Introduction To Bash Shell Scripting

References

Review

Shell Variables

Some Command Useful With Scripts

Making Scripts Colorful

Shell Functions and Scripts

Operation With Variables

  • Test Operation

  • String Operation

  • Pattern Operations

    The handy 'expr'

    Flow Control

  • break/continue

    • break [n]      Break out of loop & select

    • continue [n]   Continue next iteration of loop

  • case

      case expression in
  pattern1[|pattern11] } statements ;;
  pattern2[|pattern21] } statements ;;
  ...
esac

    User Interfaces

  • select

      select name [in list] do
  statements that can use $name
done

    • Generate a menu of each item in the list formatted with
a number for each choice

    • Prompt the user for the number

    • Store the selected choice in name.  The number is stored in REPLY

    • execute the statements in the do

    • Repeat the process again

    Command Line Options

    • shift      Shift 1st argument form the argument list

    • getopts    Used to process command line options

    • OPTIND     Variable contains number of options

    • OPTARG     Default variable for option

    • Example
  while getopts ":ab:c" opt; do
    case $opt in
      a) process_option_a;;
      b) process_option_b
         $OPTARG is the option's argument;;
      c) process_option_c;;
    esac
  done
  shift $(($OPTIND - 1))

    I/O in Scripts

    • printf        Built-in command for formatted output

    • read          Built-in command to read one line into variable(s)
  read            #Everything entered goes to REPLY
  read var        #Everything is read into var
  read a b        #Read 1st word in to a and rest into b
  read -t 300 var #Read with 300 second timeout

    • Example how to use read from $file
  while read line; do
    do_something_with_line
  done <$file

    Process Handling

    • Signals

        Name  Number  Control Character

      • EXIT     0                      Used to trap exiting script

      • HUP      1                      Logout

      • INT      2        Control-C

      • QUIT     3        Control-\

      • KILL     9  can not be ignored or trapped

      • TERM    15                      Default kill

      • TSTP    24        Control-Z

    • Traps

      • trap "" signal-list      Ignore signal

      • trap "cmds" signal-list  Execute commands if signal is caught

      • trap signal-list         Reset signal to original condition

      • trap : signal-list       (undocumented) ignore signal, pass to child
                         Signal are normally not passed to subprocesses

      • Examples
  trap 'rm tmpfile; exit' 0 1 2 #remove tmpfile on exit, logout, interrupt

  trap "echo 'You hit Control-C'" INT
  while true ; do
    sleep 60
  done

      • Example parent child process
#!/bin/bash                   #parent
echo parent running
trap 'echo parent exiting; exit' 0
trap :2
child
sleep 1000

#!/bin/bash                    #child
echo child started. pid is $$
trap 'echo child got signal 2; exit' INT
sleep 1000

    Examples Useful Scripts or Function

      

    • #  Function top5   Example how to set defaults
#  Usage   top5 {n}   #list n processes
function top5 {
  ps -ef | head -${1:-5}
}


    • #  Function hereis    Example of HERE IS FILE and handling arguments
#  Usage  hereis word1 word2 ...
function hereis {
  for name in "$@"
    do
    cat <<MSG
        This is an example of an HERE IS FILE.
        One argument is ${name}.
        The date is `date`.
MSG
  done
}


    • #  Function pick    Return selected items by user
#  Usage:           .e.g  var=`pick *`
function pick {
  for name in $@ ; do                  #for each item in argument list
    echo -n "$name (y/n/q)?" >/dev/tty #ask user to select
    read ans                           #read answer from standard in
    case $ans in                       #Check choices
      y*) echo $name;;                 #selected
      q*) break;;                      #skip rest of arguments
      *)  continue;;                   #skip item
    esac
  done
}


    • #  Function acal   Display a nicer calendar
#  but will accept Alphabetic month
function acal {
  m=""
  case $# in
  0) cal; return;;                #no arguments
  1) m=$1; y=`date +%Y`;;         #1 argument
  2) m=$1; y=$2;;                 #2 arguments
  esac

  case $m in
  Jan*|jan* ) m=1;;
  Feb*|feb* ) m=2;;
  Mar*|mar* ) m=3;;
  Apr*|apr* ) m=4;;
  May|may   } m=5;;
  Jun*|jun* ) m=6;;
  Jul*|jul* ) m=7;;
  Aug*|aug* ) m=8;;
  Sep*|sep* } m=9;;
  Oct*|oct* ) m=10;;
  Nov*|nov* ) m=11;;
  Dec*|dec* ) m=12;;
  [1-9]|1[0-2] ) ;; #numeric month
  *)        ) y=$m; m="";;
  esac
  cal $m $y
}


    • ## Function selectex - Example select
#
function selectex () {
  choices="/bin /usr /home"
  select selection in $choices; do
 if [ $selection ]; then
   ls $selection
   break
  else
    echo 'Invalid selection'
  fi
done
}


    • #  Function fwhich  Which command in $PATH is executed
#
function fwhich {
  if [[ $# -eq 0 ]] ;
    then cat << EndOfHelp  1>&2; return 2
               Usage fwhich command   #Example of parsing the $PATH
                  Return 0 - found
                  Return 1 - not found
                  Return 2 - No arguments
EndOfHelp
  fi
  for path in `echo $PATH | sed 's/^:/.:/
                              s/::/.:/g
                              s/:$/:./
                              s/:/ /g'`
  do
    if [[ -x $path/$1 ]] ;  # does executable file exists here?
      then echo $path/$1    # found it
      return 0
    fi
  done
  return 1                  # not found
}


    • # Name:         overwrite   Copy standard input to output after EOF
function overwrite {
  if [[ $# -lt 2 ]] ;
    then echo "Usage:  overwrite file command [args]" 1>&2; return 2
  fi

  file=$1; shift
  new=/tmp/overwrite1$$; old=/tmp/overwrite2$$
  trap 'rm -f $new $old; return 1' 1 2 15  # clean up files
  "$@" > $new
  if [[ $? -eq 0 ]] ;                      # collect output
    then                                   # command completed successfully
      cp $file $old                        # save original file
      trap '' 1 2 15                       # we are committed; ignore signals
      cp $new $file                        # copy new file into file
      rm -f $new $old                      # remove temp files
    else
      echo "overwrite: $1 failed, $file unchanged" 1>$2
      return 1
  fi
}


    • # Name:         zgrep
# Purpose:      caseless grep of gzip files
# Usage:        zgrep text files.gz
#
function zgrep {
 if [ $# -eq 0 ] ; then
   echo "Usage: zgrep grep_text files.gz"
    return 2
 fi
  text=$1
  shift
  while  [ $# -gt 0 ]
    do
       echo $1
       gzip -cd $1 | grep -i $text
       shift
    done
}


    • # Name:         hgrep
# Purpose:      highlighting grep
# Usage:        hgrep pattern files
#
function hgrep {
 if [ $# -lt 2 ] ; then
   echo "Usage: hgrep pattern files"
    return 2
 fi
  pattern=$1;shift
  sep=$'\001'    #note use of $' ' to create control characters
  bold=$'\e[1m'; off=$'\e[0m'
  underline=$'\e[4m'; reverse=$'\e[7m'  #other choices of highlighting
  sed -n "s${sep}${pattern}${sep}${reverse}&${off}${sep}gp" $*
}