Shell Scripting Basics

• Shell Scripting is a way to execute commands, manipulate filenames, redirect I/O, use pipes, control the environment, enable background processing, and create scripts.

Shell Programs

• The Bourne Shell (sh/bash) will be used for shell programming.

Echo Command

• The echo command displays information to standard output.
• The -n option disables output of the trailing new line.
• The -e option enables interpretations of the backslash escaped characters.

Escape Characters

• \a produces an audible alert (bell).
• \b creates a backspace.
• \c inhibits the terminating newline.
• \f generates a form feed.
• \n inserts a carriage return and a line feed (newline).
• \r creates a carriage return without the line feed.
• \t inserts a horizontal tab.
• \v generates a vertical tab.

Echo Command Usage

• To print "Hi, this is a test.": $echo “Hi, this is a test.”
• To print "Hi," on one line and "this is a test." on the next: $echo “Hi, \n this is a test.”
• To enable backslash interpretation, use the -e option: $echo -e “Hi, \n this is a test.”
• To truncate output, use \c: $echo “Hi, \n this is a test.\c”

Quoting Characters and Their Meanings

• Double quotes ("") take everything literally except $, \, and double quotation marks.
• Single quotes ('') take everything literally except single back quotation marks.
• A backslash (\) makes any character following it literal.

Shell Variables

• Shell programs use environment and local variables.
• The command set displays all variables, and unset removes a variable.

Assigning values to variables

• They have to start with a lower or uppercase letter, not a number
• There are no spaces on either side of the assignment operator

Displaying Shell Variables

• The echo command displays text and variable values.

HOME Variable

• $echo $HOME shows the HOME directory pathname.
• $pwd shows current directory pathname.
• $cd changes the directory.
• Setting $HOME temporarily changes where the system thinks home is.

Standard Shell Variables

• PS1 is set to the string used as your prompt sign.
• PS2 is set to the prompt sign that is displayed whenever you press [Return].
• CDPATH is set to a list of absolute pathnames and affects the operation of the cd command.
• SHELL is set to the full pathname of your login shell.
• TERM sets your terminal type.
• TZ sets the time zone.

Command Substitution

• Command substitution with the grave accent mark executes a command within another command.

Sequencing Commands

• Use semicolons to enter a series of commands on one command line.

Grouping Commands

• Use parentheses to group commands together.

Background Processing

• Using the ampersand symbol (&) sends a command to background for execution

Chaining command

• Standard output from one process works as standard input for the other by making use of a | between the commands

Cat Command

• cat displays text files on screen.
• cat oldfile.txt > newfile.txt copies text files.
• cat /etc/hosts /etc/resolv.conf /etc/fstab > new_file.txt combines text files.
• cat > new_file.txt creates new text files.
• -n numbers all output lines.
• -A displays non-printing characters.
• -b numbers all output lines, skipping empty lines.

Cut Command

• cut -c4 file.txt prints the fourth character of each line.
• cut -c4,6 file.txt prints the fourth and sixth characters of each line.
• cut -c4-7 file.txt prints characters four through seven of each line.
• cut -c-6 file.txt prints characters from the beginning to the sixth character of each line.
• cut -d' ' -f2 file.txt prints the second field of each line, using a space as a delimiter.
• cut -f2 file.txt prints second field

Sleep and ws commands

• sleep command delays processing by a specified number of seconds.
• wc command counts the specified words from within a file.

Ps Command

• ps determines status of active processes in the system and arranges in 4 columns
• Using ps options will return different statuses of running processes
  • -a Displays the status of all the active processes, not just the user's.
  • -f Displays a full list of information, including the full command line

Kill Command

• kill can terminate a process or send a specific signal type.

Tee Command

• tee displays the output on a screen and stores it in a file.

Grep Command

• grep searches a specified pattern in a file or list of files, filtering output.
• grep Options:
  • -c Display only the count of the matching lines in each file with a match.
  • -i Ignore the distinction between lower and upercase letters
  • -l Display the name of each file that has a match.
  • -n Display the number for each displayed line.
  • -v display only lines that do not match the pattern.

Sort Command

• sort sorts the contents of a file into alphabetical or numerical order on a line-by-line basis.
• Sort Options:
  • -b Ignore leading blanks
  • -d This will display in dictionary order
  • -f Ignores the distinction between uppercase
  • -n Numbers are sorted by their arithmetic values.
  • -o Store the output in the specified file.
  • -rReverses the order of the sort from ascending to descending order.

What is a shell script

• Shell scripts are command languages that are executed by an interpreter
• The file contains a series of commands for the shell to execute

Executing a Script

• chmod command can make a files executable.
• Current directory (.) can be adding to PATH to execute

Command Line Parameters

• $0 contains the name of the script, as it was typed on the command line.
• $1, $2,... $9 contains the first through ninth command line parameters, respectively.
• $# contains the number of command line parameters.
• $@ contains all command line parameters: "$1 $2 ... $9".
• $? contains the exit status of the last command.
• * Contains all command line parameters: "$1 $2 ... $9".

Read Command

• The read command will receive keyboard inputs

Comments and Variables

• Comments recognize that any text with # in front.
• Assign values to variables, = is utilized and there needs to be no spaces either side of it

Exit commands

• Exit terminates executions
• $? gets the exit status

If-Then-Else Construct

• if [ condition ] then, execute true-commands, else execute false-commands.
• elif can be used for additional conditions.

Testing values

• Testing different categories utilizes different parameters
• Numeric values -eq, -ne, -gt, -ge, -lt, -le
• String values =, !=, -n string, -z string
• Testing files -r , -w, -s, -f, -d

String values

• The test command compares strings
• Operators: =, !=, -n, -z

Testing files

• test uses the test file characteristics for size, type and permissions

Parameter Substitution

• Parameter substitution tests the value of a parameter and changes its value according to a specified option.
  • ${variable:-string} evaluates to variable if set and not empty, otherwise string.
  • ${variable:+string} evaluates to string if variable is set and not empty, otherwise nothing.
  • ${variable:=string} evaluates to variable if set and not empty, otherwise sets variable to string.
  • ${variable:?string} evaluates to variable if set and not empty, otherwise prints string and exits.

Arithmetic Operations

• expr performs arithmetic operations, but spaces are necessary between elements in the expression.
• Multiplication and remainder characters must be escaped.
• Grave accent marks and cause the output of expr to be substituted.

Let Command

• let is a simpler alternative to expr command and includes all the basic arithmetic operations
• You can initialize like $ x=100
• Add one $ let x=x+1

Relational Operators

• Work on both numeric and nonnumeric arguments: =, !=, <, <=, >, >=

For-in-done construct

• Executes a set of commands a specified number of times.

While-Do-Done Construct

• The while loop continues as long as the loop condition is true.

Until-Do-Done Construct

• Continues to execute the body of the loop until the condition of the loop is true.
• If the loop condition is true initially the loop will never get executed

Startup files

• System files stored in /etc/profile get executed when the OS starts
• User profile includes /etc/profile, ~/.bash_profile, ~/.bashrc, ~/.bash_login, ~/.profile
• Logout user files are stored under ~/.bash_logout

Command Lince editing

• alias used for common actions
  • shortening the names of frequently used commands -ex changing command names
• history command keeps a list of all the commands you enter during your sessions
• remove your.sh_history or file from your HOME directory.

Debugging

• The $ sh can debug the program by reading the program without running, showing the shell or running and showing what's going on.
• -n reads the commands but does not execute them -v print and displays the shell input lines as they are read. --x shows the commands and their arguments as they are executed.