How to conditionally do something if a command succeeded or failed

Question

How can I do something like this in bash?

if "`command` returns any error";
then
    echo "Returned an error"
else
    echo "Proceed..."
fi

Answer 1

How to conditionally do something if a command succeeded or failed

That's exactly what bash's if statement does:

if command ; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

Adding information from comments: you don't need to use the [ ... ] syntax in this case. [ is itself a command, very nearly equivalent to test. It's probably the most common command to use in an if, which can lead to the assumption that it's part of the shell's syntax. But if you want to test whether a command succeeded or not, use the command itself directly with if, as shown above.

Answer 2

For small things that you want to happen if a shell command works, you can use the && construct:

rm -rf somedir && trace_output "Removed the directory"

Similarly for small things that you want to happen when a shell command fails, you can use ||:

rm -rf somedir || exit_on_error "Failed to remove the directory"

Or both

rm -rf somedir && trace_output "Removed the directory" || exit_on_error "Failed to remove the directory"

It's probably unwise to do very much with these constructs, but they can on occasion make the flow of control a lot clearer.

Answer 3

Check the value of $?, which contains the result of executing the most recent command/function:

#!/bin/bash

echo "this will work"
RESULT=$?
if [ $RESULT -eq 0 ]; then
  echo success
else
  echo failed
fi

if [ $RESULT == 0 ]; then
  echo success 2
else
  echo failed 2
fi

Answer 4

This worked for me:

command && echo "OK" || echo "NOK"

if command succeeds, then echo "OK" is executed, and since it's successful, execution stops there. Otherwise, && is skipped, and echo "NOK" is executed.

Answer 5

It should be noted that if...then...fi and &&/|| type of approach deals with exit status returned by command we want to test( 0 on success ); however, some commands don't return a non-zero exit status if command failed or couldn't deal with input. This means that the usual if and &&/|| approaches won't work for those particular commands.

For instance, on Linux GNU file still exits with 0 if it received a non-existing file as argument and find couldn't locate the file user specified.

$ find . -name "not_existing_file"                                          
$ echo $?
0
$ file ./not_existing_file                                                  
./not_existing_file: cannot open `./not_existing_file' (No such file or directory)
$ echo $?
0

In such cases, one potential way we could handle the situation is by reading stderr/stdin messages, e.g. those that returned by file command, or parse output of the command like in find. For that purposes, case statement could be used.

$ file ./doesntexist  | while IFS= read -r output; do                                                                                                                  
> case "$output" in 
> *"No such file or directory"*) printf "%s\n" "This will show up if failed";;
> *) printf "%s\n" "This will show up if succeeded" ;;
> esac
> done
This will show up if failed

$ find . -name "doesn'texist" | if ! read IFS= out; then echo "File not found"; fi                                                                                     
File not found

Answer 6

The most error prone I could come up with was:

• First, get the value. Suppose you do something like:

RR=$?

Now, for not only this situation, but others you may face, consider:

defined variable:

$ AA=1 ; if (( "10#0${AA}" == 1 )) ; then echo yes ; else echo no ; fi

Answer: yes

$ AA=1 ; if (( "10#0${AA}" != 1 )) ; then echo yes ; else echo no ; fi

Answer: no

undefined variable:

$ AA=1 ; if (( "10#0${BB}" == 1 )) ; then echo yes ; else echo no ; fi

Answer: no

$ AA=1 ; if (( "10#0${BB}" != 1 )) ; then echo yes ; else echo no ; fi

Answer: yes

$ AA=1 ; if (( "10#0${BB}" == 0 )) ; then echo yes ; else echo no ; fi

Answer: yes

This prevents all kinds off errors.

You are probably aware of all the syntax, but here some tips:

• Use quotes. Avoid a "blank" to be nothing.
• The new modern notation for variables is ${variable}.
• Adding a zero concatenated before your number also avoid "no number at all".
• But wait, adding a zero makes the number become base-8. You will get an error like:
  • value too great for base (error token is "08") for numbers above 7. That is when 10# comes into play:
  • 10# forces the number to be base-10.

Answer 7

You can do this:

if ($( ping 4.4.4.4 -c1 > /dev/null )) ; then
  echo "ping response succsess!!!"
fi

Answer 8

#!/bin/bash

if command-1 ; then
   echo "command-1 succeeded and now running from this block command-2"
   command-2
else
   echo "command-1 failed and now running from this block command-3"
   command-3
fi

Answer 9

As noted elsewhere in this thread, the original question basically answers itself. Here is an illustration showing that if conditions may also be nested.

This example uses if to check if a file exists and if it is a regular file. If those conditions are true, then check whether or not it has a size greater than 0.

#!/bin/bash

echo "Which error log are you checking today? "
read answer

if [ -f /opt/logs/$answer*.errors ]
    then
        if [ -s /opt/logs/$answer*.errors ]
            then
                echo "Content is present in the $answer error log file."
            else
                echo "No errors are present in the $answer error log file."
        fi
    else
        echo "$answer does not have an error log at this time."
fi

Answer 10

This could be done simply in this way as $? gives you the status of last command executed.

So it could be

#!/bin/sh

... some command ...

if [ $? == 0 ] ; then
  echo '<the output message you want to display>'
else 
  echo '<failure message>'
fi