I used the array assignment below in order to simulate a two dimensional array:
for((i=0;i<2;i++))
do
for((j=0;j<3;j++))
do
read TWOD$i[$j]
done
done < hi.txt
The file hi.txt contains these lines:
1
2
3
4
5
6
If I use echo ${TWOD0[2]}, I can print the value 2, but if I am using a variable for the first index, bash throws a syntax error bad substitution:
for((i=0;i<2;i++))
do
printf "%s\n" "${TWOD$i[2]}"
done
Is there any way to extract the elements from the array using a variable for the first index?
You can use indirect expansion:
row="TWOD$i[2]"
printf "%s\n" ${!row}
Related
I have wrote a script that throws the output of running node processes with the cwd of that process and I store the value in an array using for loop and do echo that array.
How can I able to get the user enter the index of array regarding the output that the script throws and show the output against that input generated by user
Example Myscript
array=$(netstat -nlp | grep node)
for i in ${array[*]}
do
echo $i
done
output is something like that
1056
2064
3024
I want something more advance. I want to take input from user like
Enter the regarding index from above list = 1
And lets suppose user enter 1
Then next output should be
Your selected value is 2064
Is it possible in bash
First, you're not actually using an array, you are storing a plain string in the variable "array". The string contains words separated by whitespace, so when you supply the variable in the for statement, the unquoted value is subject to Word Splitting
You need to use the array syntax for setting the array:
array=( $(netstat -nlp | grep node) )
However, the unquoted command substitution still exposes you to Filename Expansion. The best way to store the lines of a command into an array is to use the mapfile command with a process substitution:
mapfile -t array < <(netstat -nlp | grep node)
And in the for loop, make sure you quote all the variables and use index #
for i in "${array[#]}"; do
echo "$i"
done
Notes:
arrays created with mapfile will start at index 0, so be careful of off-by-one errors
I don't know how variables are implemented in bash, but there is this oddity:
if you refer to the array without an index, you'll get the first element:
array=( "hello" "world" )
echo "$array" # ==> hello
If you refer to a plain variable with array syntax and index zero, you'll get the value:
var=1234
echo "${var[0]}" # ==> 1234
I have a sample flat file which contains the following block
test my array which array is better array huh got it?
INDIA USA SA NZ AUS ARG ARM ARZ GER BRA SPN
I also have an array(ksh_arr2) which was defined like this
ksh_arr2=$(awk '{if(NR==1){for(i=1;i<=NF;i++){if($i~/^arr/){print i}}}}' testUnix.txt)
and contains the following integers
3 5 8
Now I want to parse only those column values which are at the respective numbered positions i.e. third fifth and eighth.
I also want the outputs from the 2nd line on wards.
So I tried the following
awk '{for(i=1;i<=NF;i++){if(NR >=1 && i=${ksh_arr2[i]}) do print$i ; done}}' testUnix.txt
but it is apparently not printing the desired outputs.
What am I missing ? Please help.
How i would approach it
awk -vA="${ksh_arr2[*]}" 'BEGIN{split(A,B," ")}{for(i in B)print $B[i]}' file
Explanation
-vA="${ksh_arr2[*]}" - Set variable A to expanded ksh array
'BEGIN{split(A,B," ") - Splits the expanded array on spaces
(effictively recreating it in awk)
{for(i in B)print $B[i]} - Index in the new array print the field that is the number
contained in that index
Edit
If you want to preserve the order of the fields when printing then this would be better
awk -vA="${ksh_arr2[*]}" 'BEGIN{split(A,B," ")}{while(++i<=length(B))print $B[i]}' file
Since no sample output is shown, I don't know if this output is what you want. It is the output one gets from the code provided with the minimal changes required to get it to run:
$ awk -v k='3 5 8' 'BEGIN{split(k,a," ");} {for(i=1;i<=length(a);i++){print $a[i]}}' testUnix.txt
array
array
array
SA
AUS
ARZ
The above code prints out the selected columns in the same order supplied by the variable k.
Notes
The awk code never defined ksh_arr2. I presume that the value of this array was to be passed in from the shell. It is done here using the -v option to set the variable k to the value of ksh_arr2.
It is not possible to pass into awk an array directly. It is possible to pass in a string, as above, and then convert it to an array using the split function. Above the string k is converted to the awk array a.
awk syntax is different from shell syntax. For instance, awk does not use do or done.
Details
-v k='3 5 8'
This defines an awk variable k. To do this programmatically, replace 3 5 8 with a string or array from the shell.
BEGIN{split(k,a," ");}
This converts the space-separated values in variable k into an array named a.
for(i=1;i<=length(a);i++){print $a[i]}
This prints out each column in array a in order.
Alternate Output
If you want to keep the output from each line on a single line:
$ awk -v k='3 5 8' 'BEGIN{split(k,a," ");} {for(i=1;i<length(a);i++) printf "%s ",$a[i]; print $a[length(a)]}' testUnix.txt
array array array
SA AUS ARZ
awk 'NR>=1 { print $3 " " $5 " " $8 }' testUnix.txt
there is an array like this in bash
array_1=(1 2 3 4 5)
array_2=(6 7 8 9 0)
I have another variable that contains 1 or 2 .
array_index=1
So is it possible to create the array name using that variable - like this ?
array_${array_index}[0]
Use variable indirection to read and declare to write:
array_1=(1 2 3 4 5)
array_2=(6 7 8 9 0)
array_index=1
var="array_${array_index}[0]"
echo "The contents of $var is ${!var}"
declare "$var"="Something Else"
echo "Now $var contains ${!var} instead."
This is safer and easier to use correctly than eval.
To create array in bash, exactly to set array's value, you can use eval function, in order to make ${array_index} statement as a real index, so do as follows:
eval array_${array_index}[0]=1
To read value, also eval:
eval echo \${array_${array_index}[0]}
1
Problem is to access a particular element you have to do something like ${array[index]}. But you are also wanting to nest a variable in the array part , which bash won't understand when trying to perform the expansion since it expects array to be a variable.
So the only way I can think of doing this would be to force the array expansion to happen later than your variables. e.g.
> array_1=(1 2 3 4 5)
> array_2=(6 7 8 9 0)
> array_index=1
> eval "echo \${array_$array_index[0]}"
1
As n.m. points out in comments, eval is evil, so you should take care when using it.
Assigning arrays to variables in Bash script seems rather complicated:
a=("a" "b" "c")
b=$a
echo ${a[0]}
echo ${a[1]}
echo ${b[0]}
echo ${b[1]}
leads to
a
b
a
instead of
a
b
a
b
Why? How can I fix it?
If you want to copy a variable that holds an array to another name, you do it like this:
a=('a' 'b' 'c')
b=( "${a[#]}" )
Why?
If a is an array, $a expands to the first element in the array. That is why b in your example only has one value. In bash, variables that refer to arrays aren't assignable like pointers would be in C++ or Java. Instead variables expand (as in Parameter Expansion) into strings and those strings are copied and associated with the variable being assigned.
How can I fix it?
To copy a sparse array that contains values with spaces, the array must be copied one element at a time by the indices - which can be obtained with ${!a[#]}.
declare -a b=()
for i in ${!a[#]}; do
b[$i]="${a[$i]}"
done
From the bash man page:
It is possible to obtain the keys (indices) of an array as well as the values.
${!name[#]} and ${!name[*]} expand to the indices assigned in array variable name.
The treatment when in double quotes is similar to the expansion of the special
parameters # and * within double quotes.
Here's a script you can test on your own:
#!/bin/bash
declare -a a=();
a[1]='red hat'
a[3]='fedora core'
declare -a b=();
# Copy method that works for sparse arrays with spaces in the values.
for i in ${!a[#]}; do
b[$i]="${a[$i]}"
done
# does not work, but as LeVar Burton says ...
#b=("${a[#]}")
echo a indicies: ${!a[#]}
echo b indicies: ${!b[#]}
echo "values in b:"
for u in "${b[#]}"; do
echo $u
done
Prints:
a indicies: 1 3
b indicies: 1 3 # or 0 1 with line uncommented
values in b:
red hat
fedora core
This also works for associative arrays in bash 4, if you use declare -A (with capital A instead of lower case) when declaring the arrays.
I am trying to write code to break up a large array into many different small arrays. Eventually the array I would be passed is one of unknown size, this is just my test subject. I have gotten this far:
#!/bin/bash
num=(10 3 12 3 4 4)
inArray=${#num[#]}
numArrays=$(($inArray/2))
remain=$(($inArray%2))
echo $numArrays
echo $remain
nun=0
if test $remain -gt $nun; then
numArrays=$(($numArrays+1))
fi
array=(1 2)
j=0
for ((i=0;i<$numArrays;i++, j=j+2)); do
array=("${num[#]:$j:2}")
echo "The array says: ${array[#]}"
echo "The size? ${#array[#]}"
done
What I am really having a problem with is : I would like to make the variable 'array' be able to change names slightly every time, so each array is kept and has a unique name after the loop. I have tried making the name array_$i but that returns:
[Stephanie#~]$ ./tmp.sh
3
0
./tmp.sh: line 16: syntax error near unexpected token `"${num[#]:$j:2}"'
./tmp.sh: line 16: ` array_$i=("${num[#]:$j:2}")'
[Stephanie#RDT00069 ~]$ ./tmp.sh
3
0
./tmp.sh: line 16: syntax error near unexpected token `$i'
./tmp.sh: line 16: ` array($i)=("${num[#]:$j:2}")'
Does anyone have any advice?
Thanks
I don't think you can really avoid eval here, but you might be able to do it safely if you're careful. Here's my approach:
for name in "${!array_*}"; do # Get all names starting with array_
i="${name#array_*}" # Get the part after array_
if [[ $i != *[^0-9]* ]]; then # Check that it's a number.
printf '%s is not a valid subarray name\n' "$name"
else
# Create a variable named "statement" that contains code you want to eval.
printf -v statement 'cur_array=( "${%s[#]}" )' "$name"
eval "$statement"
# Do interesting things with $cur_array
fi
done
Before this, when you're just creating the array, you know what $name should be, so just use the printf -v part.
To make it even safer, you could save all the allowed array names in another array and check that $name is a member.
With simple variables, you can use the declare keyword to make indirect assignments:
v=foo
declare $v=5
echo $foo # Prints 5
This doesn't extend to arrays in the obvious (to me, anyway) sense:
i=2
# This produces a syntax error
declare -a array_$i=("${num[#]:$j:2}")
Instead, you can declare an empty array
declare -a array_$i
or assign items one at a time:
declare -a array_$i[0]=item1 array_$i[1]=item2
Here's an example of using a for-loop to copy, say, the 3rd
and 4th letters of a big array into a smaller one. We use
i as the dynamic part of the name of the smaller array, and
j as the index into that array.
letters=(a b c d e f)
i=1
j=0
for letter in "${letters[#]:2:2}"; do
# E.g., i=0 and j=1 would result in
# declare -a array_0[1]=c
declare -a array_$i[$j]=$letter
let j+=1
done
done
echo ${array_1[#]}; # c d
${foo[#]:x:y} gives us elements x, x+1, ..., x+y-1 from foo, and
You can wrap the whole thing inside another for-loop to accomplish the goal of splitting letters into 3 smaller arrays:
# We'll create array_0, array_1, and array_2
for i in 0 1 2; do
# Just like our subset above, but start at position i*2 instead of
# a constant.
for letter in "${letters[#]:$((i*2)):2}"; do
declare -a array_$i[$j]=$letter
done
done
Once you manage to populate your three arrays, how do you access them without eval? Bash has syntax for indirect access:
v=foo
foo=5
echo ${!v} # echoes 5!
The exclamation point says to use the word that follows as a variable whose value should be used as the name of the parameter to expand. Knowing that, you might think you could do the following, but you'd be wrong.
i=1
v=array_$i # array_1
echo ${!v[0]} # array_1[0] is c, so prints c, right? Wrong.
In the above, bash tries to find a variable called v[0] and expand it to get the name of a parameter to expand. We actually have to treat our array plus its index as a single name:
i=1
v=array_$i[0]
echo ${!v} # This does print c
This should work, but this is not a good solution, another language may be better bash does not support multi dimensional arrays
eval array_$i='('"${num[#]:$j:2}"')'
And then, for example
eval 'echo "${array_'$i'[0]}"'