Linux and UNIX Shell Basics

Aaron Toponce

Linux Guru

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What is a shell?

• Provides system interface
• Sits between user and kernel
• Loaded at bootup until shutdown
• Resides in memory
• Contains built-in commands

Interactive vs. Non-interactive

• Interactive
  • Prompts for input
  • Processes user commands
• Non-interactive
  • Shell scripts
  • No user input
  • System accounts

Login vs. Non-login

• Login
  • Start non-login shells
  • Sets the $PATH, $UID, $GID, $TERM variables
  • /etc/profile, ~/.bash_profile
  • /etc/bashrc, ~/.bashrc
  • su --login, su -l, su -
  • sudo -i
  • GUI and tty
• Non-login
  • gnome-terminal, new tabs, etc.
  • $PATH, etc., not set
  • /etc/bashrc, ~/.bashrc
  • Changing shells
  • su
  • sudo -s

Available Shells

• cat /etc/shells
• aptitude search sh | grep -i shell
• Interactive: ash, bash, csh, dash, esh, ksh, pdksh, sh, tcsh, zsh
• Non-interactive: /bin/false, /usr/sbin/nologin
• Default login: cat /etc/passwd
• Change default: chsh

Subshells

• Commands nested with `` or $()
• EG: echo "Date is `date`"
• shell built-ins vs binaries

Programming Quickstart

• #!/bin/bash
• # denotes a comment
• Script is just shell commands
• if then else, if then elif else, case
• while do done
• for do done

Shell Variables

• env vs set
• export adds 'set' variables
• syntax: variable=value
• echo $variable
• alias

File Globbing

• Simple shell metacharacters
• Not the same as REGEX!

File Globbing Continued

• \ interpret the following characeter literally
• ; separate commands
• $ Substitute variables
• ? Match single character
• * Match zero or more characters
• [abc] Match for one character from set
• [a-c] Matches a range of characters
• [!abc] Don't match from set
• (cmds) Execute commands in subshell
• {cmds} Execute commands in current shell

Q&A

Any comments, questions or rude remarks?