Tom McKeesick

20220511 Bash Practices

1. Halting execution after an error

Note: echo $? gives you the exit status of the previous command. 0 == success, 1 == failure.

set -euxo pipefail

Normal behaviour

 ☯ cat test.sh
ls 1
echo "finished"

 ☯ bash test.sh ; echo $?
ls: cannot access '1': No such file or directory
finished
0

Using set & pipefail

 ☯ cat test.sh
set -euxo pipefail
ls 1
echo "finished"

 ☯ ~/d/tmck-code.github.io bash test.sh ; echo $?
+ ls 1
ls: cannot access '1': No such file or directory

2. Checking variables

 ☯ ~ cat test.sh
set -euxo pipefail
echo "${1}"
echo "finished"

 ☯ ~ bash test.sh ; echo $?
test.sh: line 2: 1: unbound variable
1

To get more control over unbound variables, we can give the variable a default empty value, and then do an explicit check to see if the variable is empty

 ☯ ~ cat test.sh
set -euxo pipefail
[ -z "${1:-}" ] && echo "\$1 must be given!" && exit 1
echo "finished"

 ☯ ~ bash test.sh ; echo $?
+ '[' -z '' ']'
+ echo '$1 must be given!'
$1 must be given!
+ exit 1
1

3. Without printing

You might want to suppress the debug output, for brevity or to protect secret credentials from being logged.

You can either

  1. omit the x character
  2. or toggle with
    1. set -x to enable debug printing
    2. set +x to disable debug printing
 ☯ ~ cat test.sh
set -euxo pipefail
set +x
echo "without debug"
set -x

[ -z "${1:-}" ] && echo "\$1 must be given!" && exit 1
echo "finished"

 ☯ ~ bash test.sh ; echo $?
+ set +x
without debug
+ '[' -z '' ']'
+ echo '$1 must be given!'
$1 must be given!
+ exit 1
1
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