Bash: proper handling of errors

Question

I have bash script similar to:

{
  # Try block
  command 1
  command 2
  command 3
} || {
  # Catch errors
  while true; do
  {
    # Try block inside
    # Change parameters for command 1, command 2
    command 1
    command 2
    command 3
    break
  } || {
      # Change parameters and try again
      continue
  }
done
}

More or less this code works fine, but...

For some reason try sections works not as expected for me. I thought that it fails if some of my commands return not 0 code, but it's not true.

For some specific cases my command 2 returns 1 code in the first try block, but it doesn't fail and goes into catch section, command 3 executes from this try block and that's it.

Here is my question:

How to handle errors in bash? Just to play with return codes?

UPDATE:

Original code looks very similar to: The main idea is that all 3 commands should be executed one-by-one and all of them are somehow related to folder_name.

folder_name=my_folder
suffix=0
{
  cd /home/test/
  mkdir $folder_name
  ls -la $folder_name
} || {
    while true; do
      suffix=$(expr $suffix + 1)
      {
        folder_name=$folder_name_$suffix
        cd /home/test/
        mkdir $folder_name
        ls -la $folder_name
        break
      } || {
          continue
      }
    done
}

I would recommend implementing this as a script instead of writing a lengthy and less readable one liner. Hope you can understand the reason why. — Anubis
– Anubis, Commented Jul 30, 2017 at 10:13
@Anubis, I'm trying to wrap this into the script, but it looks awful and works not as I expect..That's why I published this question. — smart
– smart, Commented Jul 30, 2017 at 10:14
If you can show a simplified but working code, may be I'll be able to help. — Anubis
– Anubis, Commented Jul 30, 2017 at 10:15

Anubis · Accepted Answer · 2017-07-30 10:56:56Z

1

I see two problems here.

When grouping commands using curly braces, there must be a semicolon at the end (or a newline before the closing brace). Refer to docs here

On the other hand, you are not considering return codes anyway. You normally check the return code with something like [[ $? -eq 0 ]]. But in here, you can simply chain the commands with &&. e.g.

{
  cd /home/test/ && 
  mkdir $folder_name &&
  ls -la $folder_name;
}

However, following will do what you need.

cd /home/test/
prefix=my_folder                                                                                                                                      
suffix=0

folder_name="$prefix"
while true; do
  if [[ -e $folder_name ]]; then
    folder_name="${prefix}_$((++suffix))"
    continue
  fi  
  mkdir "$folder_name"
  if [[ $? -ne 0 ]]; then
    echo "mkdir failed"
    exit 1
  fi  
  break
done

edited Jul 30, 2017 at 10:56

answered Jul 30, 2017 at 10:47

Anubis

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4 Comments

smart Over a year ago

Well, I've replaced my code into { command 1 && command 2; echo $?} || { echo "failed"} and it returns non-zero return code. It can't print failed as expected. What is wrong here?

Anubis Over a year ago

That's wrong. Braced group's return code is the return code of the last command. In your case echo $? executes successfully and returns 0, which means the group execution was successful. Just remove it and it'll work, { comamnd 1 && command 2; } || {echo 'failed'; }. Don't forget the semicolon (or a newline).

smart Over a year ago

@Anubis, hmm..interesting. So, it's not possible to echo some success message there?

Anubis Over a year ago

Yes you can, { comamnd 1 && command 2 && echo 'success'; } || {echo 'failed'; }. Bash proceeds to the next command in an && chain only if the current command returned with 0. If any failed, the chain will break right there with the failed error code.

Petr Skocik · Accepted Answer · 2017-07-30 10:52:47Z

1

As you probably already know, you can use set -e (== set -o errexit) to simplify error handling by making the script abort on an untested failure.

Unfortunately with it, non-exec'd subshells behave differently than exec-ed shell children, because a non-exec'd subshell can see whether it's tested whereas an exec'd shell child cannot.

Consequently, this:

#!/bin/sh -e

echo "EXEC'D SHELL CHILD"
if sh -e <<'EOF'
true(){ echo true; return 0; }
false(){ echo false; return 1; }
true
false 
true
true
EOF
then
    echo SUCCESS
else
    echo FAILED: $?
fi

echo ===========
echo "NON-EXEC'D SUBSHELL"
if (
true(){ echo true; return 0; }
false(){ echo false; return 1; }
true
false 
true
true
)
then
    echo SUCCESS
else
    echo FAILED: $?
fi

Outputs:

EXEC'D SHELL CHILD
true
false
FAILED: 1
===========
NON-EXEC'D SUBSHELL
true
false
true
true
SUCCESS

(Feel free to use /bin/bash instead of /bin/sh -- it behaves exactly the same)

So you can use set -e to somewhat simplify error handling, but you still need && or || return 1 in a tested subshell (or block or function), or you need to use an exec'd shell child instead. In your case, you can do:

 sh -ec' command 1
  command 2
  command 3' || ...

or

 {  comman 1 && command 2 && command 3; } || ...

edited Jul 30, 2017 at 10:52

answered Jul 30, 2017 at 10:28

Petr Skocik

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3 Comments

smart Over a year ago

Currently, I'm using bash -x in my script with set set -o nounset set -o errexit set -o pipefail, that's why I'm not sure how it works together with -e.

Petr Skocik Over a year ago

set -o errexit == set -e. They're two ways of turning on the same feature.

Petr Skocik Over a year ago

@smart Personally, I'm quite convinced that, although I use it myself, the errexit feature is misdesigned. I think the above difference between exec'd shells and subshells/blocks/functions shouldn't exist, but the semantics of set -e are already set in stone/POSIX.

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