Fish (Unix shell)

fish is a Unix shell with a focus on interactivity and usability. Fish is designed to give the user features by default, rather than by configuration. Fish is considered an exotic shell since it does not rigorously adhere to POSIX shell standards, at the discretion of the maintainers.
Highlights.
Fish has "search as you type" automatic suggestions based on history and current directory.
This is essentially like Bash's history search, but because it is always on instead of being a separate mode, the user gets continuous feedback while writing the command line, and can select suggestions with the arrow keys, or as in Bash, press for a tab completion instead. Tab-completion is feature-rich, expanding file paths (with wildcards and brace expansion), variables, and many command specific completions. Command-specific completions, including options with descriptions, can to some extent be generated from the commands' manpages.
Fish has few syntactic rules, preferring features as commands rather than syntax. This makes features discoverable in terms of commands with options and help texts. Functions can also carry a human readable description. A special "help" command gives access to all the fish documentation in the user's web browser.
Syntax.
The syntax resembles a POSIX compatible shell (such as Bash), but deviates in important ways where the creators believe the POSIX shell was badly designed.
> set foo bar
> echo $foo
bar
> set wd (pwd)
> echo $wd
> set A 3 5 7 9 12
> set B $A[1 2]
> echo $B
3 5
> echo $A[(seq 3)]
3 5 7
> set --erase A[$B]
> echo $A
3 5 9
> for i in *.jpg
 convert $i (basename $i .jpg).png
 end
> for i in *.jpg; convert $i (basename $i .jpg).png; end
> while read line
 set arr (echo $line|tr : \n)
 echo $arr[5]
 end < /etc/passwd
No implicit subshell.
Some language constructs, like pipelines, functions and loops, have been implemented using so called subshells in other shell languages. Subshells are simply child programs that run a few commands for the shell and then exit. Unfortunately, this implementation detail typically has the side effect that any state changes made in the subshell, such as variable assignments, do not propagate to the main shell, which may surprise the user. Fish never forks off so-called subshells; all builtins are always fully functional.
> cat *.txt | read line
Variable assignment example.
This Bash example does not do what it looks like: because the loop body is a subshell, the update to codice_1 is not persistent.
found="
cat /etc/fstab | while read dev mnt rest; do
 if test "$mnt" = "/"; then
 found="$dev"
 fi
done
Workaround:
found="
while read dev mnt rest; do
 if test "$mnt" = "/"; then
 found="$dev"
 fi
done < /etc/fstab
Fish does not need a workaround:
set found "
cat /etc/fstab | while read dev mnt rest
 if test "$mnt" = "/"
 set found $dev
 end
end
Helpful error messages.
Error messages in fish are designed to actually tell the user what went wrong and what can be done about it.
> foo=bar
fish: Unknown command “foo=bar”. Did you mean “set VARIABLE VALUE”?
For information on setting variable values, see the help section on
the set command by typing “help set”.
> echo ${foo}bar
fish: Did you mean {$VARIABLE}? The '$' character begins a variable
name. A bracket, which directly followed a '$', is not allowed as a
part of a variable name, and variable names may not be zero characters
long. To learn more about variable expansion in fish, type “help
expand-variable”.
> echo $(pwd)
fish: Did you mean (COMMAND)? In fish, the '$' character is only used
for accessing variables. To learn more about command substitution in
fish, type “help expand-command-substitution”.
Universal variables.
Fish has a feature known as universal variables, which allow a user to permanently assign a value to a variable across all the user's running fish shells. The variable value is remembered across logouts and reboots, and updates are immediately propagated to all running shells.
> set --universal EDITOR emacs
> set --universal fish_color_cwd blue
Other features.
Version 2 adds: