Fish Shell Cheatsheat

fish_cheatsheet.md

fish shell cheatsheet (focused)

---

quoting

• single quotes: no expansion at all

echo '$HOME *.txt'

• double quotes: allow variable + command substitution only

echo "home is $HOME"
echo "pwd is $(pwd)"

• no quotes: full expansion (globs, variables, braces, etc.)

• variables:

set foo 1 2 3
echo $foo      # → 1 2 3 (multiple args)
echo "$foo"    # → "1 2 3" (single arg)

---

escaping

• general: \ escapes next char

echo \$HOME
echo \*
echo \(

• common sequences:

\n   newline
\t   tab
\r   carriage return
\xHH hex byte
\uXXXX unicode
\UXXXXXXXX unicode (32-bit)

• literal space:

echo hello\ world

• continuation (special case):

echo foo \
bar

---

expansions (order matters)

order:

1. command substitution
2. variable expansion
3. brace expansion
4. wildcard expansion

variable

echo $HOME

brace

echo file.{c,h}

wildcard

ls *.txt
ls **.log

• unmatched glob → command fails (not passed literally)

combining

set a x y
echo pre$a        # → prex prey
echo {$a}post     # → xpost ypost

---

command substitution

• syntax:

echo (pwd)
echo $(pwd)

• behavior:

  • splits on newline into multiple args

echo (printf "a\nb\n")   # → a b

• prevent splitting:

echo "$(pwd)"

• common pattern:

set files (ls *.txt)

• process substitution (fish-specific):

diff (cmd1 | psub) (cmd2 | psub)

---

continuations

• backslash newline:

echo foo \
bar

• no whitespace inserted

---

redirection

basic FDs

• stdin: 0
• stdout: 1
• stderr: 2

output

echo hi > out.txt        # overwrite
echo hi >> out.txt       # append

stderr

cmd 2> err.txt
cmd 2>> err.txt

stdin

cmd < input.txt
cmd <? input.txt         # fallback to /dev/null

combine stdout + stderr

cmd &> all.txt           # overwrite both
cmd &>> all.txt          # append both

equivalent:

cmd > all.txt 2>&1

redirect to FD

cmd > &2                 # stdout → stderr
cmd 2> &1                # stderr → stdout

close FD

cmd 2>&-                 # close stderr

noclobber

cmd >? file.txt          # fail if exists
cmd 2>? err.txt

---

pipes

cmd1 | cmd2
cmd 2>| less             # pipe stderr
cmd &| less              # pipe stdout+stderr

---

evaluation order (important)

• pipe is created first
• then redirections left → right

example:

cmd 2>&1 | less

---

quick examples

# safe variable usage
set f "my file.txt"
rm "$f"

# list files or nothing (no glob failure)
set files *.txt
count $files >/dev/null; and ls $files

# redirect everything
make &> build.log

# silence stdout, keep stderr
cmd > /dev/null

# silence both
cmd > /dev/null 2>&1

# stderr only through pager
make 2>| less

# inline variable override
PATH=/tmp/bin $PATH/mycmd

---