Bash script with various commands

The BASH Script file

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  other than the first line and chmod, very similar to other languages
  
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  no main()
  
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  starts from the top
  
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  in Emacs, adding the #/bin/… does make the text turn colors
  
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  can also run with sh command
  
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    sh
    

File setup

helloWorld.sh

helloWorld

helloWorld2

Shebangs

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  think of it as C++/Java includes or imports
  
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    built-in features, functions, etc…
    
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  there are different versions
  
  • 
    bash
    
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    sh
    
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    ksh
    
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    csh
    
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    etc…
    
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  watch for sh and bash
  
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    sh = Shell Command Language
    
  • 
    bash = an implementation (of many) of sh
    
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    were very similar
    
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    but bash has started to drift away from sh
    
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    will see many older sites use sh (when they mean bash)
    

Variables, Expressions and Math

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  variables have no defined type
  
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    can change (reset) at will
    
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  variable declaration (or reset!!)
  
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    a=23
    
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    a=“What up” b=36 c=Hello
    
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      no spaces around the = sign
      
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      multiple variables can be declared on one line
      
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      strings do not need “ unless it is multiple tokens
      
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  but using the variable requires a $ marker in front
  
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    echo $a
    
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      again, no space after the $
      
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  use in mathematics
  
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    all symbols are the same in other languages except multiplication
    
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      \* is used for multiplication since * is a wildcard
      
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      \ in front of parentheses \( \)
      
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    the command “expr” is used to solve integer equations
    
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  reading values from the keyboard
  
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    read command used with a variable will assign the value
    
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    read var #yes, no $
    
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  incrementing uses expr command
  
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    var=`expr $var + 1`
    
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    expr only works on integers values
    
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      reverse quotes used for nested commands in one line of script
      

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  can’t do floating point arithmetic natively in bash
  
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    have to use a command line tool “bc” or AWK
    
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      precision calculator language
      
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    var=`expr $var + 1.5` (did not work)
    
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      ` are smart quotes
      
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    answer=$(bc <<< "3.4+7/8-(5.94*3.14)")
    
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      values can be replaced with variables
      

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  echo is used in several forms to display simple text to variables
  
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  simple test examples
  
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    echo “Welcome class”
    
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    echo “$var is 40 years old”
    
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    print “Thank you very much”
    
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    echo “$var is 40 years old \c”
    
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      \c == no line break
      

If else statements

Add this below

https://www.cyberciti.biz/faq/linux-unix-shell-check-if-directory-empty/

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  same as it always was
  
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    but need to use flags for comparison
    
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  keyword then has to be on the next line by itself
  
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  overall layout is minorly different with “then” and “fi” at end
  
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    has to be on the next line
    
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  condition uses [ ]s and must be spaces before and after [ ]
  
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    no ( )s
    

[] vs [[]]

Cases statements

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  works the same way, just the overall setup is different
  

Logical Operators

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  work the same but really look like operators (functions)
  
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  Integers
  
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    require commands shown below
    
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  Strings
  
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    require [[ ]]
    
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    use same <,>, !=, = (==), etc.. symbols you are used to
    
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  for real numbers, convert to string, then compare
  

It's important to understand the difference between shell syntax and the syntax of the [ command.

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  && and || are shell operators. They are used to combine the results of two commands. Because they are shell syntax, they have special syntactical significance and cannot be used as arguments to commands.
  
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  [ is not special syntax. It's actually a command with the name [, also known as test. Since [ is just a regular command, it uses -a and -o for its and and or operators. It can't use && and || because those are shell syntax that commands don't get to see.
  

Debugging conditions

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  since the operators vary drastically, we can use the Linux command “test”
  

While loops and reading from files

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  same as many languages, will need grep/awk to break up the data for complex applications
  
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  using the read as a condition
  
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    if it can’t read any more line, then it will stop
    
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  the value read in will be stored in the variable “line” below
  
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    notice line has a $ after that line
    

Pipes

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  when one solution needs another
  
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  statement 1 would give input to statement 2 to work on
  
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  also need ` around it all to complete correctly
  

`(statement 1) | (statement 2) `

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  ASCII comparison
  
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  lots of built-in functions
  

Command line arguments

File IO Stuff

[slupoli@linux2 fileIO]$ set $term = F15

[slupoli@linux2 fileIO]$ echo "well $term" >> test.txt

[slupoli@linux2 fileIO]$ echo "well shit" >> test.txt

all of 47

exec < $filename redirects the input to grab from the filename

all of 49

3:00 in 49 uses a while loop smartly
File checking stuff

31 start at 2:30

Loops in General

break and return work as usual

4:00 in 52

complete example in 5:00 in 39

complete example in 4:00 in 40

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  For loop (range)
  
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    basic for loop starting from i until n…
    
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    can be called a range loop
    

complete example in 5:00 in 41

Grep

-i is ignoreCase

-n line number where the match was found

-c how many lines matched

-v lines that did not match

-a processing text files

46

CSH
Edit mode

http://ryanstutorials.net/bash-scripting-tutorial/

https://www.youtube.com/playlist?list=PLcUid3OP_4OW-rwv_mBHzx9MmE5TxvvcQ

https://www.youtube.com/playlist?list=PL7B7FA4E693D8E790

http://askubuntu.com/questions/229446/how-to-pass-results-of-bc-to-a-variable

CSH
CSH Stuff

http://faculty.plattsburgh.edu/jan.plaza/computing/help/tcsh.htm

set a = 100 (in CSH)

Moving within directories using foreach

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  using the normal linux commands, nothing new
  
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  but looping through when an inner set of directories many contain consistent data can be done with a loop
  
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    cd to the base directory first
    

Sources
Shebang

https://en.wikipedia.org/wiki/Shebang_%28Unix%29

http://stackoverflow.com/questions/6348902/how-can-i-add-numbers-in-a-bash-script

(comparing)

http://stackoverflow.com/questions/2237080/how-to-compare-strings-in-bash
Random values

http://tldp.org/LDP/abs/html/randomvar.html

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_02.html