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.

FromToNotes
var=valueset var value
export var=valueset -x var value
export -f functionnameredundant.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 ; fiif [ condition ] ; this ; else ; that ; endSee below for more on Fish’s multine and and or syntax.
if [[ number != number ]] ; then this ; else that ; fiif [ number -ne number ] ; this ; else ; that ; endSee below for more on Fish’s multine and and or syntax.
while condition ; do something ; donewhile condition ; something ; end
$*$argv
$1, $2$argv[1], $argv[2]But see help argparse
if [[ testthis =~ substring ]] if string match -q ‘*substring*’ testthisstring match without -r does glob style testing
if [[ testthis =~ regexpattern ]] if string match -rq regexpattern testthisstring match with -r does regex testing
[ guardcondition ] && command
[ guardcondition ] || command
works as isBut see or and and below for when it’s more complex
var=${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

Bash and PowerShell in a single script file

I’m not saying it’s all dotnet’s fault, but it was when deploying dotnetcore services to a linux VM that I thought, “what I really, really want is both bash and powershell setup scripts in a single file”. Surely a working incantation can be crafted from such arcane systems of quoting and escaping as the two languages offer?

½ an evening later :

# This file has a bash section followed by a powershell section,
# and a shared section at the end.
echo @'
' > /dev/null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Bash Start --------------------------------------------------

scriptdir="`dirname "${BASH_SOURCE[0]}"`";
echo BASH. Script is running from $scriptdir

# Bash End ----------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
echo > /dev/null <<"out-null" ###
'@ | out-null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Powershell Start --------------------------------------------

$scriptdir=$PSScriptRoot
"powershell. Script is running from $scriptdir"

# Powershell End ----------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
out-null

echo "Some lines work in both bash and powershell. Calculating scriptdir=$scriptdir, requires separate sections."

It relies on herestring quoting being different for each platform, as is the escape character ( \ vs ` ). Readibility (ha!) is very much helped by

#comments begin with a hash 

being common to both, so I can do visible dividers between the sections.

My main goal was environment variable setup before launching dotnetcore services. Sadly the incompatible syntaxes for variables and environment:

#powershell syntax
$variable="value"
$env:variable2=$value
#bash syntax
variable=value
export variable2=value 

means very little shared code inside the file, but it really cut down errors a lot just by having them in the same file. Almost-a-single-source-of-truth turned out to be much more reliable than not-at-all a single source of truth.

Bash-then-powershell was simpler than Powershell-then-bash. My state-of-the art is powershell named and validated parameters, which allows tab-completion to work in powershell.

` # \
# PowerShell Param
# every line must end in #\ except last line must end in <#\
# And, you can't use backticks in this section        #\
param( [ValidateSet('A','B')]$tabCompletionWorksHere, #\
       [switch]$andHere                               #\
     )                                               <#\
#^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `

Repo: github.com/chrisfcarroll/PowerShell-Bash-Dual-Script-Templates

Raw: Powershell-or-bash-with-parameters .

Alternatively, do everything in powershell?

Of course, sensible people would do everything in a single scripting language. But it has been well-worth having the tools for both approaches. Especially for short bootstrap scripts.

For a powershell core everywhere approach, my main adaptation is the shebang header on all .ps1 files:

#! /usr/bin/env pwsh

which tells unix machines to what kind of script it is. Powershell itself ignores it as a comment. Finally, you must also chmod a+x *.ps1 to mark them as executable.