Shell Scripting

Shell Scripting

• Scripts must be chmod 755 so they can execute

• #!/bin/bash (add to the top of the shell script so it can use the bash shell to interpret the script, you can also add python or other shell programs)

• Positional Parameters: $0 ... $9 $@ to access all 0-9 // like in a loop

• Exit Status return Codes: Range from 0 to 255: 0 = success: Other than 0 = error condition: use man or info to find meanign of exit status

&bull, $? contains the return code of the previously executed command

ls /not/here
    echo "$?"
Another Code example


    HOST="google.com"
      ping -c 1 $HOST # -c 1 means it will send 1 ping
      if["$?" -eg "0"]
      then
          echo "$HOST reachable"
      else
          "$HOST unreachable"
      fi
&& and ||
-the second statement only executes if the first one was successfull
mkdir /tmp/bak && cp test.txt /tmp/bak/
-the second statement will execute only if the first one fails
cp test.txt /tmp/bak/ || cp test.txt /tmp
• chain multiple commands together using a ; (semicolon) cp test.txt /tmp/ ; cp test.txt /bak

File Operators (test)

-d FILE -true if is a directory
-e FILE True if file exists
-f FILE True if file exist and is a regular file
-r FILE True if file is readable by you
-s FILE True if file exists and is not empty
-w FILE True if file is writable by you
-x FILE True if file is executable by you
-z FILE True if string is empty
-n FILE True if string is not empty
String1=String2 true if strings are equal
String1 != string2 True if the strings are not equal
arg1 -eq arg2 equal
arg1 -ne arg2 not equal
arg1 -lt arg2 less than
arg1 -le arg2 less than or equal to
arg1 -gt arg2 greater than
arg1 -ge arg2 greater than or equal to
read -p "Prompt to dispaly" VarableName accepting user input

for loop to rename all jpg files with the date

  #!/bin/bash
  PICTURES=$(ls *.jpg)
  DATE=$ (date +%F)

  for PICTURES IN $PICTURES
      do
          echo "Renaming ${PICTURE} to ${DATE} - ${PICTURE}"
          mv $[PICTUE} ${DATE}-${PICTURE}
  done

  // Output
  renaming bear.jpg to 2015-03-06-bear.jpg

Adding exit commands to scripts

    #!/bin/bash
      HOST="google.com"
      ping -c 1 $HOST
      IF [ "$?" -ne"0" ]
      then
          echo "$HOST unrechale"
          exit 1
      fi
      exit 0

Creating a Function

function function-name() {# code goes here}
Calling a Function
function hello() {
    echo "hello:
    }
    hello (you don't need the () just call the name of the function)
    
** Positional Parameters and passing info to a funciton


    #!/bin/bash
  function hello(){
      echo "hello $1"

  }

  hello robert # robert is passed to the function hello
  #output is hello Robert
** Outputting miltple calls to a function


    #!/bin/bahs
      funciton hello() {
      for NAME in $@
      do
          echo "Hello $NAME"
      done
  }

  hello robert bob dan

  Output
  hello robert bob dan
Using Wildcards 
* -matches zero or more characters
*.txt
a*
a*.txt
? - matches exactly one character
?.txt
a?
a?.txt
[] character class - matches any of the characters included between the brackets. Matches exactly one character.
[aeiou]* exampld: ls -la [abge]*
ca[nt] matches:(it will match either the n or the t)
can, cat, candy, catch
[!] Matches any characters NOT included between the brackets. Matches exactly one character.
[!aeiou]* would match below bacause it does not start with any of those letters
baseball, cricket
Select a range
[a-g]* matches files that start with a,b,c,d,e,f,g
[3-6]* matches all files start with 3,4,5,6
Using predefined Character Classes
[[:alpha:]]
[[:alnum:]]
[[:digit:]]
[[:lower:]]
[[:upper:]]
[[:space:]]

#!/bin/bash
cd /var/www/bash
  # there's a space after the "if", space afer ! then a space after "[", space before "]"
if [ ! -d  /var/www/bash/text/ ]
then
     mkdir /var/www/bash/text
fi
for FILE in *.txt
do
     echo "Copying $FILE"
     cp $FILE /var/www/bash/text
done
Case Statements

Simple Backup bash shell script

#!/bin/bash
tar -czf myhome_directory.tar.gz /home/linuxconfig
 
Your backup script and variables:
#!/bin/bash
 OF=myhome_directory_$(date +%Y%m%d).tar.gz
 tar -czf $OF /home/linuxconfig 
 
We can use while loop to check if file does not exists. This script will sleep until file does exists. Note bash negator "!" which negates the -e option.
#!/bin/bash
 
while [ ! -e myfile ]; do
# Sleep until file does exists/is created
sleep 1
done 

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