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cloud engineer

bash

Bash Menu and Functions

How to integrate menus and functions in your Bash script.

#!/bin/bash
function show_logs {
ls -l /var/log/
}
function show_users {
cat /etc/passwd
}
echo '--------'
echo 'My Menu'
echo '--------'
PS3='Choose an option: '
commands=("Show logs" "Show users" "Quit")
COLUMNS=0
select cmd in "${commands[@]}"; do
    case $cmd in
        "Show logs")
            echo "Showing logs ... "
            show_logs
            ;;
        "Show users")
            echo "Showing users ... "
            show_users
            ;;
	"Quit")
	    echo "Exiting ... "
	    exit
	    ;;
        *) echo "invalid option $REPLY";;
    esac
done

Result

--------
My Menu
--------
1) Show logs
2) Show users
3) Quit

Cleanup Stale CVS Snapshots

Here’s the script that looks mounts with stale file handles and removes them.

#!/bin/bash
file='/tmp/stale.txt'
df -h | grep Stale > $file
sed -i -e 's/df: ‘/'""'/g' $file
sed -i -e 's/’: Stale file handle/'""'/g' $file
while IFS= read -r line; do
  echo 'unmounting '$line
  umount $line
done < $file

Cleanup CVS Snapshot Volumes

How to unmount CVS snapshot volumes all at once using a Bash script. I ran into a scenario where I had to unmount 115 volumes. Instead of manually running umount on each one, I use the script below to unmount all 115 volumes. The script performs a grep for ‘.snapshot’ and then writes the results to a file. I then read the file line by line and perform unmount on each one.

#!/bin/bash
file='/tmp/snapshots.txt'
df -h | grep .snapshot | awk '{print $6}' > $file
while IFS= read -r line; do
  echo 'unmounting '$line
  umount $line
done < $file

Bash Spacebar Break

Here’s how to add breaks in your Bash scripts. The function is called spacebar.

function spacebar {
  echo 'Press spacebar to continue. Press Ctrl-C to exit.'
  stty -echo > /dev/null
  until read -r -n 1 -t 0.001 && [ "$REPLY" = ' ' ]; do abc=''; done
  stty echo > /dev/null
}
 
spacebar

You can call it multiple times in your script since it’s a function.

Press spacebar to continue

Here’s a little snippet of Bash code that waits for spacebar to be pressed to continue.

function spacebar {
  echo 'Press spacebar to continue. Press Ctrl-C to exit.'
  stty -echo > /dev/null
  until read -r -n 1 -t 0.001 && [ "$REPLY" = ' ' ]; do abc=''; done
  stty echo > /dev/null
}
echo 'Starting upgrade'
spacebar

Output

Starting upgrade
Press spacebar to continue. Or Ctrl-C to exit.

If you press spacebar it will continue. Ctrl-C exits the program.

Bash Reformat Numbers

How to reformat numbers in Bash.

$ n=1023555
$ m=$(numfmt --to=si n)
$ echo $m
1.1M

Other options

--to=none     # no scaling. Output: 1023555
--to=si       # International System of Units (SI) standard. Output 1.1M
--to=iec      # International Electrotechnical Commission (IEC) standard. Output: 1.1Mi 
--to=iec-i    # International Electrotechnical Commission (IEC) standard. Output: 1.1M
--to=auto     # ‘auto’ can only be used with --from.

fmtnum

Bash Script on Startup

How to add bash scripts on startup.

update-rc.d

sudo cp /path/to/yourscript.sh /etc/init.d/yourscript.sh
sudo update-rc.d /etc/init.d/yourscript.sh defaults
chmod +x /etc/init.d/yourscript.sh

Root Crontab

sudo crontab -e
@reboot /path/to/yourscript.sh

Bash input executes functions

How to execute functions in Bash based on argument.

There are 2 functions: build and terminate.

#!/bin/bash
build () {
 echo 'Building services'
}
terminate () {
 echo 'Terminating services'
}
if [ $1 == 'build' ]; then build
elif [ $1 == 'terminate' ]; then terminate
else echo 'Valid options are "build" or "terminate"'
fi

Execute a function based on argument.

$ bash test.sh build
Building services
 
$ bash test.sh terminate
Terminating services
 
$ bash test.sh anyother
Valid options are "build" or "terminate"