Syntax

Example

Explanation

~

cd ~/bin

TildeExpansion, resulting in the current user's home directory.

~

mode=$((mode & ~umask))

In an ArithmeticExpression, bitwise NOT (negation).

~user

cd ~john

Expands to the home directory of user.

`

eval `resize`

Deprecated form of CommandSubstitution. See BashFAQ/082.

!

if ! myfunc; then myerrorhandler; fi

Reverses the exit status of a pipeline.

!

[[ ! -d $d ]]

Inside [[ or [ or test, before a unary operator, reverses the check.

!

echo [!x]*

As the first character inside a [...] in a glob, reverses the set of matched characters.

!=

[ "$input" != y ]

In a test or [ command, string comparison, not equal.

!=

[[ $input != [Yy] ]

In a [[ command, a glob pattern or string comparison, not equal.

!=

(( x != 42 ))

In an ArithmeticExpression, integer comparison, not equal.

!!

!!

Repeat the previous command. (Only if HistoryExpansion is enabled.)

!word

!make

Repeat the previous command starting with word (if history expansion is enabled).

!-number

!-3

Repeat the command number entries in the past (if history expansion is enabled).

$!

pid=$!

Expands to the process ID of the most recent background pipeline.

${!var}

local arg2="${!2}"

Indirect parameter expansion; uses the value of var as a variable name and expands it.

${!arr[@]}

for i in "${!arr[@]}"

Expands to all indices of an indexed or associative array variable. The double quotes are required for associative arrays.

!(patterns)

rsync -a !(*~) "$dest"

Extended glob which matches all strings/files not matching the patterns.

"$@"

exec foo "$@"

Expands to all positional parameters, like "$1" "$2" .... The double quotes are required.

"${arr[@]}"

find "${args[@]}"

Expands to all values in an indexed array variable. The double quotes are required.

${!prefix@}

echo "${!BASH@}"

Expands to the names of variables beginning with prefix.

${var@Q}

ssh host "rm ${file@Q}"

Expands var in a quoted form which can be used in a raw bash command.

@(patterns)

[[ $input = @(foo|bar) ]]

Extended glob which matches all strings/files that match any of the patterns.

#

# look for serial number

Any word beginning with # starts a comment; the rest of the line is ignored.

$#

if (($# < 2)); then usage; fi

Expands to the number of positional parameters.

${var#prefix}

var=${var#?}

Expands to the value of var with the shortest prefix removed if present.

${var##prefix}

base=${file##*/}

Expands to the value of var with the longest prefix removed if present.

${#var}

len=${#string}

Expands to the length of var in characters (not bytes unless in the C or POSIX locale).

${#arr[@]}

size=${#arr[@]}

Expands to the number of values in an indexed or associative array variable.

$

[[ $serial =~ x[0-9]+$ ]]

In a basic/extended regular expression, matches the end of the input. Also called an anchor.

$$

kill -INT $$

Expands to the process ID of the current shell (never a subshell).

$var

cd "$foo" || exit

Expands to the value of the variable var.

${var}

echo "I like ${fruit}s"

Expands to the value of the variable var. Not a substitute for Quotes.

$(cmd)

output=$(cmd)

CommandSubstitution, expands to the standard output of cmd.

$((exp))

i=$((i+1))

ArithmeticExpression, expands to the value of exp evaluated in a math context.

%

r=$((i % 4))

In a math context, remainder (modulus).

%=

((i %= 4))

In a math context, assign a new value to a variable after performing modulus with the variable and a value.

${var%suffix}

dir=${file%/*}

Expands to the value of var with the shortest suffix removed if present.

${var%%suffix}

major=${version%%.*}

Expands to the value of var with the longest suffix removed if present.

^

if foo ^ grep bar >/dev/null

Synonym for | in the Bourne shell only (not bash or POSIX sh).

^

[[ $serial =~ ^[0-9]+z ]]

In a basic/extended regular expression, matches the start of the input. Also called an anchor.

^

echo [^x]*

As the first character inside a [...] in a glob, reverses the set of matched characters. Not POSIX.

^

x=$((y ^ 0xff))

In an ArithmeticExpression, bitwise XOR (exclusive OR).

^=

((y ^= 0xff))

In an ArithmeticExpression, assign a new value to a variable after performing bitwise XOR.

${var^}

echo "${name^}"

Expands to the value of var with the first character capitalized.

${var^pattern}

echo "${name^[aeiou]}"

Expands to the value of var with the first character capitalized only if it matches pattern.

${arr[@]^}

echo "${title[@]^}"

Expands to the values of an indexed or associative array variable with the first character of each value capitalized.

${arr[@]^pattern}

echo "${title[@]^[aeiou]}"

Expands to the values of an array variable with first characters capitalized if they match pattern.

${var^^}

caps=${name^^}

Expands to the value of var with all characters capitalized.

${var^^pattern}

var=${var^^[aeiou]}

Expands to the value of var with all characters which match pattern capitalized.

${arr[@]^^}

caps=("${arr[@]^^}")

Expands to the values of an indexed or associative array variable with all characters capitalized.

${arr[@]^^pattern}

silly=("${arr[@]^^[aeiou]}")

Expands to the values of an array variable with all characters matching pattern capitalized.

^old^new^

^.txzt^.txt^

Repeat the previous command replacing old with new (only if history expansion is enabled).

&

updatedb &

Run a pipeline in the background.

&

mode=$((mode & ~umask))

In an ArithmeticExpression, bitwise AND.

&=

((mode &= ~umask))

In an ArithmeticExpression, assign a new value to a variable after performing bitwise AND.

&&

[[ -e ~/.bashrc ]] && . ~/.bashrc

Execute the next command only if the previous command succeeded (exit status 0).

&&

(( i > 4 && i < 10 ))

In an ArithmeticExpression, logical AND.

&&

[[ -d $dir && ! -L $dir ]]

In [[, between expressions, logical AND.

*

rm *.bak

In a glob, matches any string (including the empty string).

*

[[ $serial =~ -([0-9]*)- ]]

In a basic/extended regular expression, matches 0 or more of the previous token.

*

i=$((x * y))

In a math context, multiplication.

*=

((x *= 3))

In a math context, multiply a variable by a value.

**

pow=$((2 ** n))

In a math context, exponentiation ("power").

"$*"

printf '%s\n' "$*"

Expands to a single string containing all positional parameters, with the first character of IFS between them. The quotes are required.

"${arr[*]}"

printf '%s\n' "${words[*]}"

Expands to a single string containing all values of an indexed or associative array variable, with the first character of IFS between them. The quotes are required.

(

[[ ( $a = 1 || $b = 2 ) && $c = 3 ]]

Inside [[, parentheses allow grouping to change operator precedence.

*(patterns)

var=${var##*([[:space:]])}

In an extended glob, matches 0 or more instances of the patterns.

(

[[ $line =~ foo-(bar[0-9]+) ]]

In an extended regular expression, serves two purposes: grouping, and capture grouping. In [[, captured groups are exposed in the BASH_REMATCH array variable.

\(

sed 's/a\(b[0-9]\)/c\1/'

In a basic regular expression, capture grouping. Bash does not use basic regular expressions, so this is exclusively for external commands which do, such as sed.

(cmd)

(cd "$dir" && make)

Execute cmd in a SubShell.

((exp))

((i++))

Evaluate exp as an ArithmeticExpression and exit 0 if the value is non-zero (true), or 1 if the value is 0 (false).

for ((start;cond;step))

for ((i=0; i<10; i++))

C-style for loop. start, cond and step are evaluated in math contexts.

func()

f() { ...; }

Define function func.

_

read -r var1 _

Not actually syntax, _ is simply a special parameter name. Assignments to _ do nothing, so it's common to use _ to absorb extra data fields that aren't needed.

$_

mkdir /long/path && cd "$_"

The special parameter _ holds the last argument of the previous command.

-

i=$((i - 2))

In a math context, subtraction.

-

vi foo[0-9].txt

In a glob or extended regular expression, [a-b] matches characters in the range from a to b in the current locale. Extremely unwise to use with letters outside of the C/POSIX locale. Digits tend to be safe. Remember that it's only digits, not numbers.

-

cat - file2

Not actually shell syntax. By convention, many commands that expect filenames allow - to be specified to mean standard input or standard output.

$-

if [[ $- = *i* ]]; then

The special parameter - holds all of the single-letter set options that are currently enabled.

--

rm -- "$file"

Not actually shell syntax. Signifies the end of options to most commands. If the $file begins with -, rm will not treat it as another cluster of options.

--

((i--))

In a math context, post-decrement or pre-decrement (subtracts one from a variable's value, and returns either the previous or new value, respectively).

${var-default}

${PAGER-more}

Expand a parameter, but use a default value if the parameter is unset.

${var:-default}

${PAGER:-more}

Expand a parameter, but use a default value if the parameter is unset or contains the empty string.

-=

((i -= 3))

In a math context, decrement a variable by a value.

+

i=$((i + 3))

In a math context, addition.

+

[[ $f =~ foo[0-9]+bar ]]

In an extended regular expression, matches one or more instances of the previous token. Equivalent to \{1,\} in a basic regular expression.

${var+words}

[[ ${var+defined} ]]

Expand to words if the var is set to any value; otherwise, expand to nothing.

${var:+words}

mailx ${subject:+-s "$subject"} "$rcpt"

Expand to words if the var is set to any non-empty value; otherwise, expand to nothing.

++

((i++))

In a math context, post-increment or pre-increment (adds one to a variable's value, and returns either the previous or new value, respectively).

+=

((i += 3))

In a math context, increment a variable by a value.

var+=value

output+=$line$'\n'

Appends value to the string variable var.

arr+=(words)

args+=(-o -iname "*.$ext")

Appends words to the array variable arr.

+(patterns)

var=${var##+([[:space:]])

As an extended glob, matches one or more instances of patterns.

=

[ "$input" = "" ]

In a test or [ command, string comparison.

=

[[ $input = [Yy]* ]]

In a [[ command, a glob-pattern or string comparison. Any quoted characters on the right hand side are treated as literal.

var=value

x=42

Assigns value to the string variable var.

${var=default}

: "${EDITOR=nano}"

Expands var, but if var is unset, the default value is assigned to it first.

${var:=default}

: "${EDITOR:=nano}"

Expands var, but if var is unset or contains the empty string, the default value is assigned to it first.

==

[[ $input == [Yy]* ]]

In a [[ command, a glob-pattern or string comparison. Identical to =.

==

if ((i == 3))

In a math context, integer comparison.

=~

[[ $input =~ $regex ]]

In a [[ command, match against an extended regular expression. Any quoted characters on the right hand side are treated as literal.

{ cmds }

f() { echo "f"; }

Command grouping, not a SubShell. Allows cmds to be placed anywhere a single command would normally go.

{n}

[[ $x =~ foo-[0-9]{3}-bar ]]

In an extended regular expression, matches n instances of the previous token. Equivalent to \{n\} in a basic regular expression.

{m,n}

[[ $x =~ foo-[0-9]{1,3}-bar ]]

In an extended regular expression, matches between m and n instances of the previous token. If n is omitted, treat it as infinity. Equivalent to \{m,n\} in a basic regular expression.

[chars]

rm foo[1246].txt

In a glob or basic/extended regular expression, matches any one character from chars.

[[:class:]]

var=${var##+([[:space:]])}

In a glob or basic/extended regular expression, matches any one character from the character class.

arr[index]=value

seen[$file]=1

Assigns value to the indexth element of the indexed or associative array variable arr. index is evaluated in a math context if the array is indexed.

${arr[index]}

first=${files[0]}

Expands to the indexth element of the indexed or associative array variable arr.

$[

i=$[ "$i" + 1 ]

Obsolete arithmetic expression syntax. Use $(( )) instead.

|

ls -l | more

Pipeline. The standard output of the left side is connected to the standard input of the right side.

|

bits=$((bits | 0x04))

In a math context, bitwise OR.

|

vi foo@(bar|baz).txt

In an extended glob, separates patterns within a single matching expression.

|

[[ $x =~ foo|bar ]]

In an extended regular expression, matches either of two smaller regular expressions. Has no equivalent in basic regular expressions.

|=

((flags |= 1))

In an ArithmeticExpression, assign a new value to a variable after performing bitwise OR.

|&

make |& less

Pipeline carrying standard output + standard error. Equivalent to 2>&1 |.

||

cd "$foo" || exit

Execute the second command if the first command failed (exit status nonzero).

||

[[ $a = 1 || $b = 2 ]]

In [[, logical OR.

\

sed -e 'something' \

At the end of a line, indicates that the command continues on the next line. The backslash + newline pair are removed.

\

`echo "\$4.00"

In many contexts, backslash quotes or escapes the next character, removing its special meaning.

\num

sed 's/\(a[0-9]*)\1//'

In a basic regular expression, refers to the numth capture group. Bash does not use basic regular expressions, but some external commands do, such as sed.

${var:start}

sub=${input:k}

Expands to the substring of var beginning at start, which is evaluated in a math context, zero-indexed.

${var:start:len}

c=${input:i:1}

Expands to the substring of var beginning at start and a maximum of len characters long. Both start and len are evaluated in math contexts.

;

sleep 30; play alarm.wav

Execute the second command when the first has finished, regardless of exit status.

;;

case $i in q) exit;;

Indicates the end of a command list in a case statement.

;&

case $i in a) a=1;&

Indicates the end of a command list in a case statement, but the next command list is also executed without checking the next pattern.

;;&

case $i in a) a=1;;&

Indicates the end of a command list in a case statement, but the next pattern is also checked.

"

vi "$file"

Quoting but with certain substitutions allowed.

'

vi 'my file'

Quoting with no substitutions.

$'

IFS=$' \t\n'

Quoting with only specific backslash + letter combinations substituted.

$"

printf %s\\n $"An error message"

Indicates a string to be translated. Not secure.

<

if [[ $date < 2020-01-01 ]]

In a [[ command, string (lexical) less-than comparison.

<

while ((i < 10))

In a math context, numerically (integer) less than.

<<

shifted=$((input << n))

In a math context, bitwise left shift.

<<=

((n <<= 2))

In a math context, modify a variable by bitwise left shifting.

<file

read -r pid < "$pidfile"

Standard input redirection from file.

<<delim

cat <<'EOF'

Here document. Standard input is redirected from the following lines of the script, until delim as a standalone line. If any character of delim is quoted, no substitutions are preformed in the here document. Otherwise, some substitutions occur.

<<-delim

cat <<-EOF

Here document. Standard input is redirected from the following lines with leading tabs removed. See above for more details.

<<<word

read -ra arr <<< "$line"

Here string. Standard input is redirected from word plus a newline.

<(cmd)

mapfile -t arr < <(foo)

ProcessSubstitution. cmd is executed as a background process, and the syntax is replaced by a FIFO-equivalent from which the command's standard output may be read.

n<file

while ... done 3< file

Redirect input FileDescriptor n from file.

n<&m

exec 3<&0

Duplicate input FileDescriptor m as file descriptor n.

n<&-

exec 3<&-

Close FileDescriptor n.

>

if [[ $date > 1979-12-31 ]]

In a [[ command, string (lexical) greater-than comparison.

>

while ((--i > 0))

In a math context, numerically (integer) greater than.

>>

shifted=$((input >> n))

In a math context, bitwise right shift.

>>=

((n >>= 2))

In a math context, modify a variable by bitwise right shifting.

>file

foo >/dev/null

Redirect standard output to file.

>>file

foo >> logfile

Redirect standard output to file in append mode.

&>file

foo &>/dev/null

Redirect standard output and standard error to file. Equivalent to >file 2>&1.

>&file

foo >&/dev/null

Redirect standard output and standard error to file. Equivalent to >file 2>&1. However, if file expands to a number or -, a different redirection is performed. Not recommended.

&>>file

foo &>> logfile

Redirect standard output and standard error to file in append mode. Equivalent to >>file 2>&1.

n>file

exec 3> >(timestamper)

Redirect output FileDescriptor n to file.

n>>file

set -x; exec 2>> logfile

Redirect output FileDescriptor n to file in append mode.

n>&m

foo >> logfile 2>&1

Duplicate output FileDescriptor m as file descriptor n.

n>&-

exec 3>&-

Close FileDescriptor n.

>(cmd)

exec >(timestamper)

ProcessSubstitution. cmd is executed as a background process, and the syntax is replaced by a FIFO-equivalent. The standard input of cmd is redirected from this FIFO-equivalent.

,

((a=0, b=1))

In a math context, comma separates different expressions.

.

. ~/.bashrc

Synonym for the source builtin command.

..

cd ..

Refers to the parent directory. Not precisely shell syntax.

?

vi foo-??.c

In a glob, matches any single character. Equivalent to . in a basic/extended regular expression.

?

[[ $file =~ \.txt~?$ ]]

In an extended regular expression, matches 0 or 1 instance of the previous token. Equivalent to \{0,1\} in a basic regular expression.

expr?exp2:exp3

x=$((i>0 ? 3 : 5))

In a math context, the same as C's ternary conditional operator. Only integer expressions are allowed, not strings.

/

n=$((COLUMNS / 8))

In a math context, integer division.

/=

((n /= 2))

In a math context, divide a variable by a value (integer division).

${var/search}

x=${x/junk}

Expands to the value of var with the first instance of search (a glob) removed. Degenerate/simplified form of ${var/search/replace} with an implied empty replace string.

${var//search}

x=${x//junk}

Expands to the value of var with all instances of search (a glob) removed. Degenerate/simplified form of ${var//search/replace} with an implied empty replace string.

${var/search/replace}

x=${x/foo/bar}

Expands to the value of var with the first instance of search (a glob) replaced by replace.

${var//search/replace}

x=${x//foo/bar}

Expands to the value of var with all instances of search (a glob) replaced by replace.