Shell

• Break long lines on |, &&, or || and indent the continuations.
• Don't add an extension to executable shell scripts.
• Don't put a line break before then or do, use if ...; then and while ...; do.
• Use for x; do, not for x in "$@"; do.
• Use snake_case for variable names and ALLCAPS for environment variables.
• Use single quotes for strings that don't contain escapes or variables.
• Use two-space indentation.
• Don't parse the output of ls. Understand why you shouldn't and available alternatives.
• Don't use cat to provide a file on stdin to a process that accepts file arguments itself.
• Don't use echo with options, escapes, or variables (use printf for those cases).
• Don't use a /bin/sh shebang unless you plan to test and run your script on at least: Actual Sh, Dash in POSIX-compatible mode (as it will be run on Debian), and Bash in POSIX-compatible mode (as it will be run on macOS).
• Don't use any non-POSIX features when using a /bin/sh shebang.
• If calling cd, have code to handle a failure to change directories.
• If calling rm with a variable, ensure the variable is not empty.
• Prefer "$@" over "$*" unless you know exactly what you're doing.
• Prefer awk '/re/ { ... }' to grep re | awk '{ ... }'.
• Prefer find -exec {} + to find -print0 | xargs -0.
• Prefer for loops over while read loops.
• Prefer grep -c to grep | wc -l.
• Prefer mktemp over using $$ to "uniquely" name a temporary file.
• Prefer sed '/re/!d; s//.../' to grep re | sed 's/re/.../'.
• Prefer sed 'cmd; cmd' to sed -e 'cmd' -e 'cmd'.
• Prefer checking exit statuses over output in if statements (if grep -q ...;, not if [ -n "$(grep ...)" ];).
• Prefer reading environment variables over process output ($TTY not $(tty), $PWD not $(pwd), etc).
• Use $( ... ), not backticks for capturing command output.
• Use $(( ... )), not expr for executing arithmetic expressions.
• Use 1 and 0, not true and false to represent boolean variables.
• Use find -print0 | xargs -0, not find | xargs.
• Use quotes around every "$variable" and "$( ... )" expression unless you want them to be word-split and/or interpreted as globs.
• Use the local keyword with function-scoped variables.
• Identify common problems with shellcheck.