Shell

• Command interpreter
• Accepts user commands, executes them
  • A UNIX shell commonly uses fork(), wait(), and exec() to launch user commands.
• With redirection, pipes, flow control, heredocs, scripting, job control…
  • The separation of fork and exec enables features like input/output redirection, pipes, etc., without changing anything about the programs being run.

POSIX Shell

• POSIX: Portable Operating System Interface
  • Set of standards specifying the interface between a Unix-like OS and user programs.
• A POSIX shell adheres to POSIX standard for shell syntax, built-in commands, environments variables, and other shell features.
• Provides consistent and portable environment for executing commands, scripting, and interacting with OS.

Redirection:

• The ability to change where input comes from and where output goes to in a command line.
  • Example: command > output.txt
  • Redirection achieved with <, >, and >>
• Output stdout or stderr to a file.
• Done with dup2(2)

Pipes

• Connection between two processes.
  • Example: $ echo "Hello, old friend. | cowsay
  • | character used to represent pipe.
  • echo "Hello, old friend." is passed as input to cowsay
• Output from one program becomes the input another (one way).
• pipe(2): creates a descriptor for reading and one for writing
• dup(2): sets up stdout/stdin
• File descriptors inherited across fork(2) and execve(2).

Job Control

• Ability to manage multiple processes running within shell environment.
• Command execution is normally synchronous. Shell waits (wait(2)) for completion of the child process
• & runs command in the background; Ctrl+Z moves current command to the background
• built-in jobs command lists jobs that can be foregrounded with fg or killed with %1 or %2
• Ctrl-C sends SIGTERM

Environment Variables

• Variables set in shell environment and inherited by child processes.
• Used to store information such as sysconfig, user prefs, runtime params.
• Examples:
  • PATH: Specifies directories to search for exec files
  • HOME: Specifies user home directory
  • LANG: Specifies language and locale settings

Heredocs

• Allows you to pass multiple lines of input into command or script w/o using external file.
  • i.e., can construct a temporary file in memory.
• Can be represented using <<
• Example:

cat << END
This is line 1
This is line 2
END

Flow Control

• Lets you control execution flow of a script or program based on conditions.
  • Constructs (for, while)
  • Conditionals (if, case)
  • and Function Definitions
• Allows you to make decision, repeat actions, and organize execution of commands in a script
  • i.e., sequential conditional execution

Scripting

• Lots of useful things can be expressed in shell
• Don’t want to type same commands repeatedly? Save shell commands in a shell script.
• script directly executable with #! (“shebang”)