bash script for grep with context

Faki · 05-16-2022, 02:28 PM

Am writing a bash script that uses grep to search phrases and prints them on terminal. I want the user to have the possibility of using the context options of grep (-C, -A, -B). I made an indexed array ICTX, which I want to use for making the context arguments to grep (-C NUM, -A NUM, -B NUM, -A NUM1 -B NUM2).

Would help having some advice on how I can neatly handle the making of ICTX with tho code listed below.

Code:

  while (( $# > 0 )); do
   case "$1" in
     ("-C"|"--context")
         ctx="$2" ; shift ; shift ;;
    
     ("--Bfc"|"--before-context")
         bfctx="$2" ; shift ; shift ;;

     ("--Afc"|"--after-context")
         afctx="$2" ; shift ; shift ;;
   esac
  done

  [[ -z "$ctx" ]] && ctx=8
  ictx=( -C "$ctx" )
  ictx=( -A "$afctx" )
  ictx=( -B "$bfctx" )

  grep -ni "${ictx[@]}" -e "$ptrn" -- "$fl"

teckk · 05-16-2022, 03:20 PM

$# is a special variable in bash, that expands to the number of arguments

a.sh

Code:

#!/usr/bin/bash
echo "$#"

Code:

bash -c 'echo $#'
0

bash -c 'echo $#' - A
1

bash -c 'echo $#' - A B
2

bash ./a.sh foo bar
2

Now, to the problem I see.
If ctx is zero length then ctx = 8

Code:

[[ -z "$ctx" ]] && ctx=8

echo "$ctx"
8

ictx=( -C "$ctx" )
echo "$ictx"
-C

ictx="-C "$ctx""
echo "$ictx"
-C 8

You also try and set the same variable 3 times

Code:

ictx=( -C "$ctx" )
ictx=( -A "$afctx" )
ictx=( -B "$bfctx" )

Code:

a=1
a=2
a=3
echo "$a"
3

All that I can tell you without seeing more.

Faki · 05-16-2022, 03:47 PM

Yes, I am setting the same ICTX array three times. It is currently the problem I am discussing in the question. The question is about constructing the context array ICTX in a way that does not produce conflicts (e.g. using -C with -A or -B). By default I want to use -C 8 when no options are supplied by the user (i.e. no -C, -B or -A).

michaelk · 05-16-2022, 03:58 PM

Code:

  ictx=( -C "$ctx" )
  ictx=( -A "$afctx" )
  ictx=( -B "$bfctx" )

Each line initializes the array so ictx will only contain the values -B $bfctx regardless. Instead of an array you can just concatenate the values into a single string but If any of the other options i.e afctx or bfctx are empty then that will probably cause an error.

Quote:

astring="-C $ctx -A $afctx -B $bfctx"

teckk · 05-16-2022, 04:01 PM

Not sure If I know what you want.

Example:

Code:

ctx="one"
afctx="two"
bfctx="three"

declare -A MyArray=( [C]="$ctx" [A]="$afctx" [B]="$bfctx" )

echo "${!MyArray[@]}"
echo "${MyArray[@]}"

echo ${MyArray[C]}

michaelk · 05-16-2022, 04:52 PM

May not be the best..

Code:

if  [[ $# == 0 ]]; then
    grep -ni "-C 8" -e "$ptrn" -- "$fl"
    exit
fi

  while (( $# > 0 )); do
   case "$1" in
     ("-C"|"--context")
         ctx="$2" ; shift ; shift ;;
    
     ("--Bfc"|"--before-context")
         bfctx="$2" ; shift ; shift ;;

     ("--Afc"|"--after-context")
         afctx="$2" ; shift ; shift ;;
   esac
  done

I do not know of a better way then testing each parameter to see if it is empty and constructing your grep string in the desired order.

Faki · 05-16-2022, 05:09 PM

I have done as follows as an initial try. Could you criticise please?

Code:

ictx=()
[ -n "$bfctx" ] && ictx+=( -B "$bfctx" )
[ -n "$afctx" ] && ictx+=( -A "$afctx" )

[[ -z "$ctx" ]] && ctx=8
(( ${#ictx[@]} == 0 )) && ictx+=( -C "$ctx" )

chrism01 · 05-19-2022, 11:38 PM

See this page https://wiki.bash-hackers.org/syntax..._default_value ; see subsection re default value setting.