2

I'd like to have a bash script that has allows me to call a function to see if some data is contained in a file, and if it isn't fail the main script, kind of like the following, which is simplified to keep this on point.

This doesn't work (doesn't exit the main script when the sub shell fails).

How can I write the require_line function so I could have say over 20 of them in a file like so

VALUE1=$(require_line "myKey1")
VALUE2=$(require_line "myKey2")
...

and not require an if around each one?

#!/bin/bash
set -eo pipefail

VALUE=$(require_line "myKey")

require_line(){
  local KEY=$1
  local DATA=$(cat /tmp/myfile)
  local INFO=$(echo "$DATA" | grep "$KEY")

  if [ ! -z "$INFO" ]
  then
    echo "Key not found in $DATA, key: $KEY"
    exit 1;
  fi
  echo "$INFO"
}
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2
  • You can do it in bash with set -E and an ERR trap, but I think that you're doing that wrong in the first place: better have that require_line function set the variable itself, rather than being called in a subshell: require_line(){ local -n v=$1; ...; v=$INFO; }; ... require_line VALUE1 myKey1; ... (you can also do that portably with eval instead of using variable references, but your Q is tagged bash) Commented Oct 29, 2021 at 20:42
  • Bash has to declare the function before it can be called. Commented Oct 30, 2021 at 5:03

2 Answers 2

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2

Exiting the subshell will not exit the main script as you experienced it.

I think of 3 (and a half) solutions:

  1. use set -e so that any ("untested") failing command (or subshell) would immediately exit the main script (this may be overkill or causing other troubles),
  2. send a signal from the function and catch it with a trap
  3. use || exit $? after each VALUEn=$(...) like this VALUE=$(require_line "myKey") || exit $?,
  4. combine (3.) with a (not so elegant) loop using eval.

That third one does not exactly "require an if around each one" and would still be a fairly compact syntax IMHO.

By the way, this line

echo "Key not found in $DATA, key: $KEY"

...is actually useless if you exit the whole script right after this because the sentence will be stored in the $VALUEn variable which will not be displayed.

I suggest printing to stderr like this:

echo "my error" 1>&2

Examples

Example for solution 1

#!/bin/sh
set -e

myfunc(){
        echo $1
        if [ "$1" != "OK" ] ; then exit 1 ; fi
}

VALUE1=$(myfunc "OK") 
echo $VALUE1
VALUE2=$(myfunc "NO WAY") 
echo $VALUE2

echo "main script did not exit"

$ ./test.sh
OK
zsh: exit 1     ./test.sh

But if I remove set -e from the beginning, I get:

$ ./test.sh
OK
NO WAY
main script did not exit

Example for solution 2

#!/bin/sh

trap "exit $?" USR1

myfunc(){
        echo $1
        if [ "$1" != "OK" ] ; then kill -USR1 $$ ; fi 
}

VALUE1=$(myfunc "OK") 
echo $VALUE1
VALUE2=$(myfunc "NO WAY")
echo $VALUE2

echo "main script did not exit"

$ ./test.sh
OK

Example for solution 3

#!/bin/sh

myfunc(){
        echo $1
        if [ "$1" != "OK" ] ; then exit 1 ; fi
}

VALUE1=$(myfunc "OK") || exit $?
echo $VALUE1
VALUE2=$(myfunc "NO WAY") || exit $?
echo $VALUE2

echo "main script did not exit"

$ ./test.sh
OK
zsh: exit 1     ./test.sh

Example for solution 4

#!/bin/sh

myfunc(){
        echo $1
        if [ "$1" != "OK" ] ; then exit 1 ; fi
}

I=1
for key in "OK" "OK" "NO WAY": ; do
        eval "VALUE$I=\$(myfunc \"$key\")" || exit $?
        eval "echo \$VALUE$I"
        I=$(($I+1))
done
echo "main script did not exit"

$ ./test.sh
OK
OK
zsh: exit 1     ./test.sh
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2

Let's restructure your require_line script a little to help you out.

First, we can get rid of the useless cat | grep. Second, we can use grep's intrinsic behavior to indicate success or failure of the search for KEY, as well as printing the key, if found, to stdout.

require_line(){
  local KEY="$1"
  local FILE="/tmp/myfile"

  if grep "$KEY" "$FILE"
  then
    return 0
  else
    printf 'Key not found in:\n\n"%s"\n\nKey: "%s"\n' "$(cat "$FILE")" "$KEY" >&2
    return 1
  fi
}

This leverages the built-in behavior of grep. If the key is found, grep prints the matching line, and returns success. Otherwise the else branch is taken, and a message is printed indicating that the key wasn't found. Further, in the case when grep fails, the error message is printed to stderr so that the error message won't be mistaken for a valid match found in $FILE.

You could further modify require_line to accept a filename as the $2 parameter by changing the line to read local FILE="$2" and then passing the desired filename each time you invoke require_line.

Now, with that in place....

Do you really need to store each VALUEn for KEYn, or do you just need to ensure that they are all present?

Now that you have a require_line function that cleanly returns a success or failure value, you could simply AND all your tests together. As soon as any one of them fails, the overall test will fail.

Assuming you do need the actual match values, the long-winded way to do it would be:

if value1=$(require_line "key1") &&
   value2=$(require_line "key2") &&
   value3=$(require_line "key3")
then
   printf "%s\n" "$value1" "$value2" "$value3"
else
   printf "One or more keys failed.\n" >&2
fi

That wil get tedious if you have numerous keys to check for. Using an array may be better:

#!/usr/bin/env bash

require_line(){
  local KEY="$1"
  local FILE="/tmp/myfile" # or perhaps "$2"

  if grep "$KEY" "$FILE"
  then
    return 0
  else
    printf 'Key not found in:\n\n"%s"\n\nKey: "%s"\n' "$(cat "$FILE")" "$KEY" >&2
    return 1
  fi
}

declare keys=("this" "is" "a" "test" "\." "keyN")
N=${#keys[@]}

declare values=()

j=0
while [ $j -lt $N ] && values[$j]="$(require_line "${keys[j]}")"
do
  j=$(($j+1))
done

if [ $j -lt $N ]
then
  printf 'error: found only %d keys out of %d:\n' $j $N
  printf '  "%s"\n' "${values[@]}"
fi

Running that code with some sample data:

$ cat /tmp/myfile
this is a test.
$ ./test.sh 
Key not found in:

"this is a test."

Key: "keyN"
error: found only 5 keys out of 6:
  "this is a test."
  "this is a test."
  "this is a test."
  "this is a test."
  "this is a test."
  ""

Lastly, if you really only need to verify that all the keys are present without needing to know what the match values are, the array-oriented code above could be simplified to simply loop until all keys are found, or to abort at the first key that is found to be missing.

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