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

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Archive for the ‘bash’ Category

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explainshell.com: parse and explain just about any shell command

Posted by jpluimers on 2021/02/17

I bumped into the tremendously site [WayBack] explainshell.com – match command-line arguments to their help text only after documenting the relevant cURL options of yesterdays post on checking your CertBot domain expiration dates.

The site allows put in a shell command-line to see the help text that, including matches for each argument.

It works so well because it parses both the shell command-line and the man pages, then constructs a web-page linking the relevant man page content to the shell command-line in the correct shell command-line order.

The explainshell has a counterpart showthedocs (both are open source) for explaining other languages (on the one hand more extended as it goes much deeper into parsing for instance SQL, on the other hand more limited as it only supports a few languages). More on showthedocs later.

The links

The parsing results

The first bit below is just the text output, and the second bit the screenshot, of a relatively simple command like [WayBack] explainshell.com – curl -fsSL example.org:

curl(1) -fsSL example.org
transfer a URL
 
-f, --fail
       (HTTP)  Fail  silently  (no  output at all) on server errors. This is mostly done to better enable
       scripts etc to better deal with failed attempts. In normal cases  when  a  HTTP  server  fails  to
       deliver  a  document,  it  returns an HTML document stating so (which often also describes why and
       more). This flag will prevent curl from outputting that and return error 22.

       This method is not fail-safe and there are occasions where non-successful response codes will slip
       through, especially when authentication is involved (response codes 401 and 407).
 
-s, --silent
       Silent or quiet mode. Don't show progress meter or error messages.  Makes Curl mute.
 
-S, --show-error
       When used with -s it makes curl show an error message if it fails.
 
-L, --location
       (HTTP/HTTPS) If the server reports that the requested page  has  moved  to  a  different  location
       (indicated  with  a Location: header and a 3XX response code), this option will make curl redo the
       request on the new place. If used together with -i, --include or  -I,  --head,  headers  from  all
       requested pages will be shown. When authentication is used, curl only sends its credentials to the
       initial host. If a redirect takes curl to a different host, it won't  be  able  to  intercept  the
       user+password.  See  also  --location-trusted  on  how to change this. You can limit the amount of
       redirects to follow by using the --max-redirs option.

       When curl follows a redirect and the request is not a plain GET (for example POST or PUT), it will
       do  the  following  request  with a GET if the HTTP response was 301, 302, or 303. If the response
       code was any other 3xx code, curl will re-send the following request  using  the  same  unmodified
       method.
source manpages: curl

The screenshot is even more impressive:

Read the rest of this entry »

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

bash – How to get the primary IP address of the local machine on Linux and OS X? – Stack Overflow

Posted by jpluimers on 2021/02/05

[WayBack] bash – How to get the primary IP address of the local machine on Linux and OS X? – Stack Overflow (thanks [WayBack] Collin Anderson!):

I tried on OS X 10.13.6 (macOS High Sierra) and these two work fine:

ip route get 1 | awk '{print $NF;exit}'

and

ip route get 8.8.8.8 | head -1 | cut -d' ' -f8

The first one needs the $ back-slash escaped as bash alias; the second does not need that.

Related:

–jeroen

Posted in *nix, *nix-tools, Apple, bash, Mac OS X / OS X / MacOS, macOS 10.13 High Sierra, Power User | Leave a Comment »

bash – convert comma separated values into a list of values using shell script – Stack Overflow

Posted by jpluimers on 2020/09/07

For a simple comma separated list (no quotes), I was expecting a sed script (and indeed it is possible), but tr is more elegant:

Use tr to change , into newlines:

tr , "\n" < list.txt

See https://en.wikipedia.org/wiki/Tr_(Unix)

Source: [WayBack] bash – convert comma separated values into a list of values using shell script – Stack Overflow.

–jeroen

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

find – display only files starting with . (hidden) – Unix & Linux Stack Exchange

Posted by jpluimers on 2020/08/21

find . -type f -name '\.*' -print

Must work if you want list every hidden file down in the directory hierarchy.

This sort of works on Linux, but fails on VMware ESXi (on Linux it only works when applying -maxdepth 1, deeper levels fails because they list all files where the top directory starts with a .):

If you want hidden files and hidden directories, without . and .. :

find -regex '\./\..+' -print

This works on both Linux and VMware ESXi:

If you want hidden files and hidden directories, without . and .. :

find . \( -type f -o -type d \) -name '\.*' -print

Based on:

–jeroen

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

Verifying large sets of file hashes with md5sum

Posted by jpluimers on 2020/07/24

A few tips:

  1. Recursively getting all md5 sums from a source directory:

    cd /sourceDirectory
    find -type f \( -not -name "md5sum.txt" \) -exec md5sum '{}' \; > md5sum.txt

    .

  2. Checking the sums against a target directory

    cd /targetDirectory
    md5sum -c /sourceDirectory/md5sum.txt

    .

On some systems (this was an ESXi system which can’t run stuff from the console in parallel), you could optimise this using xargs for the generation and GNU parallel for the generation and checking. Both should be very similar:

GNU parallel is written to have the same options as xargs. If you write loops in shell, you will find GNU parallel may be able to replace most of the loops and make them run faster by running several jobs in parallel.

Via:

–jeroen

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

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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