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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘bash’ Category

linux – How do I use sudo to redirect output to a location I don’t have permission to write to? – Stack Overflow

Posted by jpluimers on 2020/07/09

Various ways are explained at [WayBack] linux – How do I use sudo to redirect output to a location I don’t have permission to write to? – Stack Overflow.

Some are for simple commands and can be a one liner (for instance using tee, or executing a secondary shell).

Others are more suited for longer command sequences.

–jeroen

Posted in *nix, *nix-tools, bash, bash, Development, Power User, Scripting, Software Development | Leave a Comment »

How to return a string value from a Bash function – Stack Overflow

Posted by jpluimers on 2020/06/17

Cool: you can return strings both as a function result, and by reference: they are explained in the question, second and fourth answer of [WayBack] How to return a string value from a Bash function – Stack Overflow.

Returning them by reference has two important benefits:

  1. it is much faster (especially useful in tight loop)
  2. you can use echo (normally used to return a result) for debugging purposes

I also needed a bit of switch magic which I found at [WayBack] bash – Switch case with fallthrough? – Stack Overflow and array magic (from [WayBack] Array variables) as arrays are far more readable than indirection (on the why not, see [WayBack] BashFAQ/006 – Greg’s Wiki: How can I use variable variables (indirect variables, pointers, references) or associative arrays?).

So here is a function that returns a specific IPv4 octet.

function getIpv4Octet() {
  IPv4=$1
  octetIndex=$2
  outputVariable=$3

  slice="${IPv4}"
  count=1
  while [ "${count}" -le 4 ]
  do
    octet[${count}]="${slice%%.*}"
    slice="${slice#*.}"
    count=$((count+1))
  done
   
  case "${octetIndex}" in
    "1" | "2" | "3" | "4")
      ;;
    *)
      octetIndex="4"
      ;;
  esac
  eval $outputVariable="${octet[$octetIndex]}"
}

You call it like this:

$ getIpv4Octet "192.168.178.32" 3 result && echo ${result}
178

–jeroen

Posted in *nix, *nix-tools, bash, bash, Development, Power User, Scripting, Software Development | Leave a Comment »

how to count the length of an array defined in bash? – Unix & Linux Stack Exchange

Posted by jpluimers on 2020/06/16

I needed to enumerate all the parameters to a function and access many of them by index in the same function, so thanks to both these:

I got at this:

  declare -a arguments=("$@")
  for index in ${!arguments[@]}; do
    echo $index/${#arguments[@]}:${arguments[$index]}
  done

These are all forms of Array handling or Shell Parameter Expansion with special cases for array variables:

  • ! does indirection, in this case from the array to the index of the array
  • # gets the lengt of the parameter (for arrays: the number of elements)
  • [] acccesse an array variable using an index

Via:

–jeroen

Posted in *nix, *nix-tools, bash, bash, Development, Power User, Scripting, Software Development | Leave a Comment »

One day I will dig in the various ways that bash can do evaluation, for now: there is eval, “ and $() and I’m not sure when to choose which.

Posted by jpluimers on 2020/06/10

A while ago, I had to execute a series of aliases of which the names were stored in an array. A simple for loop with en eval call did the job.

Then I found out there are at least two more ways of evaluation in bash, so here are just a few links giving me a head start if I ever dig this up again:

Note that looping over parameters is different than over an array: [WayBack] Loop through an array of strings in Bash? – Stack Overflow

ou can use it like this:

## declare an array variable
declare -a arr=("element1" "element2" "element3")

## now loop through the above array
for i in "${arr[@]}"
do
   echo "$i"
   # or do whatever with individual element of the array
done

# You can access them using echo "${arr[0]}", "${arr[1]}" also

Also works for multi-line array declaration

declare -a arr=("element1" 
                "element2" "element3"
                "element4"
                )

–jeroen

Posted in *nix, *nix-tools, bash, bash, Development, Power User, Scripting, Software Development | Leave a Comment »

bash + sed: quadruple backslash for proper escaping within an alias

Posted by jpluimers on 2020/06/09

This is part of a bash alias where I had to use quadruple backslash in order to escape it for sed:

# The sed with quad //// is to prevent 'unterminated substitute in regular expression':
alias x='... | sed "s/=.*/ \\\\/"'

This is needed because bash will escape \\\\ into \\ which sed then escapes to \.

The easiest way to find this is to replace the sed with echo to see the expansion.

References:

–jeroen

Posted in *nix, *nix-tools, bash, bash, Development, Power User, Scripting, sed, Software Development | 2 Comments »

 
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