Bash: Store variables in string to get resolved at a later time

Question

This is a small part of the code. I am creating test_args having a variable, which needs to be resolved at a later time, when that variable is defined in that scope, here, inside a for loop.

test_args=""
test_args+='-test.coverprofile=/tmp/coverage${ii}.out'
test_dirs="a b"

ii=0
for d in ${test_dirs[@]}; do
    echo ${test_args}               # actually used as `go test ${test_args}`
    ii=$((ii+1))
done

Current output:

-test.coverprofile=/tmp/coverage${ii}.out
-test.coverprofile=/tmp/coverage${ii}.out

Expected output:

-test.coverprofile=/tmp/coverage0.out
-test.coverprofile=/tmp/coverage1.out

I am open to suggestions for better methods. I need all the files to be created with a different name, so as to prevent overwriting in the loop.

choroba · Accepted Answer · 2020-07-15 08:52:09Z

2

What you want is a template. You can use the substitution parameter expansion to achieve that:

#!/bin/bash
test_args=-test.coverprofile=/tmp/coverage%i%.out
test_dirs=(a b)

i=0
for d in "${test_dirs[@]}" ; do
    echo "${test_args/\%i%/$i}"
    ((++i))
done

answered Jul 15, 2020 at 8:52

choroba

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Comments

Chinoman10 · Accepted Answer · 2020-07-15 12:10:21Z

1

Using eval is the usual solution:

#!/bin/bash

test_args=""
test_args+='-test.coverprofile=/tmp/coverage${ii}.out'
test_dirs="a b"

ii=0
for d in ${test_dirs[@]}; do
    eval echo ${test_args}
    ii=$((ii+1))
done

Which results in:

[user@linux ~]$ ./test2.sh
-test.coverprofile=/tmp/coverage0.out
-test.coverprofile=/tmp/coverage1.out

But here's a useful discussion about this topic - I advise reading the question linked on the left and it's answer, although it doesn't quite fully apply to this use case.

edited Jul 15, 2020 at 12:10

answered Jul 15, 2020 at 9:07

Chinoman10

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2 Comments

subtleseeker Over a year ago

Is there some way to use variable indirection in my case, the way described in the link you shared?

Chinoman10 Over a year ago

I don't think it applies to your use case to be honest. You're stuck with either using 'eval' or a template as @choroba mentioned.

Suggon · Accepted Answer · 2024-07-24 08:44:35Z

0

A safer solution than using eval would be declaring the "template" variables inside a function scope, and calling the function to access them just before your main logic.

#!/bin/bash

# these will be initialized after function call
init_args() {
    test_args="-test.coverprofile=/tmp/coverage${ii}.out"
}

test_dirs="a b"
ii=0
for d in ${test_dirs[@]}; do
    init_args           # $ii is now visible to test_args for interpolation
    echo ${test_args}
    ii=$((ii+1))
done

answered Jul 24, 2024 at 8:44

Suggon

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