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S&P 4 – loops in backgrond

Came to know recently that even loops, similar to running a command, can run on background.

Adding a "&" makes a command to run in background.                                                                     Similar to that, even loops can be run on background.

#!/bin/bash

# background-loop.sh

for i in 1 2 3 4 5 6 7 8 9 10 # First loop.

  do

    echo -n “$i “

    sleep 1 # we will need to test with this sleep and without this sleep

done & # Run this loop in background.

# Will sometimes execute after second loop.

 echo first loop # This ‘echo’ sometimes will not display.

 

 for i in 11 12 13 14 15 16 17 18 19 20 # Second loop.

    do

       echo -n “$i “

       sleep 1  # we will need to test with this sleep and without this sleep

    done

echo # This ‘echo’ sometimes will not display.

 

# ======================================================

# without the sleep 1, the expected output from the script:

# 1 2 3 4 5 6 7 8 9 10

# 11 12 13 14 15 16 17 18 19 20

 

# Sometimes, though, you get:

# 11 12 13 14 15 16 17 18 19 20

# 1 2 3 4 5 6 7 8 9 10 bozo $

# (The second ‘echo’ doesn’t execute. Why?)

 

# Occasionally also:

# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

# (The first ‘echo’ doesn’t execute. Why?)

 

# Very rarely something like:

# 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20

# The foreground loop preempts the background one.

exit 0

 

with that sleep 1, its really funny.

S&P 3 - tail and head to show lines middle of the file / head - tail command to show lines in between x and y

A close friend of mine asked me - which command will show the lines in between 90 and 100?

looks like a simple question, isnt. but its a tricky one.

He gave me the choices as well:
1. tail -n 90-100 file
2. head -n 100 | tail -n 10
3. head -n 90-100 file
4. list -n 90-100 file

after seeing the choices, it is confusing, isnt?!?!

ok, lets remember these 5 rules:
rule 1. head -20 <filename> ---will show first 20 lines of the <filename>.
rule 2. tail -20 <filename> ---will show the last 20 lines of the <filename>
 
3. remember a rule, head or tail, dont work with just the numbers. the numbers should have a minus sign or plus sign.
(if you try head 20 filename, it will look for a filename 20 and secondfile filename, and it will show first ten lines from both file, provided that a file

exists with the name "20". or it will say, "20" file does not exist and it will show the first ten lines of the filename)
head <N> <filename>
tail <N> <filename>

4. in case of head, avoid "+n"

to summarize:

tail n <filename>  ==> it wont work
tail -n <filename>  =======> will show last "n" lines
tail +n <filename> (rule 5, please check below)

head -n <filename>  =======> will show first "n" lines
head +n <filename> ==> it wont work
head n <filename>  ==> it wont work
 
rule 5. tail +n <filename> will show the file starting from line n, till the end of the file. please note it looks like it works only on Posix standard tail. so,
better to avoid this as well.


ok now, to show the lines in between 90 to 100:
-- show first 100 lines, cut out and show only last ten lines(90 to 100)
head -100 <filename> | tail -10