Bash Scripting

Considerations

• It is crucial that the first line of the Bash script begins with the header #!/bin/bash.

• The extension of a file which represents a script should be .sh.

• Comments in Bash begin with # and run to the end of the line:

  echo Hello, World. # prints out "Hello, World."

• To execute a script, you must be sure your file has the permission to be executed chmod +x your_script.sh. Click here to see more about permissions.

• If you are located in the same PATH were you created the script, you execute your script by running ./your_script.sh.

Conditionals (Decision Control Structure)

Use conditionals to specify different courses of action to be taken. In this case, we have three possibilities to check a number in a range of other values:

#!/bin/bash
#Setting a value to output
output=99
#Determine an action based on the output's value
if [ $output -eq 100 ]   #if the output is equal to 100
then
  echo "The calculation reaches 100%"
else
  if [ $output -gt 100 ]   #if the output is greater than 100
  then
    echo "The calculation is greater than 100%"
  else     #only option is that the output is less than 100
  echo "The calculation is less than 100%"
  fi

Read more examples about if conditionals.

Loops (Repetitive tasks)

In a loop, commands will continue to run repeatedly until a task is executed for all elements. One useful command for loop calculations is for. Here is an example of printing numbers from 1 to 9:

#!/bin/bash
#Printing numbers from 1 to 9
for i in {1..9}; do echo $i; done

If you want to have all the numbers in the same line, add the -n option. If you want to add 4 units, use double parentheses in the operation:

#!/bin/bash
#Numbers from 1 to 9
#Add four units in the same line
for i in {1..9}; do echo -n "$((i+4))  " ;  done

• Click here to see more examples.

Working with files (combining conditionals with Bash commands)

Check the existence of a file to determine the size of the file as well the quantity of words:

#!/bin/bash
#Clear the terminal
tput clear

#Request the name of the file to be evaluated
printf "Enter the absolute path of the file, e.g. /home/user/your_file\n"
read FILE

#Evaluate the file
if [ -e $FILE ]
then
    printf "The $FILE has a size of $(du -h $FILE | awk '{print $1}') and it contains $(wc -w $FILE | awk '{print $1}') words.\n"
else
    printf "File not found, keep trying..."
fi

📝 It is important to indent the code in the if block with 4 spaces. Also include 1 space between the contents of the brackets ([ and ]) and the brackets themselves.

Working with filesystems (combining conditionals with Bash commands)

Send an email if disk usage in the system has reached 90% or more:

#!/bin/bash
#Run 'df -H' first to check filesystems and usage
#Then filter with `grep` to not consider Filesystem, tmpfs, nor cdrom using the options `-vE`
#Take only columns 5 and 1 with 'awk' and keep that output using `while read` to do an action
df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' | while read output;
do
#Print the output with 'echo', and assign to `usep` only the first column of output, taking out '%'
  echo $output
  usep=$(echo $output | awk '{ print $1}' | cut -d'%' -f1  )
#Partition is only going to take the names of your filesystems
  partition=$(echo $output | awk '{ print $2 }' )
#if the value of `usep` is greater than or equal to 90 then you will print a message and send an email to alert
  if [ $usep -ge 90 ]; then
    echo "Running out of space \"$partition ($usep%)\" on $(hostname) as on $(date)" |
     mail -s "Alert: Almost out of disk space $usep%" [e-mail]
  fi
done

You can learn more about Bash scripting by taking a look at this tutorial.