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Tiny Deathstars of Foulness

Recently someone asked me about accepting bash inputs. So I decided to take a stab at writing a little about it up. For the initial one we’ll look at accepting text input. Here, we’ll just sandwich a read statement between two echo commands. In the first echo we’ll ask for a name of a variable. Then we’ll read it in with the read command. And in the second echo we’ll write it out. Using the variable involves using the string of the variable (myvariable in this case) with a dollar sign in front of it, as in $myvariable below:

echo "Please choose a number: "
read myvariable
echo "You picked $myvariable"

Read also has a number of flags available to it:

  • -a assigns sequential indexes of the array variable
  • -d sets a delimiter to terminate the input
  • -e accepts the line.
  • -n returns after reading a specified number of characters
  • -p prompts without a trailing newline, before attempting to read any input
  • -r doesn’t use a backslash as an escape character
  • -s runs silent, which doesn’t echo text
  • -t: causes read to time out (number of seconds is right after the -t)
  • -u reads input from a file descriptor

Next, we’ll build on that read statement (note the addition of -p) and use a while to force a user to input a y or n and then parse their selection with a basic case statement:

while true;
do
read -p "Do you wish to continue?" yn
case $yn in
[Yy]* ) echo "Add your action here"; break;;
[Nn]* ) exit;;
* ) echo "Please answer yes or no.";;
esac
done

Finally, let’s look at positional parameters. Here, you can feed them at the tail end of the script, as words that are separated by spaces after the name of the script. Here, we simply just echo $0, which is the first position (aka – the name of the script you just ran) and $1 and $2 as the next two.

#!/bin/bash
echo "You Used These"
echo '$0 = ' $0
echo '$1 = ' $1
echo '$2 = ' $2

You could also take $3, $4, etc. This is different than writing flags, which requires a bit more scripting. So if you called the script with:

/path/to/script/pospar.sh test1

You would see:

You Used These
$0 = ./pospar.sh
$1 = test1

What tips/additions do you have?

February 14th, 2017

Posted In: bash, Unix

Tags: , , , , , ,

You can easily accept user provided input in bash by using the read command in bash for Linux and OS X. Here, we’ll echo out a choice to a user in a script, read the output into a variable called yn and then echo out the response:

echo "Please enter y or n: "
read yn
echo "You chose wrong: $yn"

Here, we used echo to simply write out what was chosen in the input. But we could also take this a little further and leverage a case statement to then run an action based on the choice selected:

read -p "Should the file extension change warning be disabled (y/n)? " yn
case ${yn:0:1} in
y|Y )
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
echo "The warning has been disabled"
;;
* )
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool true
echo "The warning has been enabled"
;;
esac

The options when scripting are pretty much infinite and chances are, if you’ve written any scripts, you’ll know of a better way to do this than how I’ve always done it. One of the great things about scripting is the fact that there’s always a better way. So feel free to throw any of your examples into the comments!

July 28th, 2015

Posted In: Mac OS X, Mac OS X Server, Mac Security, Ubuntu, Unix

Tags: , , , , , , , ,