=====Write Simple Bash Scripts=====

in vim enable line numbering with

<code> esc
:set nu </code>

using **awk**
awk is used to get bits of files for example 

<code>awk -F: `{print $1}` </code>

prints out the first entry  (from the $1) of a file that uses : as a separator

====Loops====

===for===
 The structure is
<code> 
for VAR in list of space separated items; do action $VAR; done 
</code>

Examples
<code>
For HOST in server a server; do echo $HOST; done

For HOST in server{a,b}; do echo $HOST; done

For HOST in server{a..b}; do echo $HOST; done

</code>
These all do the same thing

===if===

if statements use following structure

<code>
if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
else
      <STATEMENT>
      ...
      <STATEMENT>
fi
</code>

Conditions are from command **test**

<code>
[user@host ~]$ [[ 1 -eq 1 ]]; echo $?
0
[user@host ~]$ [[ 1 -ne 1 ]]; echo $?
1
[user@host ~]$ [[ 8 -gt 2 ]]; echo $?
0
[user@host ~]$ [[ 2 -ge 2 ]]; echo $?
0
[user@host ~]$ [[ 2 -lt 2 ]]; echo $?
1
[user@host ~]$ [[ 1 -lt 2 ]]; echo $?
0
The following examples demonstrate the Bash string comparison operators:

[user@host ~]$ [[ abc = abc ]]; echo $?
0
[user@host ~]$ [[ abc == def ]]; echo $?
1
[user@host ~]$ [[ abc != def ]]; echo $?
0
</code>

===while===

will run a a command while a condition is meet. 

here is an interesting example

<code>while ! test -f test_file; do sleep 1s; done </code>

this checks to see if a file called test_file exists if it does the while loop exits if not it keeps running

===case===

Case is a comparison of input with know conditions

here is an example 
<code>
read -p "give me an animal:" animal

case $animal in

   person)
        echo -e " aperson has 2 legs \n"
        
    ;;
    
    cat)
        echo -e "a cat has 4 lets \n"
    ;;
    
    *) 
        ech0 -e "this covers all other conditions \n"
        exit 1
    ;;
 esac

</code>

====Regex====

[[:linux:classnotes:RH134:bash:regex | Regular Expressions ]]