**: - Using the full pathname (/bin/date) bypasses PATH search and directly executes the specified file. 4. **Handling Multiple Matches**: - If multiple directories in PATH contain the same command name, the shell executes the first one it finds. 5. **Command Not Found**: - If a command isn\'t found in any directory listed in PATH, the shell returns a \"command not found\" error. 6. **Security Concerns**: - **Current Directory**: Avoid including the current directory (.) or adjacent colons (::) in PATH due to security risks. - Always prefix commands in the current directory with ./ to execute them explicitly. 7. **Using which and whereis**: - which identifies which directory in PATH contains a command. - whereis locates commands and their man pages in standard directories, ignoring PATH. 8. **Modifying PATH**: - Use standard shell techniques to modify PATH (export PATH=\$PATH:/new/directory). - Ensure each directory in PATH is separated correctly by colons. 9. **Shell Scripts**: - Set PATH explicitly at the start of shell scripts to ensure commands are found and executed correctly. - Failure to set PATH in scripts can lead to commands not being found, depending on the invoking environment\'s PATH. 10. **Best Practices**: - Understand how PATH works and customize it appropriately for your needs. - Avoid security risks by not including unsafe entries in PATH. Understanding these principles is crucial for effectively navigating and managing command execution in Unix-like operating systems. **Shell scripts** Shell scripting allows users to automate tasks by creating scripts containing lists of commands, executed by a shell program like bash. Scripts can be run interactively or from text files, providing flexibility and automation in system administration. **Key Points:** 1. **Interactive vs. Scripted Use:** Shells serve as interactive command interpreters for human interaction or as programming languages for executing script files stored in text format. 2. **Creating and Running Scripts:** - Scripts are lists of commands stored in text files. - Execute scripts by invoking the shell with the script file as an argument (bash script.sh or sh script.sh). - Make scripts executable (chmod +x script.sh) and run directly (./script.sh) after setting the appropriate shebang (\#!/bin/bash). 3. **Script Execution Context:** - Scripts run in their own shell process, independent of the interactive shell. - Changes to environment variables or settings in a script do not affect the interactive shell. 4. **Best Practices:** - Standard script header includes shebang (\#!/bin/bash -u), sets PATH, and specifies umask. - Use comments effectively to explain script functionality. - Store scripts in personal \~/bin directories added to \$PATH for easy execution. 5. **Handling Arguments and Variables:** - Access command-line arguments using positional parameters (\$1, \$2, etc.). - Shift parameters with shift to process multiple arguments sequentially. - Use \"\$@\" to handle multiple arguments safely. 6. **Advanced Techniques:** - Redirection (\>, \>\>) and piping (\|) allow manipulation of input and output streams. - Use for and while loops to iterate through lists of items or perform repetitive tasks. 7. **Script Security and Maintenance:** - Ensure scripts are secure by validating inputs and managing permissions (chmod). - Maintain scripts by adhering to consistent coding standards and documentation practices. **\ ** **Command Subs Arithmetic Exit** 1. **Shell Command Substitution:** - Allows substituting command output into variables or within strings using \$(command) syntax. - Example: echo \"Your userid is \$(whoami)\" - Advantages include readability, reuse of command output, and performance efficiency compared to running commands multiple times. 2. **Shell Arithmetic:** - Integer arithmetic in shell scripts is done using \$(( expression )). - Supports basic operations like addition, subtraction, multiplication (\*), division (/), and modulus (%). - Example: echo \$(( 2 + 2 \* 9 )) evaluates to 20. 3. **Decimal Point Arithmetic:** - Shell supports only integer arithmetic. For decimal calculations, use bc with quoted expressions. - Example: echo \'22/7\' \| bc -l calculates and outputs 3.14285714285714285714. 4. **Command Exit Status:** - Commands in Unix/Linux return an exit status (or return code) upon completion. - Exit status ranges from 0 (success) to 255 (various failure conditions). - Access the exit status of the last command with \$?. - Example: \$ fgrep root /etc/passwd; echo \"Exit status: \$?\" would output Exit status: 1 if no results are found. 5. **Setting Exit Status in Scripts:** - By default, a script inherits the exit status of its last executed command. - Explicitly set the script\'s exit status using exit \ within the script. - Example: exit 99 sets the script\'s exit status to 99. These concepts are fundamental for shell scripting, enabling automation, error handling, and mathematical computations within scripts effectively.