shell

Shell scripts are the essence of unix, they chain programs together treating them as a black box that takes input and produces output. Languages don't matter, as long as it supports stdin, stdout. This is an overview of the shell language. Not bash, zsh, or anything fancy. Just your regular old cross-platform sh.

variable expansion

${variable?word}    # complain if undefined
${variable-word}    # use new value if undefined
${variable+word}    # opposite of the above
${variable=word}    # use new value if undefined, and redefine
${variable:?word}   # complain if undefined or null
${variable:-word}   # use new value if undefined or null
${variable:+word}   # opposite of the above
${variable:=word}   # use new value if undefined or null, and redefine

With variable expansion the result does get evaluated immediately though, so in order to prevent that the statement must be preceded by a : (null) character:

: ${foobar:-hello}
echo "$foobar"

if-else statement

The conditional between brackets is checked for an exit code (e.g. only 0 passes the test) and then it proceeds down the logic tree. In shell everything is a string. To do equality checks there's the common = which does a string (lexical) check, and there's -eq which performs a numeric check. -eq and -ne are useful to operate based on exit codes.

if [ 'hello' = 'world' ]; then
  echo hello
else
  echo nah
fi

Or if you're used to C-syntax languages, there's the more familiar looking version:

# one line
[ "$1" -eq 0 ] && { echo hi && exit 1; }

# multi-line
[ ! "$?" -eq 0 ] && {
  echo hi
  exit 1
}

{} creates a non-subshell grouping; () creates a subshell grouping which will not exit the program if exit 1 is provided.

loop

Looping over ls output can cause trouble for filenames that contain spaces, globs or other odd characters. Instead it's safer to use a glob. In shell loops also run once if there's no match, so an extra line is needed to break the loop if no matches are found.

# loop over files in directory
for file in *.wav; do
  [ -e $file ] || break  # handle the case of no *.wav files
  echo "$file"
done
# loop over command output separated by newlines
cat <filename> | while IFS= read -r my_var; do
  echo "$s"
done

command line switches

getopt(1) is the way to handle CLI flag switches in shell. It's built into most, if not all shell distributions. It uses getopt(3) under the hood.

getopt solves the problem of flag parsing; e.g. the following are considered equivalent after being parsed by getopt:

$ cmd -aoarg file file
$ cmd -a -o arg file file
$ cmd -oarg -a file file
$ cmd -a -oarg -- file file

getopt takes argument flags as the first argument (more on colons later) and the list of arguments to apply the flags on as the second argument. It's generally recommended to pass the special variable $* as the second argument.

$ getopt <flags> <argument-list>
$ getopt fvdo:i:: $*

defining flags to parse

Using getopt requires some trickery. Arguments can be anything: from special characters (such as globs) to filenames with spaces. To make sure everything is parsed correctly and errors are handled there's a bit of boilerplate we must use:

args="$(getopt abo: $*)"
if [ $? != 0 ]; then
  echo 'Usage: ...'
  exit 2
fi
eval set -- "$args"

In the snippet above there's quite a lot going on. To check if getopt is able to parse all arguments, the exit code of the variable assignment is checked. Assigning the output to eval set directly would swallow the exit code so we must assign it to a variable before that.

After running getopt, we must redefine our arguments. set -- does this. In order to correctly parse whitespaces in arguments we must run it through eval, resulting in eval set -- to redefine our arguments.

flag types & arguments

getopt uses delimiters to signal if arguments are optional or not, and what type of arguments they take. Using the example of "option":

  • o boolean -o flag
  • o: -o takes a required argument in the form
  • o:: -o takes an optional argument

long flags

GNU getopt has support for --flag style flags (long flags), while simple getopt does not. In extended setopt long options are passed with --long while short options are passed with --option. Regular setopt has no flags and just parses short options.

This snippet detects which version is available and allows setting of the appropriate flags:

getopt -T > /dev/null
if [ $? -eq 4 ]; then
  # GNU enhanced getopt is available
  eval set -- "$(getopt --long help,output:,version --options ho:v -- "[email protected]")"
else
  # Original getopt is available
  eval set -- "$(getopt ho:v "[email protected]")"
fi

parsing arguments

Once the argument flags have been provided the actual arguments must be parsed. There are several options:

  • boolean switch
  • switch with an argument e.g. do var=$2; shift 2 ;;
  • -- delimiter, shift and signal a break to parse remaining args
  • none of the above, which means break
while true; do
  case "$1" in
    -v | --verbose ) VERBOSE=true; shift ;;
    -d | --debug ) DEBUG=true; shift ;;
    -m | --memory ) MEMORY="$2"; shift 2 ;;
    --debugfile ) DEBUGFILE="$2"; shift 2 ;;
    -- ) shift; break ;;
    * ) break ;;
  esac
done

all together now

# set CLI flags
getopt -T > /dev/null
if [ "$?" -eq 4 ]; then args="$(getopt --long help --options h -- "$*")"
else args="$(getopt h "$*")"; fi
[ ! $? -eq 0 ] && { usage && exit 2; }
eval set -- "$args"

# parse CLI flags
while true; do
  case "$1" in
    -h|--help) usage && exit 1 ;;
    -- ) shift; break ;;
    * ) break ;;
  esac
done

# assert argv count
[ "$#" != 0 ] && { usage && exit 1; }

parallel

fn1 () {
  sleep 3
  echo 'cmd1 done'
}

fn2 () {
  sleep 2
  echo 'cmd2 done'
}

(fn1 & fn2) | cat
echo 'all done'

test if command is available

#!/bin/sh
if [ "$(uname)" = "Darwin" ]; then
  which gmktemp > /dev/null || exit 1
  alias mktemp="gmktemp"   # typical action on OS X for Linux compat
fi

special variables

read from stdin

while read line; do
  echo "$line"
done

recursively read a directory

find ./templates | while read file; do
  echo "$file"
done

printf

There exist different flavors of echo, where the two main versions conflict with each other. printf is the successor to echo and is far more powerful.

To provide syntax highlighting in most editors it's preferable to use "" over '' despite character expansion not being necessary as it's handled by printf.

$ printf "hello world"                 # echo 'hello world'
$ printf "%s %s" "$var1" "$var2"       # echo contents from var1 and var2
$ printf "%b" "\x1b[1;32mhi\x1b[0m""   # echo 'hi' in green

requiring files

Sometimes a program has multiple commands, and it makes sense to split it into separate files. When a file is symlinked the paths should continue to link to the correct files. There's 1 easy trick to achieve this:

$ dirname=$(dirname "$(readlink -f "$0")")
$ cat "$dirname"/foo/bar.txt

named pipes / inter-shell communication

Named pipes are cool to create background processes with that can be addressed by name to do stuff with. It creates a physical file on the system that can be used from any shell process. The code below passes the output of pipe to cat, which then writes to out.txt. When passing in a command to pipe (ls -la in this case), it pops back out at the other side.

$ mkfifo pipe
$ cat < pipe > output.ext
$ ls -la > pipe

detect if script is executed by sudo

sudo is user 0 on the system, so we can check for that:

if [ "$(id -u)" -ne 0 ]; then
  printf "shocker(1) should be executed as sudo\n"
  exit 1
fi

readonly

Variables can be made immutable-ish by using readonly:

readonly foo='bar'

math

either bc or dc work; but $(()) seems to work on dash too

$ echo $((1 + 1))

check if variable is not set

if [ -z "$my_var" ]; then
  printf "var not set\n"
fi

find where a command is located

$ type nginx

Get unix epoch time

Time since unix epoch, unix epoch, epoch time

$ date +'%s'

Prompt

Prompt for input, create a shell CLI:

printf 'Would you like to install? (Y/n)\n'
read -r x
if [ "$x" = "y" ];then
  ...
elif [ "$x" = "" ]; then
  ...
fi

And from inside a function:

choose () {
  printf 'What kind of project do you want to create? ' > /dev/tty
  printf '(base|node|rust)\n' > /dev/tty
  printf '❯ ' > /dev/tty
  read -r project
  echo "$project"
}

See Also

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