fish shell quickstart for converting bash scripts

After some years of bash and PowerShell, and some hours of using fish, I've realised that expansion & predictive typeahead are good features in a shell, whereas “be a great programming language” is less important than I thought: because there is no need to write scripts in the language of your shell.

Fish has slicker typeahead and expansions than bash or even PowerShell. But to switch to a fish shell, you do still have to convert your profile & start-up scripts. So here's my quick-start guide for converting bash to fish.

• Do this first: at the fish prompt type help. Behold! the fish documentation in your browser is much easier to search than man pages are.
• Calmly accept that fish uses set var value instead of var=value. Roll your eyes if it helps.
• Use end everywhere that bash has fi, done, esac, braces {} etc. e.g. function definition is done with function ... end. The keywords do and then are redundant everywhere, just remove them. else has a semicolon after it. case requires a leading switch(expr).
• There is no [[ condition ]] but [ ... ] or test ... work. Type help test to see all the file and numeric tests you expect, such as if [ -f filename ] etc. string and regex conditionals are done with the string match command (see below). You can replace [[ -f this && -z that || -z other ]] with [ -f this -a -z that -o -z other ] but see below for how fish can also replace || and && constructions with or and and statements.
• But first! type help string to see the marvels of proper built-in string commands.
• Replace function parameters $*, $1, $2 etc with $argv, $argv[1], $argv[2] etc. If that makes you scowl, then type help argparse. See! That's much better than kludging about in bash.
• Remove the $ from $(subcommand) leaving just (subcommand). Inside quotes, take the subcommand outside the quote: "Today is $(date)" becomes "Today is "(date). (Recall that quotes in bash & fish don't work at all like quotes in most programming languages. Quote marks are not token delimiters and a"bc"d is a valid single token and is parsed identically to each of abcd , "abcd", abc'd').
• Replace heredocs with multi-line literal strings and standard piping syntax. However, note that if you pipe or read to a variable, the default multiline behaviour is to split on newline and generate an array. Defeat this by piping through string split0 – see https://fishshell.com/docs/current/index.html#command-substitution

Search-and-replace Script Snippets

Here is my hit-list of things to search and replace to convert a bash shell to fish. These resolved almost all of my issues in converting a few hundred lines of bash script to fish.

From	To	Notes
var=value	set var value
export var=value	set -x var value
export -f functionname	redundant.	Just remove it
alias abbr='commandstring'	(no change)	alias syntax is accepted as an abbreviation for a function definition since fish 3
command $(subshell commmand) command `subshell commmand`	command (subshell command) OR command (subshell commmand | string split0)	Just remove the $ but keep the () See below for when you want to add string split0
command "$(subshell commmand)"	command (subshell command)	Remove both the $ and the quotes ""to make this work
if [[ condition ]] ; then this ; else that ; fi	if [ condition ] ; this ; else ; that ; end	See below for more on Fish's multine and and or syntax.
if [[ number != number ]] ; then this ; else that ; fi	if [ number -ne number ] ; this ; else ; that ; end	See below for more on Fish's multine and and or syntax.
while condition ; do something ; done	while condition ; something ; end
$*	$argv
$1, $2	$argv[1], $argv[2]	But see help argparse
if [[ testthis =~ substring ]]	if string match -q '*substring*' testthis	string match without -r does glob style testing
if [[ testthis =~ regexpattern ]]	if string match -rq regexpattern testthis	string match with -r does regex testing
[ guardcondition ] && command [ guardcondition ] || command	works as is	But see or and and below for when it's more complex
var=${this:-$that}	if set -q this ; set var $this ; else ; set var $that ; end
cat > outfile <<< "heredoc" cat > outfile <<< "multiline … heredoc"	echo "multiline … heredoc" | cat > outfile	no heredocs, but multiline strings are fine NB printf is better than echo for anything complicated, in any shell.
if [[ -z $this && $that=~$pattern ]]	if [ -z $this ] ; and string match -rq $pattern $that ;
content=$(curl $url)	set content (curl $url | string split0)	without the pipe to string split0, content will be split on newlines to an array of lines.

Fish's multine and and or syntax

Fish has a multiline and and or syntax that may be clearer than && and || in both conditionals and guarded commands. It is less terse.

[ condition ]

and do this

or do that

That said, && and || are still valid in commands :

[ condition ] && do this || do that

Other gotchas

• You may have to read up on how fish does parameter expansion, and especially handling spaces, differently to bash.
• Pipe & subcommand output to multiline strings or arrays: set x (cat myfile.txt) will set x to an array of the lines of myfile.txt. To keep x as a single multine string, use string split0 : set x (cat myfile.txt | string split0)

Official tips for new fishers:

See the FAQ at https://fishshell.com/docs/3.0/faq.html