Tips for golfing in Bash

Question

What general tips do you have for golfing in Bash? I'm looking for ideas that can be applied to code golf problems in general that are at least somewhat specific to Bash (e.g. "remove comments" is not an answer). Please post one tip per answer.

Answer 1

Undocumented, but works in every version I've run into for legacy sh backwards compatibility:

for loops allow you to use { } instead of do done. E.g. replace:

for i in {1..10};do echo $i; done

with:

for i in {1..10};{ echo $i;}

Answer 2

For arithmetic expansion use $[…] instead of $((…)):

bash-4.1$ echo $((1+2*3))
7

bash-4.1$ echo $[1+2*3]
7

In arithmetic expansions don't use $:

bash-4.1$ a=1 b=2 c=3

bash-4.1$ echo $[$a+$b*$c]
7

bash-4.1$ echo $[a+b*c]
7

Arithmetic expansion is performed on indexed array subscripts, so don't use $ neither there:

bash-4.1$ a=(1 2 3) b=2 c=3

bash-4.1$ echo ${a[$c-$b]}
2

bash-4.1$ echo ${a[c-b]}
2

In arithmetic expansions don't use ${…}:

bash-4.1$ a=(1 2 3)

bash-4.1$ echo $[${a[0]}+${a[1]}*${a[2]}]
7

bash-4.1$ echo $[a[0]+a[1]*a[2]]
7

Answer 3

The normal, lengthy and boring way to define a function is

f(){ CODE;}

As this guy found out, you absolutely need the space before CODE and the semicolon after it.

This is a little trick I've learned from @DigitalTrauma:

f()(CODE)

That is two characters shorter and it works just as well, provided that you don't need to carry over any changes in variables' values after the function returns (the parentheses run the body in a subshell).

As @jimmy23013 points out in the comments, even the parentheses may be unnecessary.

The Bash Reference Manual shows that functions can be defined as follows:

name () compound-command [ redirections ]

or

function name [()] compound-command [ redirections ]

A compound command can be:

• a Looping Construct: until, while or for
• a Conditional Construct: if, case, ((...)) or [[...]]
• Grouped Commands: (...) or {...}

That means all of the following are valid:

$ f()if $1;then $2;fi
$ f()($1&&$2)
$ f()(($1))                # This one lets you assign integer values

And I've been using curly brackets like a sucker...

Answer 4

: is a command that does nothing, its exit status always succeeds, so it can be used instead of true.

Answer 5

More tips

1. Abuse the ternary operator, ((test)) && cmd1 || cmd2 or [ test ] && cmd1 || cmd2, as much as possible.

   Examples (length counts always exclude the top line):

   t="$something"
   if [ $t == "hi" ];then
   cmd1
   cmd2
   elif [ $t == "bye" ];then
   cmd3
   cmd4
   else
   cmd5
   if [ $t == "sup" ];then
   cmd6
   fi
   fi
   

   By using ternary operators only, this can easily be shortened to:

   t="$something"
   [ $t == "hi" ]&&{
   cmd1;cmd2
   }||[ $t == "bye" ]&&{
   cmd3;cmd4
   }||{
   cmd5
   [ $t == "sup" ]&&cmd6
   }
   

   As nyuszika7h pointed out in the comments, this specific example could be shortened even further using case:

   t="$something"
   case $t in "hi")cmd1;cmd2;;"bye")cmd3;cmd4;;*)cmd5;[ $t == "sup" ]&&cmd6;esac
   

2. Also, prefer parentheses to braces as much as possible. Since parentheses are a metacharacter, and not a word, they never require spaces in any context. This also means run as many commands in a subshell as possible, because curly braces (i.e. { and }) are reserved words, not meta-characters, and thus have to have whitespace on both sides to parse correctly, but meta-characters don't. I assume that you know by now that subshells don't affect the parent environment, so assuming that all the example commands can safely be run in a subshell (which isn't typical in any case), you can shorten the above code to this:

   t=$something
   [ $t == "hi" ]&&(cmd1;cmd2)||[ $t == "bye" ]&&(cmd3;cmd4)||(cmd5;[ $t == "sup" ]&&cmd6)
   

   Also, if you can't, using parentheses can still minify it some. One thing to keep in mind is that it only works for integers, which renders it useless for the purposes of this example (but it is much better than using -eq for integers).

3. One more thing, avoid quotes where possible. Using that above advice, you can further minify it. Example:

   t=$something
   [ $t == hi ]&&(cmd1;cmd2)||[ $t == bye ]&&(cmd3;cmd4)||(cmd5;[ $t == sup ]&&cmd6)
   

4. In testing conditions, prefer single brackets to double brackets as much as possible with a few exceptions. It drops two characters for free, but it isn't as robust in some cases (it's a Bash extension - see below for an example). Also, use the single equals argument rather than the double. It is a free character to drop.

   [[ $f == b ]]&&: # ... <-- Bad
   [ $f == b ]&&: # ... <-- Better
   [ $f = b ]&&: # ... <-- Best.  word splits and pathname-expands the contents of $f.  Esp. bad if it starts with -
   

   Note this caveat, especially in checking for null output or an undefined variable:

   [[ $f ]]&&:    # double quotes aren't needed inside [[, which can save chars
   [ "$f" = '' ]&&: <-- This is significantly longer
   [ -n "$f" ]&&:
   

   In all technicality, this specific example would be best with case ... in:

   t=$something
   case $t in hi)cmd1;cmd2;;bye)cmd3;cmd4;;*)cmd5;[ $t == sup ]&&cmd6;esac
   

So, the moral of this post is this:

1. Abuse the boolean operators as much as possible, and always use them instead of if/if-else/etc. constructs.
2. Use parentheses as much as possible and run as many segments as possible in subshells because parentheses are meta-characters and not reserved words.
3. Avoid quotes as much as physically possible.
4. Check out case ... in, since it may save quite a few bytes, particularly in string matching.

P.S.: Here's a list of meta-characters recognized in Bash regardless of context (and can separate words):

< > ( ) ; & | <space> <tab>

---

EDIT: As manatwork pointed out, the double parenthesis test only works for integers. Also, indirectly, I found that you need to have whitespace surrounding the == operator. Corrected my post above.

I also was too lazy to recalculate the length of each segment, so I simply removed them. It should be easy enough to find a string length calculator online if necessary.

Answer 6

Instead of grep -E, grep -F, grep -r, use egrep, fgrep, rgrep, saving two chars. The shorter ones are deprecated but work fine.

(You did ask for one tip per answer!)

Answer 7

Element 0 of an array may be accessed with the variable name only, a five byte saving over explicitly specifying an index of 0:

$ a=(code golf)
$ echo ${a[0]}
code
$ echo $a
code
$ 

Answer 8

If you need to pass the content of a variable to STDIN of the next process in a pipeline, it is common to echo the variable into a pipeline. But you can achieve the same thing with a <<< bash here string:

$ s="code golf"
$ echo "$s"|cut -b4-6
e g
$ cut -b4-6<<<"$s"
e g
$ 

Answer 9

One-line for loops

An arithmetic expression concatenated with a range expansion will be evaluated for each item in the range. For example the following:

: $[expression]{0..9}

will evaluate expression 10 times.

This is often significantly shorter than the equivalent for loop:

for((;10>n++;expression with n)){ :;}
: $[expression with ++n]{0..9}

If you don't mind command not found errors, you can remove the inital :. For iterations larger than 10, you can also use character ranges, for example {A..z} will iterate 58 times.

As a practical example, the following both produce the first 50 triangular numbers, each on their own line:

for((;50>d++;)){ echo $[n+=d];} # 31 bytes
printf %d\\n $[n+=++d]{A..r}    # 28 bytes

Answer 10

Avoid $( ...command... ), there is an alternative which saves one char and does the same thing:

` ...command... `

Answer 11

A shorter syntax for infinite loops (which can be escaped with break or exit statements) is

for((;;)){ code;}

This is shorter than while true; and while :;.

If you don't need break (with exit as the only way to escape), you can use a recursive function instead.

f(){ code;f;};f

If you do need break, but you don't need exit and you don't need to carry over any variable modification outside the loop, you can use a recursive function with parentheses around the body, which run the function body in a subshell.

f()(code;f);f

Answer 12

Use if to group commands

Compared to this tip which removes the if at all, this should only work better in some very rare cases, such as when you need the return values from the if.

If you have a command group which ends with a if, like these:

a&&{ b;if c;then d;else e;fi;}
a&&(b;if c;then d;else e;fi)

You can wrap the commands before if in the condition instead:

a&&if b;c;then d;else e;fi

Or if your function ends with a if:

f(){ a;if b;then c;else d;fi;}

You can remove the braces:

f()if a;b;then c;else d;fi

Answer 13

Use arithmetic (( ... )) for conditions

You could replace:

if [ $i -gt 5 ] ; then
    echo Do something with i greater than 5
fi

by

if((i>5));then
    echo Do something with i greater than 5
fi

(Note: There is no space after if)

or even

((i>5))&&{
    echo Do something with i greater than 5
}

... or if only one command

((i>5))&&echo Echo or do something with i greater than 5

Further: Hide variable setting in arithmetic construct:

((i>5?c=1:0))&&echo Nothing relevant there...
# ...
((c))&&echo Doing something else if i was greater than 5

or same

((c=i>5?c=0,1:0))&&echo Nothing relevant there...
# ...
((c))&&echo Doing something else if i was greater than 5

... where if i > 5, then c = 1 (not 0;)

Answer 14

Loop over arguments

As noted in Bash “for” loop without a “in foo bar…” part, the in "$@;" in for x in "$@;" is redundant.

From help for:

for: for NAME [in WORDS ... ] ; do COMMANDS; done
    Execute commands for each member in a list.

    The `for' loop executes a sequence of commands for each member in a
    list of items.  If `in WORDS ...;' is not present, then `in "$@"' is
    assumed.  For each element in WORDS, NAME is set to that element, and
    the COMMANDS are executed.

    Exit Status:
    Returns the status of the last command executed.

For example, if we want to square all numbers given positional arguments to a Bash script or a function, we can do this.

for n;{ echo $[n*n];}

Try it online!

Answer 15

Alternative to cat

Say you are trying to read a file and use it in something else. What you might do is:

echo foo `cat bar`

If the contents of bar was foobar, this would print foo foobar.

However, there is an alternative if you are using this method, which saves 3 bytes:

echo foo `<bar`

Answer 16

Alternatives to head

line is three bytes shorter than head -1, but is being deprecated.

sed q is two bytes shorter than head -1.

sed 9q is one byte shorter than head -9.

Answer 17

tr -cd is shorter than grep -o

For example, if you need to count spaces, grep -o <char> (print only the matched) gives 10 bytes while tr -cd <char> (delete complement of <char>) gives 9.

# 16 bytes
grep -o \ |wc -l
# 15 bytes
tr -cd \ |wc -c

(source)

Note that they both give slightly different outputs. grep -o returns line separated results while tr -cd gives them all on the same line, so tr might not always be favourable.

Answer 18

Shorten file names

In a recent challenge I was trying to read the file /sys/class/power_supply/BAT1/capacity, however this can be shortened to /*/*/*/*/capac*y as no other file exists with that format.

For example, if you had a directory foo/ containing the files foo, bar, foobar, barfoo and you wanted to reference the file foo/barfoo, you can use foo/barf* to save a byte.

The * represents "anything", and is equivalent to the regex .*.

Answer 19

Use [ instead of [[ and test when possible

Example:

[ -n $x ]

Use = instead of == for comparison

Example:

[ $x = y ]

Note that you must have spaces around the equals sign or else it won't work. Same applies to == based on my tests.

Answer 20

Use tail recursion to make loops shorter:

These are equivalent in behavior (though probably not in memory/PID usage):

while :;do body; done
f()(body;f);f
body;exec $0
body;$0

And these are roughly equivalent:

while condition; do body; done
f()(body;condition&&f);f
body;condition&&exec $0
body;condition&&$0

(technically the last three will always execute the body at least once)

Using $0 requires your script to be in a file, not pasted into the bash prompt.

Eventually your stack might overflow, but you save some bytes.

Answer 21

Sometimes it is shorter to use the expr builtin for displaying the result of a simple arithmetic expression instead of the usual echo $[ ]. For example:

expr $1 % 2

is one byte shorter than:

echo $[$1%2]

Answer 22

Use a pipe to the : command instead of /dev/null. The : built-in will eat all its input.

Answer 23

split has another (deprecated, but nobody cares) syntax for splitting input into sections of N lines each: instead of split -lN you can use split -N e.g. split -9.

Answer 24

Expand away the tests

Essentially, the shell is a kind of macro language, or at least a hybrid or some kind. Every command-line can be basically broken into two parts: the parsing/input part and the expansion/output part.

The first part is what most people focus on because it's the most simple: you see what you get. The second part is what many avoid ever even trying to understand very well and is why people say things like eval is evil and always quote your expansions - people want the result of the first part to equal the first. That's ok - but it leads to unnecessarily long code branches and tons of extraneous testing.

Expansions are self-testing. The ${param[[:]#%+-=?]word} forms are more than enough to validate the contents of a parameter, are nestable, and are all based around evaluating for NUL - which is what most people expect of tests anyway. + can be especially handy in loops:

r()while IFS= read -r r&&"${r:+set}" -- "$@" "${r:=$*}";do :;done 2>&-

IFS=x
printf %s\\n some lines\ of input here '' some more|{ r;echo "$r"; }

---

somexlines ofxinputxhere

...while read pulls in not blank lines "${r:+set}" expands to "set" and the positionals get $r appended. But when a blank line is read, $r is empty and "${r:+set}" expands to "" - which is an invalid command. But because the command-line is expanded before the "" null command is searched, "${r:=$*}" takes the values of all of the positionals concatenated on the first byte in $IFS as well. r() could be called again in |{ compound command ;} w/ a different value for $IFS to get the next input paragraph as well, since it is illegal for a shell's read to buffer beyond the next \newline in input.

Answer 25

Quotes can be omitted when printing strings.

echo "example"
echo example

Output in SM-T335 LTE, Android 5.1.1:

u0_a177@milletlte:/ $ echo "example"
example
u0_a177@milletlte:/ $ echo example
example

Answer 26

When assigning noncontinuous array items, you can still skip the successive indices of continuous chunks:

bash-4.4$ a=([1]=1 [2]=2 [3]=3 [21]=1 [22]=2 [23]=3 [31]=1)

bash-4.4$ b=([1]=1 2 3 [21]=1 2 3 [31]=1)

The result is the same:

bash-4.4$ declare -p a b
declare -a a=([1]="1" [2]="2" [3]="3" [21]="1" [22]="2" [23]="3" [31]="1")
declare -a b=([1]="1" [2]="2" [3]="3" [21]="1" [22]="2" [23]="3" [31]="1")

According to man bash:

Arrays are assigned to using compound assignments of the form name=(value1 ... valuen), where each value is of the form [subscript]=string. Indexed array assignments do not require anything but string. When assigning to indexed arrays, if the optional brackets and subscript are supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one.

Answer 27

Print the first word in a string

If the string is in the variable a and doesn't contain escape and format characters (\ and %), use this:

printf $a

But it would be longer than the following code if it is needed to save the result into a variable instead of printing:

x=($a)
$x

Answer 28

Use pwd instead of echo to generate a line of output

Need to put a line on stdout but don't care about the contents, and want to restrict your answer to shell builtins? pwd is a byte shorter than echo.

Answer 29

Doing 2 embed loop with 1 for instruction:

for ((l=i=0;l<=99;i=i>98?l++,0:++i)) ;do
    printf "I: %2d, L: %2d\n" $i $l
done |
    tee >(wc) | (head -n4;echo ...;tail -n 5)
I:  0, L:  0
I:  1, L:  0
I:  2, L:  0
I:  3, L:  0
...
I: 96, L: 99
I: 97, L: 99
I: 98, L: 99
I: 99, L: 99
  10000   40000  130000

Answer 30

In a case statement, it is valid to omit the last ';;' - which saves 2 bytes:

#!/bin/sh

case $I in
0)cmdA;;
1)cmdB;;
*)cmdC
esac

Answer 31

Assign and Print quoted strings

If you want to assign a quoted string to a variable, and then print the value of that variable, then the usual way to do that would be:

a="Programming Puzzles & Code Golf";echo $a

If a was previously unset, this may be shortened to:

echo ${a=Programming Puzzles & Code Golf}

If a was previously set, then this should be used instead:

echo ${a+Programming Puzzles & Code Golf}

Note this is only useful if the string requires quotes (e.g. contains whitespace). Without quotes, a=123;echo $a is just as short.

Answer 32

This work under simple shell

cd `mktemp -d` &&>'c'a't'
ln -s /proc/loadavg /proc/uptime .

Then now

procs=$(*)
echo $procs
0.30 0.08 0.03 1/612 31671 322787.60 1259967.99

Nota: Of course, >cat could be written >$(echo -e \\0143\\0141t)

Care, from there, you could encouter some issues due to Locales!

Goto last demo using LC_ALL=C

Same way:

mv cat grep
mv loadavg Mem
ln -s /proc/meminfo Zdatas
rm uptime

mems=$(*)
echo $mems 
MemTotal: 16386788 kB MemFree: 9816320 kB MemAvailable: 12144892 kB

or worst...

mv grep sed
mv Mem s+\\\(Mem\\\|Swap\\\).\*\:++p\;d

then

memsw=$(*)
echo $memsw 
16386788 kB 9834080 kB 12163596 kB 0 kB 20971516 kB 20971516 kB

So simple is this!

declare -p procs mems memsw
declare -- procs="0.14 0.12 0.05 1/612 32078
324221.56 1265616.26"
declare -- mems="MemTotal:       16386788 kB
MemFree:         9832212 kB
MemAvailable:   12161688 kB"
declare -- memsw="       16386788 kB
         9834080 kB
   12163596 kB
            0 kB
      20971516 kB
       20971516 kB"

Last test using LANG=C

(or not)

cd `mktemp -d` &&>'c'a't'
ln -s /proc/meminfo zdatas
cp c* y+-+-+\;s+\\\(Mem\\\|Swap\\\).\*\:\ \*++p\;d
rename 'y/act/esd/' ???
*

may render something like

378908 kB
31940 kB
217924 kB
0 kB
102396 kB
102396 kB

Or

cd `mktemp -d`
ln -s +++\;s+\\\(Mem\\\|Swap\\\).\*\:\ \*++p\;d /proc/me*o ed .
rename 's/^/chr(113+(20>length?1.3*length:8))/e' *
*

Answer 33

Changing behaviour of += by setting integer flag

Try this many times:

unset i
i=1
for a in {1..10} ;do
    i+=1
    ((RANDOM%9)) || declare -i i
  done
echo $i
11117

or in one line

unset i;i=1;for a in {1..10};do i+=1;((RANDOM%9))||declare -i i;done;echo $i

This could answer something between 20 to 11111111111...

Answer 34

It is almost always better to receive input as arguments to a script or function instead of on stdin. Examples:

Input: a string:

# == stdin ==
read a;echo $a
echo `<&0`
# == arguments ==
echo "$1"       # if $1 can have globbing characters/newlines/tabs
echo $1         # if $1 does not have any problematic characters

Input: a list of strings:

# == stdin ==
while read s; do echo $s; done          # one-time use, split on newlines
read -a a;for s in ${a[@]};{echo $s;}   # read one line from stdin; splits on spaces
mapfile a;for s in "${a[@]}";{echo $s;} # read to EOF, split on newlines
# == arguments ==
for s;{ echo $s;}

The exception: a list of strings and other arguments:

read i;while read s; do echo ${s[$i]}; done   # both on stdin
i=$1;shift;for s;{ echo ${s[$i]};}            # both as arguments
read i;for s;{ echo ${s[$i]};}                # list as arguments, other as stdin

Additionally, read will mangle backslashes without -r, and will stop on newlines without -d '', making it even more expensive in worst case scenarios.

Answer 35

To print a range of characters (shorter than echo {a..z}|tr -d ' '):

printf "%s" {a..z}

Print range in reverse order (shorter than printf "%s" {a..z}|rev):

printf "%s" {z..a}