CS 480 M01 Lecture Notes 8 - Sep 19, 2022

Ch 7. Scripting and the Shell

• standardize and automate through scripts ... - structured, reproducible, and replicable admin changes
      X checklists or memory
  Plus it saves time & serves as a documentation
  Useful Commands: awk, cut, find, grep, head, sed, sort, tail, tee, uniq, wc, ...
  
• Scripting Philosophy
  • write microscripts - many .... "How can I avoid to deal with this again?"
    in ~/bin , functions in .bash_profile ... , & aliases
    
  • learn few tools well: shell, editor, scripting language
  • automate everything
  • don't optimize too soon
  • pick the right (scripting) language (semi related ... python vs perl former student's opinion/email)
  • follow best practices:
    • usage message (print it out if wrong arguments and exit; plus implement --help)
    • input validation
    • meaningful exit code (0=success, nonzero=failure)
    • appropriate naming conventions for variables, scripts, and routines
    • variable names reflecting stored values but short: number_of_lines_of_input X n_in_lines
    • style guide
    • comment block at the top (what it does and expects, author, date, ...)
    • good comments
    • not suid (hard to make it correctly and safe, use sudo instead)
    • STDERR/STDOUT
    • no half-baked scripts
    • ...
  • clarity and programmer efficiency over computational efficiency
• Shell Basics
  • Command Editing:
    • all basic emacs commands available : [Ctrl-E], [Ctrl-A], [Ctrl-F], [Ctrl-B], [Ctrl-P], [Ctrl-N], [Alt-F], [Alt-B], ...
      (try   emacs -nw   then   [TAB] few times to Emac tutorial and hit [ENTER])
    • changing to vi-mode: set -o vi / set -o emacs
  • Pipes and redirection, Variables and quoting
    • STDIN, STDOUT, SDTERR - three communication channels available for every process
      set by kernel, normally keyboard and screen
      in UNIX / Linux they are always file descriptors 0, 1, 2
    • shell interprets < , > , >> as instruction to reroute standard input/output
      • echo "test" > /tmp/myfile
      • mailx -s "test mail" itest@cs.nmsu.edu < /tmp/myfile
      redirecting SDTERR: >& (both) or 2> (only STDERR)
      find / -mount -name core -ls > /tmp/corefiles 2> /tmp/corefiles_errors
find / -mount -name core -ls > /tmp/corefiles 2> /dev/null

      Above is true for bash BUT different shells (csh, tcsh, ..) may behave differently ! :
      (find / -mount -name core -ls > /tmp/corefiles_csh) >& /tmp/corefiles_errors_csh
      

    • Pipes:
      • ps aux | grep ssh
      • cut -d ":" -f 7 < /etc/passwd | sort
cut -d ":" -f 7 < /etc/passwd | sort -u
cut -d ":" -f 7 < /etc/passwd | sort | uniq
cut -d ":" -f 7 < /etc/passwd | sort | uniq -c
        
    • && : execute a second command only if the previous command completes successfully
      mkdir /tmp/foo && cd /tmp/foo
echo $?
mkdir /tmp/foo && cd /tmp/foo
echo $?
    • || : execute a second command only if the previous command fails
      mkdir /tmp/foo || cd /tmp/foo
echo $?
    • Next: Variables:
    • ...