Introducing the shell

The shell is a user interface between you and the operating system. The most common is the bash shell (Born Again SHell).

The shell performs a number of tasks in a way that the builtins don’t know they happen before they are executed, such as:

  • evaluates special characters for pattern matching or globbing (wildcards)
  • handles pipes
  • redirects stdin and stdout

The term shell can reference a few things:

  1. The concept of the linux shell, as in “run the command in the shell”
  2. A specific instance of the shell running on your machine

Pattern matching

Pattern matching or globbing are when you use the asterisk (*) character to match any character pattern.

  • if there are no matches, the shell tries to match the pattern literally
  • the shell always evalutates the pattern before passing the result to the command
grep one *.txt          # * matches any number of characters
grep one on?.txt        # ? matches any one character
grep one file[12345]    # match a single character within the set bracket []
grep one file[1-5]      # range syntax, equivalent to [12345]
grep one file*[0246]    # combination syntax, match any even number
ls *.doc                # no match, tries to match literally
ls: cannot access '*.doc': No such file or directory

Variables

A running shell can define variables that store values. Each variable has a name and a value.

Print a variable value with printenv or by prepending the variable name with a $:

  • When you use the $ syntax, the shell evaluates the variable before passing it to the command
printenv HOME   # /home/elinux
printenv USER   # elinux
echo $HOME      # /home/elinux
echo $USER      # elinux

Defining variables

There are pre-defined and user-defined variables. Variables like HOME and USER are pre-defined by the shell. By convention, they are uppercase.

User-defined variables work the same as pre-defined vars:

  • Do not put a space on either side of the equals sign
name=value
$name

dev=$HOME/Development/
cd $dev
~/Development

Aliases

An alias is a name that stands in for a command, much like a variable is a name that stands in for a value:

  • called shadowing because an alias command takes precendence over the built-in command. The alias “shadows” the original command
alias                   # list all aliases in current shell
alias ll                # get value of ll
unalias ll              # delete ll alias

alias ll="ls -lh"       # create alias
ll
total 36K
-rw-rw-r-- 1 elinux elinux  325 Mar 25 18:45 animals.txt
-rw-rw-r-- 1 elinux elinux   93 Mar 27 13:29 grades
...

Redirecting input and output

Input redirection is when the shell redirects stdin to come from a file rather than the keyboard.

  • there are three unique streams: stdin, stdout, and stderr
wc < file.txt                       # input redirection
wc < file.txt > output              # redirect input and output
cp nothing file.txt 2> errors.log   # redirect stderr
cp nothing file.txt 2>> errors.log  # append stderr
cp nothing file.txt &> errors.log   # redirect stdout and stderr

Quotes and backslashes

You can change how expressions are evaluated with single quotes, double quotes, and backslashes:

  • single quotes: evaluates every character literally, including $ and \
  • double quotes: recognize some characters, such as $
  • backslash: escape character that tells the shell to treat the next character literally.
    • Can also place at the end of the line to create multi-line commands for readability
    • can also “unshadow” an alias, meaning that if a command has an alias, it runs the original command
echo \$HOME
echo "This is \"the most\" example"
\ll                     # unalias
ll: command not found

PATH

A list of directories in memory that the shell searches for binaries when you execute a command. Also called the search path:

  • if a command is in multiple dirs in the search path, your machine runs the first one it encounters. This lets you override system programs with your own by placing binaries in user dirs that are searched before system dirs
  • which locates a program in the search path
  • type can locate builtins, aliases, functions, and programs
echo $PATH | tr : "\n"          # print dirs on own line

Environment

The shell’s environment consists of variables that store the search path, cwd, preferred text editor, customized shell prompt, etc.

  • When the shell exits, the environment is destroyed
  • Define your environment in startup files
  • .bashrc (dot bash R-C)