# Tips for golfing in Bash

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.

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;}

• what shell is the is sh and what shell allows this for syntax? it is expressly allowed in zsh. – mikeserv Dec 23 '15 at 19:55
• @mikeserv Bash. I remember reading somewhere that this syntax was allowed in some old sh and that Bash also allows it because of that, though sadly I don't have a citation. – Digital Trauma Dec 23 '15 at 19:57
• ahh... csh, probably - that's how they worked in that shell. – mikeserv Dec 23 '15 at 20:02
• for((;i++<10)){ echo $i;} is shorter than for i in {1..10};{ echo$i;} – Evan Krall Jan 19 '17 at 4:58
• for((;${#r}>i++;)){ echo${r:i-1:1};} iterates over each character of a string $r – roblogic Mar 23 '19 at 12:27 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 $[a[0]+a[1]*a[2]] 7  • Replacing while((i--)), which works, with while[i--] or while$[i--] did not work for me. GNU bash, version 4.3.46(1) – Glenn Randers-Pehrson Nov 6 '16 at 22:54
• Correct, @GlennRanders-Pehrson. That is not supposed to work. – manatwork Nov 7 '16 at 9:08
• y=bc<<<"($x*2.2)%10/1" ... example of using bc for non-integer calculations... note the /1 at the end truncates the resulting decimal to an int. – roblogic Mar 23 '19 at 12:39 • s=$((i%2>0?s+x:s+y)) ... example of using ternary operator in bash arithmetic. It's shorter than if..then..else or [ ] && || – roblogic Mar 23 '19 at 12:43
• @manatwork Thanks. They must have removed it. I'm on GNU bash, version 5.0.2(1)-release (x86_64-apple-darwin16.7.0) and it's not in mine. – Jonah Apr 3 '19 at 16:40

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...

• Note that you can also use f()while ... f()if ... and other compound commands. – jimmy23013 Nov 7 '14 at 6:13
• This one surprised me because I thought f()CODE was legal. It turns out that f()echo hi is legal in pdksh and zsh, but not in bash. – kernigh Nov 7 '14 at 15:50
• its especiallly useful w/ for since it defaults to positionals:f()for x do : $x; done;set -x *;PS4=$* f "$@" or something. – mikeserv Dec 23 '15 at 20:21 : is a command that does nothing, its exit status always succeeds, so it can be used instead of true. • Using a subshell and piping it would use about the same number of bytes, but piping it would be more practical. – ckjbgames Jan 24 '17 at 16:05 • Except when you do :(){:|:} – enedil Jul 17 '17 at 12:50 • Well at that point : isn't the same command anymore, this kind of fork bomb just defines a function called : and calls it twice. a(){a|a} is equivalent – Zev Isert Dec 5 '20 at 7:59 More tips 1. Abuse the ternary operator, ((test)) && cmd1 || cmd2 or [ test ] && cmd1 || cmd2, as much as possible. Examples (length counts always exclude the top line): t="$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):

&lt; &gt; ( ) ; & | &lt;space&gt; &lt;tab&gt;


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.

• Sorry to say, but you have some serious errors there. [ $t=="hi" ] will always evaluate to 0, as it is parsed as [ -n "STRING" ]. (($t=="hi")) will always evaluate to 0 as long as $t has non-numerical value, as strings are forced into integers in arithmetic evaluations. Some test cases: pastebin.com/WefDzWbL – manatwork Feb 15 '14 at 13:30 • @manatwork Thanks for the catch. I'll update accordingly. – Isiah Meadows Feb 17 '14 at 3:55 • Using a case would be shorter here. Also, you don't need a space before }, but you do after {. – nyuszika7h Jun 23 '14 at 15:33 • Why would = be less robust than ==? = is mandated by POSIX, == isn't. – Dennis Sep 5 '14 at 4:28 • The question does ask for one tip per answer... – Toby Speight Aug 22 '16 at 11:21 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
$ • Since we're golfing, s=code\ golf, echo$s| and <<<$s (keeping in mind that the latter two work only because there are no repeated spaces, etc.). – Dennis Nov 7 '14 at 3:29 # 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  • you can also iterate backwards: for((;0<i--;)){ f;} – roblogic Mar 23 '19 at 13:21 • Oftentimes you can reuse the range in the loop by producing an evalable string. For example, your triangular numbers code could be turned into eval echo\$[n+={1..50}]\; for 26 bytes – Jo King May 20 at 2:27

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

• Too bad there's no Pgrep for grep -P. Although I see how it could be easily confused with pgrep, which is used to look up processes. – nyuszika7h Apr 21 '14 at 19:21
• @nyuszika7h I personally use grep -o a lot – user16402 Apr 22 '14 at 6:55
• Wouldn't it save 3 including the space? – ckjbgames Jan 24 '17 at 16:04

Avoid $( ...command... ), there is an alternative which saves one char and does the same thing:  ...command...   • Sometimes $( ) is needed if you have nested command substitutions; otherwise you'd have to escape the inner  – Digital Trauma Apr 13 '14 at 4:18
• These technically do different things, I've had to use the backticks instead of $() when I wanted to run the substitution on my machine instead of the scp target machine, for example. In most cases they're identical. – undergroundmonorail Apr 21 '14 at 19:31 • @undergroundmonorail: you never need backticks. Anything they can do, $() can do if you quote things properly. (unless you need your command to survive something that munges $ but not backticks). There are some subtle differences in quoting things inside them. mywiki.wooledge.org/BashFAQ/082 explains some differences. Unless you're golfing, never use backticks. – Peter Cordes Sep 24 '15 at 7:09 • @PeterCordes I'm sure there was a way but everything I tried at the time didn't work. Even if backticks weren't the best solution, I was glad I knew about them because it was the only solution I had. ¯\_(ツ)_/¯ – undergroundmonorail Sep 24 '15 at 7:43 • @DigitalTrauma Sample of nesting: echo bc <<<"\date +%s\-12" ... (It's hard to post sample containing backtick in comment, there! ;) – F. Hauri Jan 24 '17 at 17:22 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
$ # 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  • You could use the ternary operator [test] &&$if_true || $else in these functions and save some bytes. – ckjbgames Jan 24 '17 at 15:09 • Also, you don't need spaces around && and || – roblogic Mar 23 '19 at 8:18 ### Use arithmetic (( ... )) for conditions You could replace: if [$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;)

• You could save 2 bytes by using [ ] instead of (()) . – ckjbgames Jan 24 '17 at 15:06
• @ckjbgames Are you sure of that!? Wich bash version are you using? – F. Hauri Jan 24 '17 at 15:33
• @FHauri I guess it would be about the same in terms of bytes. – ckjbgames Jan 24 '17 at 15:56
• @ckjbgames With [ ] you need the dollar sign for variable. I don't see how you could do same with same or smaller length by using [ ]. – F. Hauri Jan 24 '17 at 16:55
• Bonus Tip: if the first line of a for loop begins with ((...)), no newline or space is necessary. E.g. for((;10>n++;)){((n%3))&&echo $n;} Try it online! – primo Dec 8 '18 at 9:41 A shorter syntax for infinite loops (which can be escaped with break or exit statements) is for((;;)){ code;}  This is shorter than while true; and while :;. If you don't need break (with exit as the only way to escape), you can use a recursive function instead. f(){ code;f;};f  If you do need break, but you don't need exit and you don't need to carry over any variable modification outside the loop, you can use a recursive function with parentheses around the body, which run the function body in a subshell. f()(code;f);f  # Loop over arguments As noted in Bash “for” loop without a “in foo bar…” part, the in "$@;" 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 $[n*n];}  Try it online! # Alternative to cat Say you are trying to read a file and use it in something else. What you might do is: echo foo cat bar  If the contents of bar was foobar, this would print foo foobar. However, there is an alternative if you are using this method, which saves 3 bytes: echo foo <bar  • Is there a reason why <bar by itself does not work but placing it in backticks does? – user41805 Oct 19 '18 at 18:05 • @Cowsquack Yes. The < puts a file to a command, but in this case it puts it into standard output due to a quirk. The backticks evaluate this together. – Okx Oct 19 '18 at 19:44 • Is there a shorter way to read from standard input other than cat? – Joel Oct 3 '19 at 21:21 • @Joel dd can do sometimes – Ulysse BN Oct 15 '20 at 8:29 # Use [ instead of [[ and test when possible Example: [ -n$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. • The [ vs. [[ may depend on the amount of required quotes: pastebin.com/UPAGWbDQ – manatwork Apr 22 '14 at 5:56 • @manatwork That's a good point. – nyuszika7h Apr 22 '14 at 19:02 • General rule: [...] == /bin/test, but [[...]] != /bin/test and one should never prefer [...] over [[...]] outside of codegolf – cat Dec 26 '15 at 22:23 # 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. • Although doomed, we can still use for a while line from util-linux package to read a single line. – manatwork Jan 28 '17 at 12:48 ## 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. # 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 .*. Use a pipe to the : command instead of /dev/null. The : built-in will eat all its input. • No, it will crash the program with SIGPIPE in most cases. echo a|tee /dev/stderr|: will not print anything. – jimmy23013 Nov 7 '14 at 6:27 • There is a race: echo a|tee /dev/stderr|: did print a on my computer, but elsewhere SIGPIPE might kill tee first. It might depend on version of tee. – kernigh Nov 7 '14 at 15:56 • Yes, it's a SIGPIPE problem: tee >(:) < <(seq 1 10) will work, but tee /dev/stderr | : won't. Even a() { :;};tee /dev/stderr < <(seq 1 10)| a don't print anything. – F. Hauri Jan 24 '17 at 17:20 • @user16402 - you should have a fccing name to my purview... anyway, the : intrinsic eats not at all... if you supposit input to colon you might flood a pipe to in out error... but you can float a redirect by a colon, or drop a process with it... :| while i>&$(($??!$?:${#?})) command shit; do [ -s testitsoutput ]; done or however that pseudo suggestion applies... also, are you aware youre nearly so ghosty as me? ... avoid at all costs the < <(psycho shit i can alias to crazy math eat your world; okay? anyway, ksh93 has a separate but equal composite char placement) – mikeserv Sep 10 '19 at 2:04 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. ### 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. 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.

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]


# 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.

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


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$declare -p a b declare -a a=([1]="1" [2]="2" [3]="3" [21]="1" [22]="2" [23]="3" [31]="1") declare -a b=([1]="1" [2]="2" [3]="3" [21]="1" [22]="2" [23]="3" [31]="1")  According to man bash: Arrays are assigned to using compound assignments of the form name=(value1 ... valuen), where each value is of the form [subscript]=string. Indexed array assignments do not require anything but string. When assigning to indexed arrays, if the optional brackets and subscript are supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. • Helpful to add: uninitialised elements will expand to 0 in arithmetic expansions and "" in other expansions. – Digital Trauma Jun 29 '17 at 15:47 ## Print the first word in a string If the string is in the variable a and doesn't contain escape and format characters (\ and %), use this: printf$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


## 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


# 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.

• \${a+foo} doesn't set a`. – GammaFunction Oct 20 '19 at 6:28