bash script, to use a variable in a variable

Question

In order to obtain the results of consecutive numbers, a bash script was created as follows. However, the results did not come out as intended.

#!/bin/bash

test="i am $i"

for i in {1..10}
do
        echo "$test"
done

result

sh test.sh

i am 
i am 
i am 
i am 
i am 
i am 
i am 
i am 
i am 
i am

But the result I want is... As shown below, how do we deal with the variables to get the results?

i am 1
i am 2
i am 3 
i am 4
i am 5
i am 6
i am 7
i am 8
i am 9
i am 10

$i is expanded as part of assignment, use echo i am $i instead. — oguz ismail
– oguz ismail, Commented Sep 8, 2021 at 5:27
Variables contain static strings, not dynamic instructions for generating strings, so fundamentally this won't work. But this seems like an XY problem. Depending on what the bigger picture is (what the larger problem you're trying to solve is), a function might be a good solution, or maybe a printf format string, or... I dunno. What's the actual goal here? — Gordon Davisson
– Gordon Davisson, Commented Sep 8, 2021 at 6:21

Saeed · Accepted Answer · 2021-09-08 06:45:02Z

1

Use $i outside of the variable

#!/bin/bash

test="i am "

for i in {1..10}
do
  echo $test $i
done

Also you can use ${i} inside of the variable

#!/bin/bash

for i in {1..10}
do
  test="i am ${i}"
  echo $test
done

The result is:

i am  1
i am  2
i am  3
i am  4
i am  5
i am  6
i am  7
i am  8
i am  9
i am  10

Or you can replace substr with anything you want inside.

For example

#!/bin/bash
test="I am SUBSTR"

for i in {1..10}
do
  echo ${test/SUBSTR/$i}
done

When you have multiple variables, I know this solution:

#!/bin/bash
test="I am SUBSTR and STR2"

for i in {1..10}
do
  o=${test/SUBSTR/$i}
  echo ${o/STR2/$i*$i}
done

Using sed also can help

#!/bin/bash
test="I am SUBSTR and STR2"

for i in {1..10}
do
  echo $test | sed  -e 's/SUBSTR/'$i'/;s/STR2/'$i++'/'
done

edited Sep 8, 2021 at 6:45

answered Sep 8, 2021 at 5:31

Saeed

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6 Comments

Saeed Over a year ago

Can't you split the parts before and after that variable? part1 $variable part2 ??

user1819769 Over a year ago

Thank you. You should use $i in the middle of the sentence. We want to use $i as the where condition for the DB query. Is there any way to declare and use the $test externally?

Saeed Over a year ago

I updated my answer, You can replace your substr with the variable. @user1819769

user1819769 Over a year ago

Thank you once again. Then, how can we use test="select * from TABLE where A=$i and B="abcd"?

Saeed Over a year ago

define test like test="select * from TABLE where A=COUNTER and B="abcd" and in loop replace COUNTER with $i. Did you try this? @user1819769

|

ceving · Accepted Answer · 2021-09-08 06:46:11Z

0

You need to write a function in order to delay the evaluation.

#!/bin/bash

message () { echo "i am $1"; }

for i in {1..10}
do
  message $i
done

Or the following, if you just want to craft the message.

#!/bin/bash

message () { echo "i am $1"; }

for i in {1..10}
do
  test="$(message $i)"
  echo "$test"
done

answered Sep 8, 2021 at 6:46

ceving

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Comments

yanyong · Accepted Answer · 2021-09-09 01:51:03Z

0

#!/bin/bash

test='i am $i'

for i in {1..10}
do
  eval echo "$test"
done

or

#!/bin/bash

test='echo "i am $i"'

for i in {1..10}
do
  eval "$test"
done

Example:

yanyong@master:~$ test='echo "i am $i"'; for i in {1..10}; do eval "$test"; done
i am 1
i am 2
i am 3
i am 4
i am 5
i am 6
i am 7
i am 8
i am 9
i am 10

References:

https://man7.org/linux/man-pages/man1/eval.1p.html

edited Sep 9, 2021 at 1:51

answered Sep 8, 2021 at 10:41

yanyong

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