How do I iterate over a range of numbers defined by variables in Bash

Question

How do I iterate over a range of numbers in Bash when the range is given by a variable   I know I can do this  called  sequence expression  in the Bash documentation     for i in  1  5   do echo  i  done   Which gives      1    2    3    4    5   Yet  how can I replace either of the range endpoints with a variable  This doesn t work   END 5 for i in  1   END   do echo  i  done   Which prints       1  5

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discussion  Using seq is fine  as Jiaaro suggested  Pax Diablo suggested a Bash loop to avoid calling a subprocess  with the additional advantage of being more memory friendly if  END is too large  Zathrus spotted a typical bug in the loop implementation  and also hinted that since i is a text variable  continuous conversions to-and-fro numbers are performed with an associated slow-down   integer arithmetic  This is an improved version of the Bash loop   typeset -i i END let END 5 i 1 while   i lt  END    do     echo  i             let i   done   If the only thing that we want is the echo  then we could write echo    i       ephemient taught me something  Bash allows for   expr expr expr   constructs  Since I ve never read the whole man page for Bash  like I ve done with the Korn shell  ksh  man page  and that was a long time ago   I missed that   So   typeset -i i END   Let s be explicit for   i 1 i lt  END   i    do echo  i  done   seems to be the most memory-efficient way  it won t be necessary to allocate memory to consume seq s output  which could be a problem if END is very large   although probably not the    fastest      the initial question  eschercycle noted that the  a  b  Bash notation works only with literals  true  accordingly to the Bash manual  One can overcome this obstacle with a single  internal  fork   without an exec    as is the case with calling seq  which being another image requires a fork exec    for i in   eval echo   1   END     do   Both eval and echo are Bash builtins  but a fork   is required for the command substitution  the        construct

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If you re on BSD   OS X you can use jot instead of seq   for i in   jot  END   do echo  i  done

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If you need it prefix than you might like this   for   i 7 i lt  12 i      do echo  printf   2 0d n   i  sed  s   0    done   that will yield  07 08 09 10 11 12

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If you want to stay as close as possible to the brace-expression syntax  try out the range function from bash-tricks  range bash   For example  all of the following will do the exact same thing as echo  1  10    source range bash one 1 ten 10  range   one   ten  range  one  ten range  1   ten  range  1  10    It tries to support the native bash syntax with as few  gotchas  as possible  not only are variables supported  but the often-undesirable behavior of invalid ranges being supplied as strings  e g  for i in  1  a   do echo  i  done  is prevented as well   The other answers will work in most cases  but they all have at least one of the following drawbacks    Many of them use subshells  which can harm performance and may not be possible on some systems  Many of them rely on external programs  Even seq is a binary which must be installed to be used  must be loaded by bash  and must contain the program you expect  for it to work in this case  Ubiquitous or not  that s a lot more to rely on than just the Bash language itself  Solutions that do use only native Bash functionality  like  ephemient s  will not work on alphabetic ranges  like  a  z   brace expansion will  The question was about ranges of numbers  though  so this is a quibble  Most of them aren t visually similar to the  1  10  brace-expanded range syntax  so programs that use both may be a tiny bit harder to read   bobbogo s answer uses some of the familiar syntax  but does something unexpected if the  END variable is not a valid range  bookend  for the other side of the range  If END a  for example  an error will not occur and the verbatim value  1  a  will be echoed  This is the default behavior of Bash  as well--it is just often unexpected    Disclaimer  I am the author of the linked code

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This works fine in bash   END 5 i 1   while     i -le  END      do     echo  i       i   i   1   done

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The POSIX way  If you care about portability  use the example from the POSIX standard   i 2 end 5 while    i -le  end    do     echo  i     i     i 1   done   Output   2 3 4 5   Things which are not POSIX          without dollar  although it is a common extension as mentioned by POSIX itself        is enough here  See also  What is the difference between single and double square brackets in Bash  for        seq  GNU Coreutils   start  end   and that cannot work with variables as mentioned by the Bash manual  let i i 1  POSIX 7 2  Shell Command Language does not contain the word let  and it fails on bash --posix 4 3 42 the dollar at i  i 1 might be required  but I m not sure  POSIX 7 2 6 4 Arithmetic Expansion says      If the shell variable x contains a value that forms a valid integer constant  optionally including a leading plus or minus sign  then the arithmetic expansions     x    and      x    shall return the same value    but reading it literally that does not imply that    x 1   expands since x 1 is not a variable

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If you re doing shell commands and you  like I  have a fetish for pipelining  this one is good   seq 1  END   xargs -I    echo

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for i in   seq 1  END   do echo  i  done  edit  I prefer seq over the other methods because I can actually remember it

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This works in Bash and Korn  also can go from higher to lower numbers  Probably not fastest or prettiest but works well enough  Handles negatives too   function num range        Return a range of whole numbers from beginning value to ending value        gt  gt  gt  num range start end      start  Whole number to start with       end  Whole number to end with     typeset s e v    s   1     e   2     if      e   gt     s      then       v   s        while      v   lt     e      do          echo   v             v v 1         done    elif      e   lt    s      then       v   s        while      v   gt     e      do          echo   v             v v-1         done    fi    function test num range      num range 1 3   egrep  1 2 3    assert lc 3    num range 1 3   head -1   assert eq 1    num range -1 1   head -1   assert eq  -1     num range 3 1   egrep  1 2 3    assert lc 3    num range 3 1   head -1   assert eq 3    num range 1 -1   tail -1   assert eq  -1

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I ve combined a few of the ideas here and measured performance   TL DR Takeaways    seq and      are really fast for and while loops are slow      is slow for           loops are slower        is even slower Worrying about N numbers in memory  seq or       is silly  at least up to 1 million     These are not conclusions  You would have to look at the C code behind each of these to draw conclusions  This is more about how we tend to use each of these mechanisms for looping over code  Most single operations are close enough to being the same speed that it s not going to matter in most cases  But a mechanism like for    i 1  i lt  1000000  i      is many operations as you can visually see  It is also many more operations per loop than you get from for i in   seq 1 1000000   And that may not be obvious to you  which is why doing tests like this is valuable   Demos    show that seq is fast   time  seq 1 1000000   wc   1000000 1000000 6888894  real    0m0 227s user    0m0 239s sys     0m0 008s    show that      is fast   time  echo  1  1000000    wc         1 1000000 6888896  real    0m1 778s user    0m1 735s sys     0m0 072s    Show that for loops  even with a   noop  are slow   time  for i in  1  1000000    do    done   wc         0       0       0  real    0m3 642s user    0m3 582s sys 0m0 057s    show that echo is slow   time  for i in  1  1000000    do echo  i  done   wc   1000000 1000000 6888896  real    0m7 480s user    0m6 803s sys     0m2 580s    time  for i in   seq 1 1000000    do echo  i  done   wc   1000000 1000000 6888894  real    0m7 029s user    0m6 335s sys     0m2 666s    show that C-style for loops are slower   time  for    i 1  i lt  1000000  i        do echo  i  done   wc   1000000 1000000 6888896  real    0m12 391s user    0m11 069s sys     0m3 437s    show that arithmetic expansion is even slower   time  i 1  e 1000000  while    i -le  e    do echo  i  i     i 1    done   wc   1000000 1000000 6888896  real    0m19 696s user    0m18 017s sys     0m3 806s    time  i 1  e 1000000  while    i -le  e    do echo  i    i i 1    done   wc   1000000 1000000 6888896  real    0m18 629s user    0m16 843s sys     0m3 936s    time  i 1  e 1000000  while    i -le  e    do echo    i      done   wc   1000000 1000000 6888896  real    0m17 012s user    0m15 319s sys     0m3 906s    even a noop is slow   time  i 1  e 1000000  while      i     -le  e    do    done   wc         0       0       0  real    0m12 679s user    0m11 658s sys 0m1 004s

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You can use  for i in   seq  END   do echo  i  done

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There are many ways to do this  however the ones I prefer is given below  Using seq     Synopsis from man seq     seq  -w   -f format   -s string   -t string   first  incr   last      Syntax   Full command seq first incr last   first is starting number in the sequence  is optional  by default 1  incr is increment  is optional  by default 1  last is the last number in the sequence   Example     seq 1 2 10 1 3 5 7 9   Only with first and last     seq 1 5 1 2 3 4 5   Only with last     seq 5 1 2 3 4 5   Using  first  last  incr   Here first and last are mandatory and incr is optional  Using just first and last    echo  1  5  1 2 3 4 5   Using incr    echo  1  10  2  1 3 5 7 9   You can use this even for characters like below    echo  a  z  a b c d e f g h i j k l m n o p q r s t u v w x y z

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Another layer of indirection   for i in   eval echo  1   END    do

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Replace    with         tmpstart 0  tmpend 4   for    i  tmpstart  i lt   tmpend  i        do  echo  i   done   Yields   0 1 2 3 4

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The seq method is the simplest  but Bash has built-in arithmetic evaluation  END 5 for   i 1 i lt  END i      do     echo  i done      gt  outputs 1 2 3 4 5 on separate lines  The for   expr1 expr2 expr3    construct works just like for  expr1 expr2 expr3  in C and similar languages  and like other   expr   cases  Bash treats them as arithmetic

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This is another way   end 5 for i in   bash -c  echo  1    end      do echo  i  done

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Here is why the original expression didn t work   From man bash      Brace expansion is performed before   any other expansions  and any   characters special to  other    expansions  are    preserved in the   result   It is strictly textual   Bash   does not apply any syntactic   interpretation to the context of   the expansion or the text between the   braces    So  brace expansion is something done early as a purely textual macro operation  before parameter expansion   Shells are highly optimized hybrids between macro processors and more formal programming languages   In order to optimize the typical use cases  the language is made rather more complex and some limitations are accepted      Recommendation   I would suggest sticking with Posix1 features  This means using for i in  lt list gt   do  if the list is already known  otherwise  use while or seq  as in      bin sh  limit 4  i 1  while    i -le  limit    do   echo  i   i     i   1   done   Or ----------------------- for i in   seq 1  limit   do   echo  i done    1  Bash is a great shell and I use it interactively  but I don t put bash-isms into my scripts   Scripts might need a faster shell  a more secure one  a more embedded-style one  They might need to run on whatever is installed as  bin sh  and then there are all the usual pro-standards arguments  Remember shellshock  aka bashdoor

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The seq method is the simplest  but Bash has built-in arithmetic evaluation  END 5 for   i 1 i lt  END i      do     echo  i done      gt  outputs 1 2 3 4 5 on separate lines  The for   expr1 expr2 expr3    construct works just like for  expr1 expr2 expr3  in C and similar languages  and like other   expr   cases  Bash treats them as arithmetic

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These are all nice but seq is supposedly deprecated and most only work with numeric ranges   If you enclose your for loop in double quotes  the start and end variables will be dereferenced when you echo the string  and you can ship the string right back to BASH for execution   i needs to be escaped with   s so it is NOT evaluated before being sent to the subshell   RANGE START a RANGE END z echo -e  for i in   RANGE START   RANGE END   do echo     i   done    bash   This output can also be assigned to a variable   VAR  echo -e  for i in   RANGE START   RANGE END   do echo     i   done    bash    The only  overhead  this should generate should be the second instance of bash so it should be suitable for intensive operations

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I know this question is about bash  but - just for the record - ksh93 is smarter and implements it as expected     ksh -c  i 5  for x in  1   i   do echo   x   done  1 2 3 4 5   ksh -c  echo  KSH VERSION  Version JM 93u  2012-02-29    bash -c  i 5  for x in  1   i   do echo   x   done   1  5

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if you don t wanna use  seq  or  eval  or jot or arithmetic expansion format eg  for   i 1 i lt  END i      or other loops eg  while  and you don t wanna  printf  and happy to  echo  only  then this simple workaround might fit your budget  a 1  b 5  d  for i in    a     b    do echo -n  quot  i quot   done   echo  quot  d quot    bash PS  My bash doesn t have  seq  command anyway  Tested on Mac OSX 10 6 8  Bash 3 2 48

[bash] How do I iterate over a range of numbers defined by variables in Bash?

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