ZLBashExample

###Getting started ####Creating bash scripts First, create a file using your favorite text editor

$ vim testscript

######Skeleton Script There are two conventions, that although not necessarily required, should be followed

# You must tell the thing running the script where to find the bash interpreter 
# Usually /usr/bin/bash works fine, but in the case that bash is not in the /usr/bin directory
# we want to use /usr/bin/env bash. Where env is a program used for finding other programs, in our case - bash.

#!/usr/bin/env bash

# Lots of great code here

# Exit with 0 for success, or any other number up to 255 for failure
exit 0

######Permissions Now we have a bash file, but we can't run it yet (you can try if you want using ./testscript but you'll get a permission denied error). This is because the file does not have the execute permission set. We can allow execution of this file by the user in this way:

$ chmod u+x testscript

####Running If you try to run your script now by simply entering the name of the script it won't work.

$ testscript

When you try to run a script, bash refers to a value ($PATH) that points to one or more directories that may contain the bash script. If the script is found in none of the directories then it won't be executed. The current directory is not (and should never) be part of the $PATH.

There are a couple ways to address this. We can either run the script from the current directory using one of two methods, or we can move it to a directory that $PATH points for easier usage.

######Running in current directory We can run the script by simply telling bash to run it explicity

$ bash testscript

Or we can tell it to search in the current directory instead of using the $PATH value

$ ./testscript

######Moving to a PATH directory If we're planning on using the script often or from places other than the current directory, we can put the script in one of the directories PATH points to for ease of use.

# This will probably be a place your $PATH points to, but examine it first (using echo $PATH) to be sure
$ mv testscript /usr/local/bin

Now when you type

$ testscript

into your terminal, it will search through all the directories in $PATH, find your script, and run it.

###Basic Syntax ####Variables #####Setting/Getting

#!/usr/bin/env bash
# Variables can be set like this
myVar="Hello world"

# And expanded like this 
echo $myVar

# prints "Hello world"

exit 0

#####Expansion Variables not inside of quotes will be expanded

$ echo This is my variable $myVar
# prints This is my variable Hello world

Variables inside single quotes will not be expanded

$ echo 'This is my variable $myVar
# prints This is my variable $myVar

Variables inside double quotes will be expanded

$ echo "This is my variable $myVar"
# prints This is my variable Hello World

####Space Characters Spaces are very important in bash - my act as delimiters. For example, if you try to set a variable like this:

$ aVar = "Hello"
# prints -bash: myvar: command not found

Bash assumed that aVar was a script you wanted to run, with = and "Hello" as arguments. This is because it uses space charaters to separate commands. Instead, use:

# aVar="Hello"

######Parameter Separation We have a function called print that prints each parameter on it's own line. Assume we want to print the word "Hello"

$ print Hello
Hello

Good, that works. Now let's print "Hello World"

$ print Hello World
Hello
World

That doesn't seem right, Hello World should be one the same line - or should it. This program prints each parameter on it's own line, and since space characters are the delimiters between paramters it thinks that "Hello" and "World" are two separate paramters. So how do we fix this - we simply use double quotes

$ print "Hello World"
Hello World

On space charaters and double quotes It is generally advised to use double quotes very liberally. They will help us avoid common problems with parameter separation and expansion.

###Conditionals #####Conditional Blocks

#!/usr/bin/env bash
myName="Zack Liston"
yourName="John Doe"

# Note that the spacing here is important
if [[ $myName = $yourName ]]
  then echo "You're the same!"
elif [[ "Zack Liston" = $myName ]]
  then echo "Hello $myName"
else 
  echo "Something else"
fi

exit 0

#####Conditional Loops ######While

#!/usr/bin/env bash
while true
do echo "Infinite loop
done

######For

#!/usr/bin/env bash

for (( i=10; i > 0; i-- ))
  do echo "Loop one number is $i"
done

for i in 10 9 8 7 6 5 4 3 2 1
  do echo "Loop two number is $i"
done

for i in {10..1}
  do echo "Loop three number is $i"
done
exit 0

###Parameters Parameters can be accessed individually like so

$ script param1 param2 param3 param4
# $1 is "param1"
# $2 is "param2"
# etc, if there are more than 9 parameters the 9+n parameters must be accessed like so ${10} 
# Otherwise if accessed like this $10 it will subsitite $1 for the first parameter and append 0

Parameters can be accessed as a group like so

$ script param1 param2 param3 param4
# $@ is a collection of all the parameters. We usually want to surrond it in double quotes to 
# promote space safety. 

We can also get the number of parameters

$ script param1 param2 param3 param4
# $# is the number of parameters, in this case 4

###File descriptors - Input/Output ####File Descriptors Bash has three open file descriptors by default

• Standard Input - File Descriptor 0
• Standard Output - File Descriptor 1
• Standard Error - File Descriptor 2

######Standard Input - stdin By default is connected to the keyboard.

######Standard Output - stdout By default is connected to the screen. This is the actual result of a program

######Standard Error - stderr By default is connected to the screen. This includes all messages, not just errors.

####Redirection Redirection is one of the most useful tools in bash. It allows us to get/put information from anywhere to anywhere. For example if I wanted to see all the files in the current directory I could do this:

$ ls

And since stdin is by default set to the screen, then we see the result. However, if we want to write the result to a file we can do it as simple as this

$ ls > files.txt

If you open files.txt you'll see the all the files in the directory.
Now if we want to also send stderr to files.txt we can easily do that

$ ls > files.txt 2>&1

We can reference the file descriptors by their numbers. Remember that stdout is 1 and stderr is 2. The syntax above is saying send stdout to the same place stdin is going.
Note that the > creates a new file or overwrites the existing one. If we want to append instead we can use this

$ ls >> files.txt
# appends the output to files.txt instead of replacing it. 

####Pipes One of the most important capabilities of bash is being able to send the output of one script to the input of another. We can do this using pipes. For example, if we want to get any file/directory in the current directory that matches "Desktop":

$ ls -l | grep "Desktop"

This feature is the glue of bash. It allows us to string together many small programs to get the result we want, which is what bash is in a nutshell.
Sometimes we may want to send the output to another program, but also save it. We can do that using the tee command.

$ ls -l | tee files.txt | grep "Desktop" > result.txt
# This first gets all the files in the directory, then saves it it files.txt, then sends
# it to grep, which finds which ones have "Desktop" and save those to result.txt

###Basic Tools ####Grep Grep is a very powerful tool for searching. This is its most basic format, it returns the filename and the line that matches the expression

$ grep <expression> <files to search>

# to find all C files in the current directory that have the term "print"

$ grep "print" *.c

To get just the names of the files matching the expression provide the -l option

$ grep -l "print" *.c

To ignore the case provide the -i option

$ grep -i "print" *.c

To filter out results you don't want to see provide the -v option. For example, we want to see what files contain print but not printable. Not that we must pipe this to another grep.

$ grep -i "print" *.c | grep -v "printable"

######Regular Expressions We can grep either by string, or a regular expression.

####Awk Awk is used to select which output we want and discard the rest. Awk is a language and itself, and can be highly customized. We use the basic format:

$ awk '{<action>}' <inputfile(s)>

# or using the output of another program

$ <otherprogram> | awk '{<action>}'

For example, if we want the permissions of all the files in the directory (the permissions are the first field) we can pipe ls to awk

$ ls -l | awk '{print $1}' 

The action of awk is wide open, allowing us to do nearly anything. One simple example is reversing all of the words in each line:

$ awk '{
  for (i=NF; i>0; i--) {
    printf "%s ", $i;
  }
  printf "\n"
}' <inputfile> 

NF is a variable containing the number of fields. Note that within awk we do not need the $ before NF to access it. $NF refers to the last field.

Another example. Suppose we want to get aggregate size of all the files in a directory, we can use awk.

# We know that the first field of ls -l contains the size of the file
$ ls -l | awk '{sum += $5} END {print sum}'

####Sort Simply enough, this sorts a file or input. The real power in sort lies in it's options. It's used like this;

$ sort -<options> <inputfile>

Several important options include -r which sorts by reverse order

$ sort -r myfile.txt

And -f which ignores case when sorting.

$ sort -f myfile.txt

Note that by default sort sorts by alphabetical value. So if you have a collection of numbers it sorts them by they're alphabetical values rather than their numeric values. So 31 would come after 300 for example. We solve this problem with the -n option

$ sort -n numbers.txt