Linux Bash Script Error Handling

Question

I use bash scripts in Linux to Migrate Database output files and I was wondering of a way to handle errors when executing a Linux command in a bash script.

For example normally when I want to loop through files in a directory I will write it like this

# list files and grep results for .sql extension
for FILE in `ls | grep ".sql"`
do
    echo "found file: $FILE"
done

Which works perfectly because grep returns the file name if it has a .sql extension or returns nothing

I was wondering how to use a Linux command that returns a result or an error such as

ls ./*.sql

which returns the name of the file but if it doesn't find a file it returns the error

ls: ./*.sql: No such file or directory

Is it possible to check if a Linux command is returning an error in a bash script?

get out of the habit of using ALL_CAPS_VARS: some day you'll use PATH and then wonder why your script is broken. — glenn jackman
– glenn jackman, Commented Mar 20, 2015 at 15:11
Normally i name my variables LV_STR or LV_FILE, im not thaaat stupid to name a var PATH — TheLovelySausage
– TheLovelySausage, Commented Mar 20, 2015 at 20:15

anubhava · Accepted Answer · 2015-03-20 14:12:24Z

2

First of all you don't really need ls command here. You can just do:

# list files and grep results for .sql extension
for file in *.sql; do
    echo "found file: $file"
done

Then to avoid getting single iteration if *.sql files don't exist use:

shopt -s nullglob
# list files and grep results for .sql extension
for file in *.sql; do
    echo "found file: $file"
done

In general you can check $? after running any command to check for status of most recently executed command.

answered Mar 20, 2015 at 14:12

anubhava

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speakr · Accepted Answer · 2015-03-20 14:17:46Z

2

You can check for errors with using && and ||:

ls ./*.sql && echo "some files exist" || echo "no such files exist"

You can get the error code of the last run program with checking $?:

ls ./*.sql ; echo $?

edited Mar 20, 2015 at 14:17

answered Mar 20, 2015 at 14:11

speakr

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ILostMySpoon · Accepted Answer · 2015-03-20 14:16:47Z

1

You can check the exit code of the last run command with the $? variable in bash. Conventionally, 0 is returned for successful executions and 1 or higher for failed executions. As an example:

$ touch /root/test 
touch: cannot touch ‘/root/test’: Permission denied
$ echo $?
1

You can incorporate this in your bash scripts like so:

#!/bin/bash

touch /root/test

if [ $? -eq 0 ]
then
  echo "Successfully created file"
else
  echo "Could not create file"
fi

answered Mar 20, 2015 at 14:16

ILostMySpoon

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