0105 | Shells

Shells

Common Command Line Shells

• korn shell | ksh
• c shell | csh
  • popular with programmers
• upgraded c shell | tcsh
  • popular with programmers
• z-shell | zsh
  • more modern
  • default in kali
• some commands like cd are built into the shell
  • like file and path expansions (* -- wildcard)
  • the shell first interprets the wildcard, then substitutes the names and sends them to ls
    • ls file*.txt

Environment Variables

• each shell has it's own copy of the variables
  • changes in one, will NOT affect the others
• list current vars in current shell
  • printenv
• variable format
  • <var-name>=<var-value>
• by convention var name is all UPPERCASE
• PATH variable
  • the list of the directories that are searched in order to find commands that are to be executed
• command execution in shell
  • 1 -- type the command
  • 2 -- shell checks if it's a built-in command
  • 3 -- if not built-in command, search PATH directories for the command
  • 4 -- if command exists in multiple directories, first is executed
• global vs local environmental variables
  • global vars can be accessed by anything executing in that shell
    • like scripts and by and sub-shells
  • naming
    • global variables = environmental variables
    • local variables = shell variables
  • example | local variable

    # create local val
    COUNT_LOCAL=42
    echo $COUNT_LOCAL	# will print the value 
    echo COUNT_LOCAL 	# will print the string
    # creating subshell
    bash
    echo $COUNT_LOCAL	# is not defined
    exit
  • example | global variable

    # create global variable
    export COUNT_GLOBAL=55
    echo $COUNT_GLOBAL
    # create sub-shell
    bash
    echo $COUNT_GLOBAL 	# var is defined
    exit			  
    	
    # unset the variable
    unset COUNT_GLOBAL
    echo $COUNT_GLOBAL 	# no longer defined

Startup Files

".bashrc" and alias

• when you start a new bash shell, the shell is configured via startup files
• 3 types of shells
  • interactive -- we can execute commands
  • non login -- not a shell that was created as a result of a successful login
• adding an alias for rm

  vim .bashrc
  # r:recursive; f:force; i:interactive
  alias del='rm -rfi'
  alias c='clear'
  # save the file  
  	
  # rerun all the commands in the .bashrc file
  source .bashrc

Redirecting Input and Output

• 3 conceptual files are always open
  • stdin | 0
  • stdout | 1
  • stderr | 2
• redirecting output
  • ls /etc/ > ~/dir-contents
• append content to existing files
  • ls /etc/ >> ~/dir-contents
• redirecting input
  • head < /etc/passwd
  • shell takes the entire content of the file and sends it to the head utility
• redirecting error to "black hole"
  • find / -name 'sample.txt' 2> /dev/null
• redirecting multiple at the same time

  # redirecting both output and error
  find / -name 'sample.txt' &> all.txt
  # or -- the SAME
  find / -name 'sample.txt' > all.txt 2>&1
  
  # redirecting to different locations
  find / -name 'sample.txt' > location.txt 2> /dev/null

Pipes

• connect the stdout of one command to the stdin of an other command
• examples

  # look through the directory listing
  ls -l /etc/ | less
  ls -l /etc/ | head -n 20 | tail -n 5
  find / -name 'sample.txt' | less
• if you want to redirect the stderr too --> use "|&"
  • find / -name 'sample.txt' |& less

Command History

• previously executed commands
  • history
• execute previously executed command
  • !199
• indirectly excute last command
  • !-1
  • !!
• executing the last cat command
  • !cat
• modifying the previously executed command
  • ^file1.txt^fileA.txt
  • text between the first set of carrots --> the previous command
  • between the next --> the replacement text
• history size is determined by HISTSIZE and HISTFILESIZE in ".bashrc"

Command Substitution

back-ticks (`) and $(...)

• examples | with back ticks to execute those inside first
  • once the inside is executed, it will substitute the content of the file with the command

  # use backticks `
  ls -l `cat file-list.txt`
  # or -- the SAME -- (preferred)
  ls -l $(cat file-list.txt)