Jang Rush

Posted on May 24, 2020 • Originally published at mmap.page on May 24, 2020

Dive into Fish

Introduction

Fish is the Friendly Interactive Shell.It also offers a relative clean syntax for shell scripting.(I myself find that rc has a more clean syntax.Fish’s tab completion is powerful.Though plan9 fans may say that we should implement tab completion interminal emulator.)

Help

help command or command --help.

Syntax overview

command [argument ...]

Commands and arguments are separated by the space character ( ).Every command ends with either a newline or a semicolon (;).

Quotes and escaping

Variable expansions take place in double quoted strings butnot in single quoted ones.

Escapes in single quotes: \’ and .

Escapes in double quotes: \”, \, and \$.

$ - ? * ~ # ( ) { } [] < > ^ & ; ' "

These characters and the space character have special meanings in fish.We need to escape them by prepending them with a backslash.We escape a backslach with another backslash ().

IO redirection and piping

standard input: <SOURCE_FILE
standard output: >DESTINATION
standard error: ^DESTINATION
standard output, appended: >>DESTINATION_FILE
standard error, appended: ^^DESTINATION_FILE

DESTINATION can be one of the following:

A filename.
An ampersand (&) followed by the number of another file descriptor.
An ampersand followed by a minus sign (&-). The file descriptorwill be closed.

For example, redirect both standard output and standard errorto the file all_output.txt:

echo Hello >all_output.txt ^&1

Any FD can be redirected in an arbitrary way by prefixing the redirection with the number of the FD:

N<DESTINATION
N>DESTINATION
N>>DESTINATION_FILE

Prepend the desired FD number and then outputredirect symbol to the pipe. For example:

make fish 2>| less

Jobs

command &: run command in background.
fg and bg: switch between foreground and background.
jobs: list jobs.

Functions and conditional structure

See help page for function, if, switch, and, or, for, while, begin, end, etc.

Notes on wrapper functions:

Don’t forget $argv.
Prefix the call to the command with the word ‘command’. Failing to do so will cause infinite recursion bugs.

Autosuggestions

fish suggests commands as you type, based on command history, completions, and valid file paths. As you type commands, you will see a completion offered after the cursor, in a muted gray color.

To accept the autosuggestion (replacing the command line contents), hit right arrow or Control-E.

Tab completion

Fish tab completion list features descriptions of the completions andit is scrollableby arrow keys, page up/page keys,the tab key or the space bar.Pressing any other key will exit the list and insertthe pressed key into the command line.

fish provides completions for:

Builtins, functions and regular programs.
Environment variable names.
Usernames in tilde expansion.
Filenames, even with wildcards.
Job id, job name and process names in process expansion.

fish provides a large number of program specific completions.

You can write your own builtin.See help complete and /usr/share/fish/completions.

Set variables

set name value1 value2 ...

Options:

--local
--global
--export
--universal

When calling a function, all current local variables temporarily disappear.

Special variables

BROWSER
CDPATH
fish_greeting
LANG, LC_ALL, LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC and LC_TIME.
fish_user_paths (An array of directories that are prepended to PATH. This can be a universal variable.)
PATH
umask
_, the name of the currently running comman.
argv
history
HOME
PWD
status
USER

Command line expansions

Wildcards:

?, *, and ** match non-hidden file names.

Use . as the first character to match hidden files.

Command substitution:

(command)

Variable expansion:

$variable
$array # --> multiple arguments
"$array" # --> one argument

Brace expansion:

{a,b,c}
{$USER}san # Neat, isn't it?

Home directory expansion:

~
~user

Process expansion:

%self # shell pid
%id # group id
%string
%user # pids of processes owned by user

Arrays:

$PATH[3] # indices start at 1
(seq 10)[2..5]
(seq 10)[2..5 1..3]
(seq 10)[-1..1] # Reverse output
$$foo[1] # the same as ${$foo[1]}

History

Press ‘up’ and ‘down’ to search commands containing the string entered.Press Alt-up and Alt-down, match against the token under the cursor.Press escape to abort.