Linux Bash strict mode set commands

Links: - 104 Linux Index
- Linux - Bash strict mode

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Rules

set -e

• The set -e option instructs bash to immediately exit if any command has a non-zero exit status.
  • The obvious exception here are commands executed inside loop conditions, conditionals or as arguments to logic operators (||, &&).
• Example:

echo "hello worlds"
cd nonexistent
echo "after"

# Output:
# hello worlds
# ./test.sh: line 4: cd: nonexistent: No such file or directory
# after

set -e
echo "hello worlds"
cd nonexistent
echo "after"

# Output:
# hello worlds
# ./test.sh: line 4: cd: nonexistent: No such file or directory

• Notice that bash scripts stops at the point when we get a return code of 1 (cd nonexistent).

set -u

• set -u affects variables.
• When set, a reference to any variable you haven't previously defined - with the exceptions of $* and $@ - is an error, and causes the program to immediately exit.
  • Languages like Python, C, Java and more all behave the same way, for all sorts of good reasons. One is so typos don't create new variables without you realising it.

set -u

hello=10
echo "test script"
echo $Hello

# Output:
test script
./test.sh: line 5: Hello: unbound variable

• Without the -u option, this will be a silent error.
• But with the -u option, the script exits on that line with an exit code of 1.

set -o pipefail

• This setting prevents errors in a pipeline from being masked.
• If any command in a pipeline fails, that return code will be used as the return code of the whole pipeline.
• By default, the pipeline's return code is that of the last command - even if it succeeds.

ls -alh | cd nonexistent | echo "hello"
echo $?

# Output:
# ./test.sh: line 3: cd: nonexistent: No such file or directory
# hello
# 0

• Notice that even though cd nonexistent failed since the last command in the pipeline was successful the exit code of the whole pipe was 0.

set -o pipefail
ls -alh | cd nonexistent | echo "hello"
echo $?

# Output:
# ./test.sh: line 3: cd: nonexistent: No such file or directory
# hello
# 1

• Notice that now the exit code is 1 since cd nonexistent failed.
  • Still it won't quit the program.
  • We need set -e along with it to quit the program if we encounter any exit code other than 0.

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Last updated: 2022-12-19