Nuances in Shell Scripting

Shell’s just another programming language.

Variables

• Variables are denoted by $
• foo = bar fails since spaces denote arguments
• echo '$foo' outputs $foo
• echo "$foo" outputs bar

Reserved Variables

• Reserve variables include, but are not limited to:
  • $0: name of the script
  • $1 to $9: arguments passed to the script
• $#: number of arguments passed to the script
• $@: all arguments passed to the script
• $?: exit status of the last command
• $_: last argument of the previous command; can also be accessed by Esc + . or alt + .
• $$: process ID (pid) of the current script
• !!: rerun the entire last command

Command and Process Substitutions

• Command/process substitutions are like f-strings in Python
• Difference between command and process substitution is that the latter will place the output in a temporary file
• Command substitution: $(command)
• Process substitution: <(command)

Comparisons and Brace Expansions

• When doing comparisons in bash, use [[ ]] rather than [ ]; apparently, chances of mistakes are lower this way
• Use {} for common substrings in commands
  • convert image.{png,jpg}
  • cp /path/to/project/{foo,bar,baz}.sh /newpath
  • mv *{.py,.sh} folder
  • touch {foo,bar}/{a..h}

Functions vs Scripts

• Functions are executed in the current environment, whereas scripts aren’t
• Thus, functions can modify environment variables whereas scripts can’t
• Scripts can only access environment variables that are exported with export