Posts Tagged ‘fish’

Friendly Interactive Shell

Saturday, May 8th, 2010

fish (Friendly interactive shell) is a UNIX command shell. Its name is an acronym for friendly interactive shell.

fish focuses on interactive use, discoverability, and user friendliness. The design goal of fish is to give the user a rich set of powerful features in a way that is easy to discover, remember, and use. fish features a user-friendly and powerful tab completion, including descriptions of every completion, tab-completion of strings with wildcards, and many command specific completions. It also features an extensive and discoverable help system. A special help command gives access to all the fish documentation in your preferred web browser.

The fish syntax is slightly different from other shell script languages. These changes were made to make the language more powerful as well as to make the language small and easy to learn. One obvious difference between fish and other command line interpreters like bash is that the contents of a variable is not subject to token separation, meaning that there is rarely a need to enclose variable dereferences within quotes.

# Variable assignment, set the variable ‘foo’ to the value ‘bar’.
# Don’t use the = operator, since it is inherently whitespace sensitive.
# Also, the set command can easily be extended to work with arrays, scoping, etc.
> set foo bar
> echo $foo

# Command substitution, assign the output of the command ‘pwd’ into the variable ‘wd’.
# Don’t use “ since they can’t be nested and look too much like like ‘ ‘.
# Don’t use $() since $ is only used for variable expansion in fish.
> set wd (pwd)
> echo $wd

# Array variables. ‘A’ becomes an array with the values 3, 5, 7, 9, 12
> set A 3 5 7 9 12
> echo $A[(seq 3)]
3 5 7
# Array slicing. ‘B’ becomes the first and second element of ‘A’
> set B $A[1 2]
> echo $B
3 5
# Erase the third and fifth elements of ‘A’
> set -e A[$B]; echo $A
3 5 9

# for-loop, convert jpegs to pngs
> for i in *.jpg; convert $i (basename $i .jpg).png; end

# while-loop, read lines /etc/passwd and output the fifth
# colon-separated field from the file. This should be
# the user description.
> cat /etc/passwd|while read line; set arr (echo $line|tr : \n); echo $arr; end
One important difference between fish and other shells is the lack of subshells. Many tasks like pipelines, functions and loops are implemented using so called subshells in other languages. Subshells are simply child programs that run a few commands for the shell and then exit. Unfortunately, changes made inside a subshell do not have any effect in the main shell, meaning that actions such as variable assignments and the use of many builtin functions do not work as expected. Fish never forks off so called subshells, so all builtins are always fully functional.

# This will not work in most other shells, since the ‘read’ builtin
# Will run in its own subshell. fish and zsh work as expected.
> cat *.txt | read line

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

> 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 instantly propagated to all running shells.

# This will make emacs the default text editor. The ‘-U’ tells fish to
# make this a universal variable.
> set -U EDITOR vi

# This command will make the current working directory part of the fish
# prompt turn blue on all running fish instances.
> set -U fish_color_cwd blue

Other features
Advanced tab completion.
Syntax highlighting with extensive error checking.
Support for the X clipboard.
Smart terminal handling based on terminfo.
Searchable command history.