Introduction to bash

• A "shell" is a program that allows a user to interact with a computer's services.
• A shell is traditionally run in a terminal window, and interaction is by a text-based command-line interface (CLI).
• A graphical shell (GUI), e.g., Mac's Finder, Linux Gnome, provides a pointy-clicky cover over the CLI, and often offers less direct access to the computer's services.
• In a UNIX CLI the user provides commands to get things done.
• which tells if a command is builtin to the shell or an executable
  help lists all the builtin commands

• After a user logs into a UNIX system, the shell program specified in their password entry is run (stored at /etc/passwd, use getent passwd user_name to extract it on CSIF), and it is the shell that provides the prompt.
  • A shell may be run in other contexts, e.g., in a shell script or as a program like any other.
• A bash login shell begins by executing commands from the system files /etc/profile and then from one of the user's personal initialization files: ~/.bash_profile, ~/.bash_login, or ~/.profile.
• Sample configuration file for bash
  • .bash_profile (standard code that exports Anaconda's path to the environment)

• echo is used for output, e.g.,
  echo "Hello World!"

• Many characters have special meaning in most shells, including space $ ( ) [ ] * & | < > ; \ ` ' "
• Command arguments may be enclosed in 'single quotes' or in "double quotes" to create a single argument with spaces in it. Example: File names with spaces
• Backslash \ can be used to suppress the special meaning of individual characters, e.g., \' is a quote, \\ is a backslash. Example: Filenames with $s in them

• The shell maintains a list of variables, each of which has as value a list of zero or more words. The values of shell variables can be set with the export or simple assignment (e.g., VAR=value) and cleared with the unset command.
• Values of variables are accessed by adding a $ prefix (same for environment variables)

• These tell the shell to replace input commands.
• Typically defined in the user's shell resource file

  alias ls='ls -F'
  alias h='history'
  alias vi='vim'
  alias ispell='ispell -t -d english -B -S!*' 

• Use the alias and unalias commands to set, examine, and clear aliases.

• Every process starts with a standard input (stdin), standard output (stdout), and standard error (stderr) stream, each connected to some device.
• The standard stream of a command may be redirected.
• > name - The file name is used as standard output
• >> name - Like >, but appends output to the end of name.
• A series of commands joined by | characters forms a pipeline. The stdout of the command in a pipeline is connected to the stdin of the next.
  • stderr output may be directed through a pipe with the stdout using |& (in newer bash versions)

• A shell script is a sequence of shell commands
• Lines starting with # are comments
• $1, $2, etc., are arguments to the script
• Two ways to run scripts
  • source <script file> or ./<script file>
  • Add a line that specifies which shell should be used to execute the commands on its first line, with a leading #!, e.g., #!/bin/bash. Make the file executable (with chmod +x your_file_name).
• LookingForInts - Sample bash script

Exercises

• How many builtin commands are there in your shell?
• What does the source command do?

Exam Style Questions

• Give the command to set a shell variable named NewPath to the current value of the PATH variable.
  NewPath=$PATH
• Explain how to make a shell script into a standalone executable.
• Explain the effect of putting the following lines in your .bash_profile file:

  alias vi='vim'
  alias pico='vi' 

• Explain the underlying mechanism that causes a command such as

  ls -la | grep ecs34 

  to send the output from ls to the input of grep.