cli.md

class: center, middle, title title: CLI-fu

Commandline-fu

---

Execution model

• Programs
  • Input
    • One file
    • Two arrays of strings
  • Output
    • Two files
    • An integer

---

Files

• In system calls, files are just integers
• STDIN (file descriptor 0)
• STDOUT (file descriptor 1)
• STDERR (file descriptor 2)

// writes to stdout
write(1, "Hello, world!\n", 14);

# writes to stdout
puts "Hello, world!"

---

Two arrays of strings - #1: ARGV

• ARGV (vector of arguments)
• Name of program is the first argument

int main(int argc, char **argv) {
  int i;
  for (i=0; i<argc; i++) {
    printf("[%d]=%s\n", i, argv[i]);
  }
}

puts "[0]=#{$0}"
ARGV.each_with_index do |a,i|
  puts "[#{i}]=#{a}"
end

---

Two arrays of strings - #2: ENV

• Treated as set of key-value pairs
• List of strings split on first "=" character

int main(int argc, char **argv, char **env) { // not portable
  int i;
  for (i=0; env[i]; i++) {
    printf("ENV[%d]: %s\n", i, env[i]);
  }
}

---

Output an integer

• "Exit status", "Error code"
• 0 = everything OK
• not 0 (often 1) = something went wrong

int main(void) {
  return 1;
}

$ true ; echo $?
# => 0
$ false ; echo $?
# => 1

---

How shells work

• Find the command(s) to run
• Wire up standard files
• Execute the pipeline
• Do control flow based on exit codes

$ /bin/echo pizza | grep z
# /bin/echo = fully qualified, no need to search
# grep = check each \`:`-separated entry in \`$PATH`
# open pipe = set of two files (UNIX sockets)
# run /bin/echo with fd1 = write end of pipe
# run /bin/grep with fd0 = read end of pipe

---

Portable shell has very few constructs

if # condition
then # command
elif # condition
then # command
else # command
fi

case ... in
  pattern)
    # command
    ;;
esac

while # condition
do # command
done

for x in ...
do # command
done

---

Surprise! "condition" = command

if # command
then # command
elif # command
then # command
else # command
fi

case ... in
  pattern)
    # command
    ;;
esac

while # command
do # command
done

for x in ...
do # command
done

---

Double surprise! "[" is a command

$ which [
# => /bin/[

$ if [ -z "$var" ]
# if $var is empty:
# runs ["[", "-z", "", "]"]

• Use [[ ... ]] if your shell supports it
• POSIX allows [[ and ]] to have different semantics
• bash and zsh both do this
• More resilient (syntax-enhanced)

$ var="directory with spaces"
$ [ -d $var ]
# dash: 2: [: directory: unexpected operator
# bash: [: too many arguments
## works fine in zsh... where arg splitting is different

---

Shell logic and variables

• Can set variables
• ENV variables are inherited
• export creates new ENV vars
• && and || (&& binds tightly)

---

Quoting

• Backslash = escape character

• Single quotes retain everything

• Double quotes retain everything except:

  • $ => parameter expansion
  • ` => command substitution
  • Backslash only escapes:

    $ ` " \ <newline>
    

• Always need to be quoted:

  | & ; < > ( ) $ ` \ " ' <space> <tab> <newline>
  

• Sometimes need to be quoted:

   * ? [ # ~ = %
  

• HEREDOCs

---

Commands

• awk
• sed
• xargs
• jq