Coding Bash vs. Perl vs. Python (by Evan Liu)

#1.1. create the script file named /tmp/tst
cat>/tmp/tst <<"EOD"
  echo "\$0 returns program name:          $0"
  echo "\$1 returns the 1st argument:      $1"
  echo "\$* returns the argument list:     $*"
  echo "\$@ returns the argument list:     $@"
  echo "\$$ returns the shell PID:         $$"
  echo "\$_ returns the lastest command:   $_"
  echo "\${#@} returns the argument number:${#@}"
  echo "\${!#} returns the last argument:  ${!#}"
  echo "\$# returns the argument num:      $#"
  echo "\$- returns the shell options list:$-"
  echo "\$OPTARG returns the next element of argv:$OPTARG"
  echo "\$OPTIND returns the next index of argv:  $OPTIND"
EOD

#2.2. invoke the script file to test special variables
bash /tmp/tst a{1..3}

#3.3. create a function named func1
func1(){
  echo "\$0 returns program name:          $0"
  echo "\$1 returns the 1st argument:      $1"
  echo "\$* returns the argument list:     $*"
  echo "\$@ returns the argument list:     $@"
  echo "\$$ returns the shell PID:         $$"
  echo "\$_ returns the lastest command:   $_"
  echo "\${#@} returns the argument number:${#@}"
  echo "\${!#} returns the last argument:  ${!#}"
  echo "\$# returns the argument num:      $#"
  return 0 #zero is right,function will be called as subshell
}

#4.3. invoke the function to test the special variables
func1 p{1..4}  #or func p1 p2 p3 p4
echo "the return value is $?"

-------output----------

#1.1. create the script file named /tmp/tst

#2.2. invoke the script file to test special variables
$0 returns program name:          /tmp/tst
$1 returns the 1st argument:      a1
$* returns the argument list:     a1 a2 a3
$@ returns the argument list:     a1 a2 a3
$$ returns the shell PID:         18805
$_ returns the lastest command:   $$ returns the shell PID:         18805
${#@} returns the argument number:3
${!#} returns the last argument:  a3
$# returns the argument num:      3
$- returns the shell options list:hB
$OPTARG returns the next element of argv:
$OPTIND returns the next index of argv:  1

#3.3. create a function named func1

#4.3. invoke the function to test the special variables
$0 returns program name:          bash
$1 returns the 1st argument:      p1
$* returns the argument list:     p1 p2 p3 p4
$@ returns the argument list:     p1 p2 p3 p4
$$ returns the shell PID:         18803
$_ returns the lastest command:   $$ returns the shell PID:         18803
${#@} returns the argument number:4
${!#} returns the last argument:  p4
$# returns the argument num:      4
the return value is 0

#1.1. create the script file named /tmp/tst
cat>/tmp/tst <<'EOF'
use feature ":5.10";
say "the script name :\$0 -->$0";
say "the argument list:\@ARGV -->@ARGV";
say "the argument 1st :\$ARGV[0]  -->$ARGV[0]";
say "the argument num :\scalar(\@ARGV) -->".scalar(@ARGV);
say "the argument num :0+\@ARGV -->".(0+@ARGV);
say "the argument num :1+\$#ARGV -->".(1+$#ARGV);
EOF

#2.2. invoke the script file to test special variables
perl /tmp/tst arg1 arg2 arg3


-------output----------

the script name :$0 -->/tmp/tst
the argument list:@ARGV -->arg1 arg2 arg3
the argument 1st :$ARGV[0]  -->arg1
the argument num :scalar(@ARGV) -->3
the argument num :0+@ARGV -->3
the argument num :1+$#ARGV -->3

#1.1. create the script file named /tmp/tst
cat >/tmp/tst<<'EOF'
import sys
import os
print "the argument array: sys.argv -->", sys.argv
print "the script name: sys.argv[0] -->", sys.argv[0]
print "the script name: __file__ -->", __file__
print "the array of path: sys.path -->", sys.path[0]
print "the script path: sys.path[0] -->", sys.path[0]
EOF

#2.2. invoke the script file to test special variables
python /tmp/tst arg1 arg2 arg3

-------output----------

the argument array: sys.argv --> ['/tmp/tst', 'arg1', 'arg2', 'arg3']
the script name: sys.argv[0] --> /tmp/tst
the script name: __file__ --> /tmp/tst
the array of path: sys.path --> /tmp
the script path: sys.path[0] --> /tmp

#1.1. Copy and paste all the content before the '---output--

' part in linux bash prompt:
bash <<"EOF"
echo "the login name: \$LOGNAME -->$LOGNAME"
echo "the current working dir: \$PWD -->$PWD"
echo "the user default working dir: \$HOME -->$HOME"
echo "the users mail file full path: \$MAIL -->$MAIL"
echo "the current shell: \$SHELL -->$SHELL"
echo "the shell search path: \$PATH -->$PATH"
echo "the default locale: \$LANG -->$LANG"
echo "the current terminal type: \$TERM -->$TERM"
echo "the primary system prompt: \$PS1 -->$PS1"
echo "the secondary system prompt: \$PS2 -->$PS2"
echo "the Internal field separator: \$IFS  -->$IFS"
#  echo "the files colour: \$LS_COLORS -->$LS_COLORS"
EOF

#1.1. Copy and paste all the content before the '---output--

' part in linux bash prompt:
#  paste below in bash shell
script=$(cat <<'EOF'
use feature ":5.10";
say "\$ENV{'LOGNAME'} -->$ENV{'LOGNAME'}";
say "\$ENV{'PWD'} -->$ENV{'PWD'}";
EOF
)
#  run the script in bash shell
perl -e "$script"

#1.1. Copy and paste all the content before the '---output--

' part in linux bash prompt:
#  paste below in bash shell
python <<'EOF'
import os
print "os.getenv('LOGNAME')-->"+os.getenv('LOGNAME')
print "os.environ['PWD']-->"+os.environ['PWD'] 
#  print all, commented for too many output
#  for key in os.environ.keys():
#  print "%s  %s" %(key,os.environ[key])
EOF

bash <<"EOF"
  echo "current bash PID: \$\$--> $$"
  pgrep "$0"
  echo "parent PID of the bash: \$PPID --> $PPID"
  sleep 1 &
  echo "last background process PID: \$! --> $!"
  cat /etc/noexists 2>/dev/null
  echo "exit status of the last command: \$? --> $?"
EOF

-------output----------

current bash PID: $$--> 18811
18811
29861
31291
32127
56685
64466
64734
parent PID of the bash: $PPID --> 18810
last background process PID: $! --> 18813
exit status of the last command: $? --> 1

#1.1. print out process related variables
#  paste below in bash shell
script=$(cat <<'EOF'
use feature ":5.10";
say "current script PID: \$\$ -->$$";
system "cat /etc/noexists 2>/dev/null";
say "system call exit status: \$? -->$?";
open(FH,'<',"/etc/noexists") or 
say "error message: \$! -->$!";
foreach (@ARGV) {
say "default arg in loop or grep:\$_ --> $_";
}
sub func1 {
say "subroutine argument list: \@_ --> @_";
}
func1(@ARGV);
func1("p1", "p2", "p3");
EOF
)
#  run the script in bash shell
perl -e "$script" arg1 arg2 arg3

-------output----------

current script PID: $$ -->18816
system call exit status: $? -->256
error message: $! -->No such file or directory
default arg in loop or grep:$_ --> arg1
default arg in loop or grep:$_ --> arg2
default arg in loop or grep:$_ --> arg3
subroutine argument list: @_ --> arg1 arg2 arg3
subroutine argument list: @_ --> p1 p2 p3

#  todo

-------output----------

#  todo