CODEWITHSUNDEEP
linuxbashshell
sudhir tataraju
sudhir tataraju

Reputation: 1379

Execute command substitutions in input read from a file

In shell script how to make script read commands in input file string

Example 1 (script1.sh):

a="google.analytics.account.id=`read a`"
echo $a

Example 2 (script2.sh):

cat script2.sh

a=`head -1 input.txt`
echo $a

Sample input.txt

google.analytics.account.id=`read a`

If I run script1.sh the read command is working fine, but when I am running script2.sh, the read command is not executed, but is printed as part of the output.

So I want script2.sh to have the same output as script1.sh.

Upvotes: 0

Views: 320

Answers (2)

ULick
ULick

Reputation: 999

In script1.sh the first line is evaluated, therefore the read a is executed and replaced in the string.

In script 2.sh the first line is evaluated, therefore the resulting string from execution of head is put into the variable a.

There is no re-evaluation done on the resulting string. If you add the evaluation with eval $a and the first line in input.txt is exactly as the first line of script1.sh (actually the a="..." is missing) then you might get the same result. The heredoc, as CharlesDuffy suggested, seems more accurate.

Upvotes: 0

Charles Duffy
Charles Duffy

Reputation: 295650

Your input.txt contents are effectively executed as a script here; only do this if you entirely trust those contents to run arbitrary commands on your machine. That said:

#!/usr/bin/env bash
#              ^^^^- not /bin/sh; needed for $'' and $(<...) syntax.

# generate a random sigil that's unlikely to exist inside your script.txt
# maybe even sigil="EOF-$(uuidgen)" if you're guaranteed to have it.
sigil="EOF-025CAF93-9479-4EDE-97D9-483A3D5472F3"

# generate a shell script which includes your input file as a heredoc
script="cat <<$sigil"$'\n'"$(<input.txt)"$'\n'"$sigil"

# run that script
eval "$script"

Upvotes: 1

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