Basic Shell (Console) operation for beginners

[Bruce B]

Just for fun, how fast can your computer execute 1,000,000 commands?

The script will execute
date 2x
: 999,998x

: is a command which says do nothing and give an successful error
code which would be 0

Code: Select all

#!/bin/bash

date
for ((i=1;i<999999;i++));do
:
done
date

It takes my midrange machine about 17 seconds

Script name: millionx (attached)

Isn't there a command for timing execution? If so, what's its name?

~

EDIT - an improved millionx file is available down a few posts, better to
use it.

[Bruce B]

The if statement

This script gives examples of

• the if statement
  conditional checks and branching according to the condition
  using echo -n for echoing output without a linefeed
  how to get user input from within the program
  how to quote variables in on screen output
  using echo to print and format the output
  the 'exec' command to start a new script and terminate the existing script

I recommend downloading, reading, studying and running this script
You will be using the if statement and other commands in the script
over and over again when writing your scripts.

The 'for loop' is another which need be committed to memory.

Code: Select all

#!/bin/bash

echo -n "Please enter a primary color: "
read clr
echo

if [ "$clr" = "red" ] ; then
	echo " You entered a valid primary color \"$clr\""
elif [ "$clr" = "green" ] ; then
	echo " You entered a valid primary color \"$clr\""
elif [ "$clr" = "blue" ] ; then
	echo " You entered a valid primary color \"$clr\""
elif [ "$clr" = "yellow" ] ; then
	echo " You entered a primary color used in painting \"$clr\""
else
	echo " \"$clr\" is not known to the program as a primary color"
	echo " We use red, green, and blue as primary colors"	
fi	

echo
echo -n "Again (y,n)? "
read -n 1 a
echo

if [ "$a" = "y" ] ; then 
	echo 
	exec $0

fi

echo

script name: colors, file attached

~

[Bruce B]

In a recent post I presented an idea of 'something in - something out'

We all know the concept unconsciously. We press the 'm' key, this is input.
We expect to see 'm' echoed as output on the screen. When it doesn't happen,
we think something is wrong. What is wrong can be conceptualized
as an input/output problem.

In this post I want to talk about thinking in terms of true and false.
If we are sane, we don't think in absolutes. In programming, true
and false is often all we are making our decisions on. If you program
for several years, you will probably lose sanity.

My advice is, after the sanity is lost, be calm, quite, soft spoken,
articulate and whatever you do, don't appear to be a danger to
others. Chances are they won't lock you up, if you take my advice.

Let's look at our if statement from a true / false perspective.

Code: Select all

if [ "$clr" = "red" ] ; then
	if false no commands execute
	if true the commands execute
elif [ "$clr" = "green" ] ; then
	the elif is shorthand for 'else if'
	the only chance of the elif running commands is if the 'if' was not true
	the elif can only execute if it is true
	if false nothing happens
elif [ "$clr" = "blue" ] ; then
	same as above
elif [ "$clr" = "yellow" ] ; then
	same as above above
else
	if none of our if or elif statements were true
	the else commands will execute
	they will only execute under these circumstances
fi	

We have five possible conditions, but only one will execute commands

We only need the 'elif' and 'else' - if we need them.

The basic if statement is like this

Code: Select all

if [ "3" = "4" ] ; then
	echo 3 equals 4
fi

3 doesn't equal 4 so nothing happens, because it is a false condition

Code: Select all

if [ "4" = "4" ] ; then
	echo 4 equals 4
fi

Above we have a true condition, so the command runs.

Code: Select all

if [ "3" = "4" ] ; then
	echo 3 equals 4
else
	echo 3 does not equal 4
fi

See how it works, the first test is false, we programmed in the else.
The else executes because the first 'if test' was false. If it were true
the else would not execute commands.

~

[Bruce B]

Piping commands through scripts - introduction

We can make some very useful scripts (and C programs) for use as
pipes.

I'll give a couple bare bones examples

Filename: upperc
contents:
tr "a-z" "A-Z"

Filename lowerc
contents:
tr "A-Z" "a-z"

To make the pipes, put the corresponding line at the top of the file,
name the file and make it executable.

Then if you want to change case of a string of text or even a file, you
can pipe the text through the executable text file.

You can actually pipe through some complex scripts, but this is just
an introductory to the concept.

~

[Bruce B]

About Characters

Our characters are not really characters from the computer's perspective.
They are binary values.

There are currently eight people in the whole world, aliens maybe, who
can read binary strings.

Thousands of propeller heads can read octal and hex.

Normal people read decimal math. Although hex is very useful in
programming.

Each character we see doesn't exist except by translation from binary to
character.

Sometimes we want to know the value of our characters, especially in C
and probably other programs.

The following program will display characters and their decimal
equivalents in your bash shell.

You might want to keep the program for future reference.

After the program runs, you can scroll up to see what scrolled off screen.

Code: Select all

#!/bin/bash

for ((i=33;i<127;i++)) ; do
	printf "\x$(printf %x $i )"
	echo "  has decimal value of $i"
done

Script file attached

~

[Bruce B]

A C type 'for loop' construct

I've used this construct twice so far, without explanation.

for ((i=33;i<127;i++))

i=33 initializes the variable i with the value of 33
i<127 terminates the loop after count reaches 126
i++ increments the value by one on each iteration

You can use this construct when you need a loop that counts. It is not
used much in bash, but used extensively in C

Another way is use the normal for loop like this

for i in 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 and etc.

But that is work, we want better ways to improve our laziness.

Try this on the command line

# myvar=`echo {33..126}` (those are braces and backticks)

then

# echo $myvar

After setting the variable, you could then use the conventional
for construct

for i in $myvar

~

[jpeps]

Isn't there a command for timing execution? If so, what's its name?

~

"timer" script; put in $PATH

Code: Select all

#!/bin/sh

#### begin with:  "timer & " 
###  end with:    "kill `pidof timer` " 

SEC="0"

echo -en "\nWorking..............."
while [ "$0" ]; do
  for ((i=0;i<1000001;i++)); do 
  
    SEC="$(( $SEC+1 ))"
 [ ${SEC} -gt 0 -a ${SEC} -lt 11 ] &&  echo -en "$SEC"   
 [ ${SEC} -gt 10 -a ${SEC} -lt 101 ] &&  echo -en "\b$SEC"   
 [ ${SEC} -gt 100 -a ${SEC} -lt 1001 ] &&  echo -en "\b\b$SEC"   
 [ ${SEC} -gt 1000 -a ${SEC} -lt 10001 ] &&  echo -en "\b\b\b$SEC"   
 [ ${SEC} -gt 10000 -a ${SEC} -lt 100001 ]  &&  echo -en "\b\b\b\b$SEC"   
 [ ${SEC} -gt 100000 -a ${SEC} -lt 1000001 ] &&  echo -en "\b\b\b\b\b$SEC"   
    echo -en "\b"
    sleep 1
   done
done

Then:

Code: Select all

#!/bin/bash 

date
exec timer &
for ((i=1;i<99999;i++));do
:
done
date
kill `pidof timer`

[Moose On The Loose]

A C type 'for loop' construct

# myvar=`{33..126}` (those are braces and backticks)

Code: Select all

# myvar=`{1..10}`
bash: 1: command not found


# myvar=`echo {33..126}`
# echo $myvar
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126
#

[Bruce B]

Moose On The Loose,

Thanks, I fixed the original post.

Bruce

I could say, I'm doing this to demonstrate that FOSS is a better way of
development because of peer review. But probably, I had insomnia. Also
known as sleeping at the keyboard.

[Bruce B]

jpeps,

The timer script is a keeper. Thanks.

I found a perfect second timer for us. I'll post and explain it later.

As mentioned, I taught myself, mostly through books. My dad was/is a
computer scientist, which I found helpful. Still, most of what I learned is
self taught.

What I found over and over is the authors would do OK for a few chapters,
then get over my head. I call it an easy gradient followed by too steep a
gradient.

I would like to teach a lot of basics, both concepts and language.

I really don't want to do what others did to me, which is hit me with a
steep gradient.

You know enough, you may not catch it if the gradient was too steep.

I would like however some PMs advising me about the gradient and if the
teaching is happening without too much brain strain.

Bruce

I also wonder if it is a good thing to listen to Pink Floyd at 77% volume
while doing this.

~

[amigo]

'time' is the command for timing operations. If you need to time the toal of a bunch of repitions, then put the repeating vommands in a function and then: 'time func_name' should give the total of all the loops.

[jpeps]

'time' is the command for timing operations. If you need to time the toal of a bunch of repitions, then put the repeating vommands in a function and then: 'time func_name' should give the total of all the loops.

also can run "time myscript"

[PupGeek]

What I found over and over is the authors would do OK for a few chapters, then get over my head.

Same here.... every time..... and it always gets crazy right around variables and expressions. And arrays? forget it.

I think there are not enough examples to cover every scenario and that some of the examples are not very interesting as well. When I was in college for electronics, we (meaning the professor and the students) engineered and built a laboratory power supply right from the ground up. We all had a voice and applied our theoretical knowledge and lab experiences for this project. I think this would be a good approach to learning programming too. Kinda what I tried to do when starting this thread.

oh, and as for aliases being portable, could you not define them in the script you want to share? I know this would only be advantageous in long scripts that use the alias(es) numerous times.

[Bruce B]

I tried the time command and couldn't figure out how to apply to the
script. Some examples please?

Fortunately, I found Lupu 5.20 replaced the BusyBox date extension with
the 'real' date command, which has a second timer.

This makes the math super easy, just subtract the start time from the
finish time. No hour or minute roll overs to complicate things.

Code: Select all

#!/bin/bash

echo Executing one million commands
start=`date +%s`
for ((i=1;i<999999;i++));do
:
done
stop=`date +%s`
total=`expr $stop - $start`
echo "Total time: $total seconds"

If script doesn't right work on your Puppy, I can upload the real date
command.

~

[Bruce B]

PupGeek,

After reading your post, I played some with using aliases for adding color
to text output. I wasn't pleased.

I then made functions and the results looked great.

Adding colored output gives a more professional look to the script. But
colors are a hassle, unless made easy, which I think I did.

You can give the functions the names you want, in this script, I used this pattern:

whi = normal white
bwhi = bright white
and so on
std = return to normal

Script usage examples

cya
echo Puppy Linux
std
# or use ;
gre;echo Lupu 5.20;std

#or

bmag;echo -n "Puppy Linux ";bwhi;echo version 5.20;std
red;echo "-- Read Warning --"
bred;echo "-- Read Warning --";std


The functions

Code: Select all

function bla() {
    echo -en "\033[30m"
}
function red() {
    echo -en "\033[31m"
}
function gre() {
    echo -en "\033[32m"
}
function yel() {
    echo -en "\033[33m"
}
function blu() {
    echo -en "\033[34m"
}
function mag() {
    echo -en "\033[35m"
}
function cya() {
    echo -en "\033[36m"
}
function whi() {
    echo -en "\033[37m"
}
function std() {
    echo -en "\033[0m"
}

function bred() {
    echo -en "\033[31;1m"
}
function bgre() {
    echo -en "\033[32;1m"
}
function byel() {
    echo -en "\033[33;1m"
}
function bblu() {
    echo -en "\033[34;1m"
}
function bmag() {
    echo -en "\033[35;1m"
}
function bcya() {
    echo -en "\033[36;1m"
}
function bwhi() {
    echo -en "\033[37;1m"
}

You can copy and paste the text to the top of your file OR you can import
the functions.

Import Example: . /root/bin/clrs

File attached: clrs

[Bruce B]

Our character sets have more characters than our keyboards. Some of
these characters are very useful in presenting our posts and documents
more correctly or better formatted.

echo "¢ £ ¤ ¥ § © « ® ² ³ ¶ · ¹ º » ¼ ½ ¾ ÷ ø"


I called the script xtrachars

We can run the script, it will output to the screen, then select the
characters and click middle mouse button to paste them into our
document. To use for later copying and pasting.

I'm interested in finding some ambitious person to modify the script so
that it pipes the output to the clipboard. This way all we have to do is run
it and click middle mouse button, thus saving some extra work.

~

[Bruce B]

Introducing more fun stuff

» while loop
» integer math using the echo command
» x instead of quotes, explained

Code: Select all

#!/bin/bash

# Count by two until we reach 20, then quit
# A trivial program

while [ x$cnt != x20 ] ; do
	cnt=`echo $((cnt + 2))`
	echo $cnt
done

The variable cnt has not been initialized prior to the
start of the loop. This means the loop will run at least one time.

It also means the variable cnt doesn't exist at all
when the loop starts.

In previous examples I used quotes

[ "$cnt" != "20" ]

The reason for the quotes was to have something there which is equal on
both sides of our comparison. We are comparing the value in $cnt with
the value 20

In this example I used x in place of quotes for exactly the same reason, I
used quotes. Note: it is easier to type one lowercase x than two " "

[ x$cnt != x20 ]

What if I didn't use "" or x or something?

Considering that $cnt doesn't exist at the beginning of the loop our test
would look like this:

[ != 20 ]

!= is a comparison operator and there is nothing to compare to. Bash will
see the error, complain and quit.

[ x$cnt != x20 ] is actually [ x != x20 ] , we have something for Bash to
compare and that's all it wants.

This is why we use x's , quotes or something of your choice in these test
statements.

Now you know the echo command can also perform integer math. And the
example shows how it can be done. There are a variety of ways to do
math in our scripts. All in due time.

~

[Bruce B]

Introducing the time saving 'printf' statement

For this post, I took a very small snippet of xine -help output.

It has indents to maintain and characters which would throw echo off, if echo were not properly quoted.

Typically bash programmers use the echo command for on screen display.
But when you have a lot of text to display there is a much easier way.

First example: echo command on each line with quotes on non empty
lines. The echo and its quotes do not display on the output.

Code: Select all

echo "Usage: xine [OPTIONS]... [MRL]"
echo
echo "OPTIONS are:"
echo "  -v, --version                Display version."
echo "      --verbose [=level]       Set verbosity level. Default is 1."
echo "  -c, --config <file>          Use config file instead of default one."
echo "  -V, --video-driver <drv>     Select video driver by id. Available drivers:"
echo "                               dxr3 aadxr3 xv SyncFB opengl raw xshm aa caca"
echo "  -A, --audio-driver <drv>     Select audio driver by id. Available drivers:"
echo "                               null alsa oss esd file none"
echo "  -u, --spu-channel <#>        Select SPU (subtitle) channel '#'."
echo "  -a, --audio-channel <#>      Select audio channel '#'."
echo "  -p, --auto-play [opt]        Play on start. Can be followed by:"
echo "                    'f': in fullscreen mode."
echo "                    'h': hide GUI (panel, etc.)."
echo "                    'w': hide video window."
echo "                    'q': quit when play is done."
echo "                    'd': retrieve playlist from DVD. (deprecated. use -s DVD)"
echo "                    'v': retrieve playlist from VCD. (deprecated. use -s VCD)"
echo "                    'F': in xinerama fullscreen mode."

Second example using the bash printf statement. It requires one quote
at the beginning and one quote at the end. The formatting is maintained.
The printf and the two quotes do not display on the output.

Code: Select all

printf "
Usage: xine [OPTIONS]... [MRL]

OPTIONS are:
  -v, --version                Display version.
      --verbose [=level]       Set verbosity level. Default is 1.
  -c, --config <file>          Use config file instead of default one.
  -V, --video-driver <drv>     Select video driver by id. Available drivers:
                               dxr3 aadxr3 xv SyncFB opengl raw xshm aa caca
  -A, --audio-driver <drv>     Select audio driver by id. Available drivers:
                               null alsa oss esd file none
  -u, --spu-channel <#>        Select SPU (subtitle) channel '#'.
  -a, --audio-channel <#>      Select audio channel '#'.
  -p, --auto-play [opt]        Play on start. Can be followed by:
                    'f': in fullscreen mode.
                    'h': hide GUI (panel, etc.).
                    'w': hide video window.
                    'q': quit when play is done.
                    'd': retrieve playlist from DVD. (deprecated. use -s DVD)
                    'v': retrieve playlist from VCD. (deprecated. use -s VCD)
                    'F': in xinerama fullscreen mode.
  -s, --auto-scan <plugin>     auto-scan play list from <plugin>
  -f, --fullscreen             start in fullscreen mode,
  -F, --xineramafull           start in xinerama fullscreen (display on several screens),
"

~

[jpeps]

I tried the time command and couldn't figure out how to apply to the
script. Some examples please?

Code: Select all

#!/bin/bash 

date
function million {
for ((i=1;i<999999;i++));do
:
done
}
time million
echo
date

[Bruce B]

Jpeps,

Thanks for coming to the rescue. I'll call suicide prevention and tell them
it was just a false alarm.

Have you ever worked with other programmers in a community?

I haven't.

Would you like to do a fairly small group project, here online?

If so, would you be willing to be the benevolent dictator?

Let me know, if you are interested. If so I'll define the project I have in
mind, if you don't like the project, you can change your mind.

Bruce

~