CS 480 M01 Lecture Notes 9 - Sep 21, 2022

Last Time: Ch 7. Scripting and the Shell Part 1

• Useful Commands: awk, cut, find, grep, head, sed, sort, tail, tee, uniq, wc, ...
• Scripting Philosophy
  microscripts 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
  follow best practices: usage message, input validation, meaningful exit code (0=success, nonzero=failure), ...
  clarity and programmer efficiency over computational efficiency
• Shell Basics
  Command Editing (emacs commands, changing to vi)
  Pipes and redirection, Variables and quoting
  STDIN, STDOUT, SDTERR in UNIX / Linux always file descriptors 0, 1, 2
< , > , >> to reroute standard input/output
  redirecting SDTERR: >& (both) or 2> (only STDERR)
  Pipes ('|'),
  && and || ( execute 2nd command if 1st succeds / fails )
  

Ch 7. Scripting and the Shell Continued

• Shell Basics Continued
  • Pipes and Redirection, Variables and Quoting Continued
    • || : execute a second command only if the previous command fails
      ls -al /tmp/foo
cd /tmp/foo || mkdir /tmp/foo; cd /tmp/foo; ls -al
ls -l /tmp >& output || echo "error: cannot list files..."
ls -l /root >& output2 || echo "error: cannot list files..."
    • Variables:
      Variable names - unmarked in assignments ; prefixed with $ when their values referenced
      myvar='/etc'   or   myvar=/etc
      but watch for spaces:   myvar='sth with spaces'   or   myvar=sth\ with\ spaces
echo $myvar2   vs   echo $myvar/2   vs   echo ${myvar}2  
      

      in csh or tcsh : set myvar=/etc
echo $myvar
ls -l ${myvar}/passwd
ls -l $myvar/passwd

      n=1
m=$((n + 1 ))
echo "n=$n, m=$m"

    • Enviroment Variables:
      printenv, env
    • Filters: any pgm that reads STDIN and writes STDOUT can be used as a filter (part of a pipeline)
      Common filter commands: cut, sort, uniq, wc, tee, head, tail, grep (perl can be used instead usually...)
      Read, study, and try the many examples in the textbook!
      find /etc -name passwd 2> /dev/null | tee /dev/tty | wc -l
ps aux | grep -v grep | grep `echo $$`
$?
echo $?
echo $?
• Sh Scripting
  • From commands to scripts:

    tail -13 /etc/hosts
    tail -13 /etc/hosts | grep -v "^#"
    tail -13 /etc/hosts | grep -v "^#" | perl -ane 'print $F[1],"\n"'
    tail -13 /etc/hosts | grep -v "^#" | perl -pe 's/^\S* *//'
    tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}'
    tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}' | while read stl do echo $st; done
    tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}' | while read st; do echo $st; done
    tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}' | while read st; do echo "ping -c 1 $st | grep 0%"; done
    tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}' | while read st; do echo "ping -c 1 $st | grep 0%"; done | sh - x
    tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}' | while read st; do echo "ping -c 1 $st | grep 0%"; done | sh -x
    		

    also

    for st in `tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}'`; do echo "ping -c 1 $st | grep 0%"; done | sh -x
    		

    or

    for st in `tail -13 /etc/hosts | grep -v "^#" | awk '{print $2}'`; do echo $st; ping -c 1 $st | grep 0%; done
    		

    while loop with counter:

    	 n=1; while [ $n -lt 5 ]; do echo $n; n=$((n + 1)); done
    		

  • Spaces in file names: quote or escape, ! shell scripts need to be ready ...

       find   pathname(s)   other_args   -print0   |    xargs   -0   ls   -l
    or (? better - compare ...)
       find   pathname(s)   other_args   -exec   ls   -l   {}   \;

    
    find . -type f -mtime -10 -name \*.pl -print0 | xargs -0 chmod a+x
find . -type f -mtime -10 -name \*.pl -exec chmod go-x {} \; -ls

  • Command-line arguments, functions, and control flow

    
    #!/bin/bash
    show_usage(){
    	...
    }
    if [ $# -ne 2 ]; then 
    	show_usage
    else 
    	if [ -d $1 ]; then  # 1st argument, $0 - invoked script name
    	elif [ ... ] && [ ... ] ;  then 
    	else 
    		
    	fi
    fi
    	

  
  

  	var1=1; var2=2; if [ $var1 -lt $var 2 ]; then echo "$var1 is less then $var2"; else echo "$var1 is NOT less then $var2"; fi
  	var1=1; var2=2; if [ $var1 \< $var 2 ]; then echo "$var1 is less then $var2"; else echo "$var1 is NOT less then $var2"; fi
  	var1=a1; var2=a2; if [ $var1 \< $var 2 ]; then echo "$var1 is less then $var2"; else echo "$var1 is NOT less then $var2"; fi
  	

  
  
• Regular Expressions: read the book
  filename matching and expansion (shell globbing) is NOT regular expression


• Python, Ruby, GIT ...


• Simple Shell Script Example:
  rotate.sh

  #!/bin/sh
  cd /tmp
  for i in  9 8 7 6 5 4 3 2 1 0 ;  do
          j=`expr ${i} + 1`
          echo "$i -> $j"
          if [ -f log.${i} ]; then 
                  mv log.${i} log.${j}
          fi
  done
  if [ -f log ]; then 
          mv log log.0
  else
          echo "no previous log file found ..."
  fi
  touch log
  chmod 600 log
  

  The 3rd line could also be

  for i in `seq 9 -1 0`; do
  

  The 4th line could also be

  j=$((i+1))
  

  rotate.csh

  #!/bin/csh
  cd /tmp
  foreach i (9 8 7 6 5 4 3 2 1 0 )
          set j = `expr $i + 1`
          echo "$i -> $j"
          if ( -f log.${i} ) then 
                  mv log.${i} log.${j}
          endif
  end
  if ( -f log ) then 
          mv log log.0
  else
          echo "no previous log file found ..."
  endif
  touch log
  chmod 600 log