01-Exit Statuses and Return Codes.md

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Exit Statuses and Return Codes

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Exit Status / Return Code

• every command returns an exit status
• exit status aka return code
• the codes range from 0 to 255
• 0 = the command ran successfully
• anything other than 0 is an error or warning
• these codes can be used for error checking
• you can use man or info to find the meaning of an exit status
• ex:

man ping

• ex: grep will return 1 if no search pattern is found
• $? contains the return code of the previously executed command

ls /not/here/
echo "$?"
# it will output 2

• example of using an error code:

#!/bin/bash

HOST="google.com"

ping -c 1 $HOST

if [ "$?" -eq "0" ]
then
  echo "$HOST reachable."
else
  echo "$HOST unreachable."
fi

• another example - only checking for an error:

#!/bin/bash

HOST="google.com"

ping -c 1 $HOST

if [ "$?" -ne "0" ]
then
  echo "$HOST unreachable."
fi

• another example - assign the return code to a variable:

#!/bin/bash

HOST="google.com"

ping -c 1 $HOST
RETURN_CODE=$?

if [ "$RETURN_CODE" -ne "0" ]
then
  echo "$HOST unreachable."
fi

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Using && or || with exit statuses

• you can also use && or || with return codes
• && = and

# both will run
# the second command will only run if the first command succeeds 
mkdir /tmp/bak && cp test.txt /tmp/bak

• || = or

# if the first command suceeds then the second command will not run
# if the first command fails then the second command will be run
cp test.txt /tmp/bak/ || cp test.txt /tmp

and &&

• example of using &&

#!/bin/bash
HOST="google.com"
ping -c 1 $HOST && echo "$HOST reachable."

or ||

• example of using ||

#!/bin/bash
HOST="google.com"
ping -c 1 $HOST || echo "$HOST unreachable."

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Chaining multible commands together on a single line with a semicolon

• you can chain several seperate commands together with a semicolon to ensure they all get executed
• the semi colon does not check the exit status of the previous command

cp test.txt /tmp/bak/ ; cp test.txt /tmp

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Creating an exit status for your shell script

• your shell script returns an exit status when its run
• you can control the exit status in the script
• by default the exit code will be whatever code the last command returns
• to control the exit status use the exit command
• example:

# end of script
exit 0

• whenever the exit code is reached the shell script will stop running
• example of using an exit code in a script:

#!/bin/bash
HOST="google.com"
ping -c 1 $HOST

if [ "$?" -ne "0" ]
then
  echo "$HOST unreachable."
  exit 1
fi

exit 0

• if another script is calling your script the exit codes can be useful tools