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README

Demonstrate how to use shell options in shell scripts.

Bash

Bourne Again Shell.

List options

Set or unset values of shell options and positional parameters.

Flag Description
allexport Automatically exports all variables created or modified.
emacs Enables an Emacs-style command line editing interface.
errexit Exits the shell immediately if a command exits with a non-zero status.
errtrace The ERR trap is inherited by shell functions.
functrace The DEBUG and RETURN traps are inherited by shell functions.
histexpand Enables history expansion, allowing re-use of commands from history.
history Saves commands in the command history.
ignoreeof Prevents exiting the shell using EOF (End Of File) characters like Ctrl+D.
keyword (Non-standard option, availability varies.)
monitor Enables job control, running processes in the background.
noclobber Prevents overwriting of existing files by redirection (e.g., >).
noexec Reads and checks commands without executing them.
nolog (Non-standard option, availability varies.)
notify Provides immediate job notification for background jobs.
onecmd Runs only one command and then exits.
physical Avoids resolving symbolic links when performing commands like cd.
posix Enables POSIX compliance mode.
privileged Enables privileged mode.
verbose Prints shell input lines as they are read.
vi Enables a Vi-style command line editing interface.
xtrace Prints commands and their arguments as they are executed.
braceexpand Enables brace expansion, allowing for generating arbitrary strings.
hashall Remembers the full pathname of commands to avoid searching the PATH each time.
interactive-comments Enables the use of comments (#) in interactive shells.
noglob Disables filename expansion (globbing).
nounset Treats unset variables as an error when performing parameter expansion.
pipefail Ensures the exit status of a pipeline is the status of the last command to exit with a non-zero status, or zero if all commands exit successfully. 
# show bash options
bash

# help for set
help set
man bash

set -o
shopt | grep "on$"
# print out current options
echo $-

Auto-exporting variables

Read an env file and export each entry.

# -a  Mark variables which are modified or created for export.
set -a
. ./.env.template
set +a
env

No clobbering of files

# noclobber is same as -C
# -C  If set, disallow existing regular files to be overwritten
#           by redirection of output.
set +o noclobber
set -o 
set -o noclobber

Debugging

Use the following shebang for debugging

#!/bin/bash -x
# -x  Print commands and their arguments as they are executed.

Pipefail and scripts

There are a common set of options that scripts set to prevent hard to understand errors.
However, not everyone is convinced with -e option as mentioned here in Why doesn't set -e (or set -o errexit, or trap ERR) do what I expected?.

# -e  Exit immediately if a command exits with a non-zero status.
# -u  Treat unset variables as an error when substituting.
# -f  Disable file name generation (globbing).
# -o option-name
# pipefail     the return value of a pipeline is the status of
#              the last command to exit with a non-zero status,
#              or zero if no command exited with a non-zero status
set -euf -o pipefail

Pushing and popping options

NOTE: The reason we have to save both $- and set +o is because the set +o is run as a subprocess and not all options are inherited.

# boilerplate saving and restoring options
local saveOptions=$-
local original_state=$(set +o | sort)
printf "$saveOptions\n"
printf "$original_state\n"

# override flags on and off
set +e
set -x
this_should_fail "test_error"
echo "exitcode: $?"

# restore the original shell options
eval "$original_state"
set -$saveOptions

Tests to show how it works.

# show options and follow examples
./options_test.sh --help 

# pushing and popping options
./options_test.sh --debug --skip_error --error  
./options_test.sh --skip_error --error

ZSH

Zee Shell.

# show zsh options
zsh 

man zsh

# list options 
set -o | sort 
# list options as "set" commands 
set +o | sort 
setopt  
setopt extendedglob
ls ^d*.txt
unsetopt extendedglob

Resources

  • The Set Builtin here
  • Clobbering here
  • How can I list Bash's options for the current shell? here
  • How to restore the value of shell options like set -x? here
  • Why doesn't set -e (or set -o errexit, or trap ERR) do what I expected? here