How to perform a for loop on each character in a string in Bash

Question

I have a variable like this   words            I want to make a for loop on each of the characters  one at a time  e g  first character      then character      character      etc   The only way I know is to output each character to separate line in a file  then use while read line  but this seems very inefficient    How can I process each character in a string through a for loop

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With sed on dash shell of LANG en US UTF-8  I got the followings working right     echo                  sed -e  s        1 n g                           and    echo  Hello world    sed -e  s        1 n g  H e l l o  w o r l d   Thus  output can be looped with while read       do       done  edited for sample text translate into English                  is zh TW UTF-8 encoding for              How are you  doing                a normal space character             Happy new year           a double-byte-sized full-stop followed by text description

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It is also possible to split the string into a character array using fold and then iterate over this array   for char in  echo            fold -w1   do     echo  char done

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You can use a C-style for loop   foo string for    i 0  i lt    foo   i       do   echo    foo  i 1   done      foo  expands to the length of foo    foo  i 1  expands to the substring starting at position  i of length 1

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TEXT  hello world  for i in  1     TEXT    do    echo   TEXT i   done   where  1  N  is an inclusive range     TEXT  is a number of letters in a string    TEXT i    - you can get char from string like an item from an array

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bin bash  word   echo  Your Message   fold -w 1   for letter in   word    do echo  quot   letter  is a letter quot   done  Here is the output  Y is a letter o is a letter u is a letter r is a letter M is a letter e is a letter s is a letter s is a letter a is a letter g is a letter e is a letter

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Another way is   Characters  TESTING  index 1 while    index -le    Characters    do     echo   Characters    cut -c  index -  index      index   expr  index   1  done

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I believe there is still no ideal solution that would correctly preserve all whitespace characters and is fast enough  so I ll post my answer  Using   foo  i 1  works  but is very slow  which is especially noticeable with large strings  as I will show below   My idea is an expansion of a method proposed by Six  which involves read -n1  with some changes to keep all characters and work correctly for any string   while IFS    read -r -d    -n 1 char  do           do something with  char done  lt   lt  printf  s   string     How it works    IFS    - Redefining internal field separator to empty string prevents stripping of spaces and tabs  Doing it on a same line as read means that it will not affect other shell commands  -r - Means  raw   which prevents read from treating   at the end of the line as a special line concatenation character  -d    - Passing empty string as a delimiter prevents read from stripping newline characters  Actually means that null byte is used as a delimiter  -d    is equal to -d    0   -n 1 - Means that one character at a time will be read  printf  s   string  - Using printf instead of echo -n is safer  because echo treats -n and -e as options  If you pass  -e  as a string  echo will not print anything   lt   lt       - Passing string to the loop using process substitution  If you use here-strings instead  done  lt  lt  lt    string    an extra newline character is appended at the end  Also  passing string through a pipe  printf  s   string    while      would make the loop run in a subshell  which means all variable operations are local within the loop    Now  let s test the performance with a huge string  I used the following file as a source  https   www kernel org doc Documentation kbuild makefiles txt The following script was called through time command      bin bash    Saving contents of the file into a variable named  string     This is for test purposes only  In real code  you should use    done  lt   filename   construct if you wish to read from a file    Using  string    cat makefiles txt    would strip trailing newlines  IFS    read -r -d    string  lt  makefiles txt  while IFS    read -r -d    -n 1 char  do           remake the string by adding one character at a time         new string    char  done  lt   lt  printf  s   string      confirm that new string is identical to the original diff -u makefiles txt  lt  printf  s   new string     And the result is     time   test sh  real    0m1 161s user    0m1 036s sys     0m0 116s   As we can see  it is quite fast  Next  I replaced the loop with one that uses parameter expansion   for    i 0   i lt    string   i       do     new string     string  i 1   done   The output shows exactly how bad the performance loss is     time   test sh  real    2m38 540s user    2m34 916s sys     0m3 576s   The exact numbers may very on different systems  but the overall picture should be similar

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I m surprised no one has mentioned the obvious bash solution utilizing only while and read    while read -n1 character  do     echo   character  done  lt   lt  echo -n   words     Note the use of echo -n to avoid the extraneous newline at the end  printf is another good option and may be more suitable for your particular needs  If you want to ignore whitespace then replace   words  with    words         Another option is fold  Please note however that it should never be fed into a for loop  Rather  use a while loop as follows   while read char  do     echo   char  done  lt   lt  fold -w1  lt  lt  lt   words     The primary benefit to using the external fold command  of the coreutils package  would be brevity  You can feed it s output to another command such as xargs  part of the findutils package  as follows   fold -w1  lt  lt  lt   words    xargs -I  -- echo     You ll want to replace the echo command used in the example above with the command you d like to run against each character  Note that xargs will discard whitespace by default  You can use -d   n  to disable that behavior    Internationalization  I just tested fold with some of the Asian characters and realized it doesn t have Unicode support  So while it is fine for ASCII needs  it won t work for everyone  In that case there are some alternatives   I d probably replace fold -w1 with an awk array   awk  BEGIN FS      for  i 1 i lt  NF i    print  i     Or the grep command mentioned in another answer   grep -o      Performance  FYI  I benchmarked the 3 aforementioned options  The first two were fast  nearly tying  with the fold loop slightly faster than the while loop  Unsurprisingly xargs was the slowest    75x slower   Here is the  abbreviated  test code   words   python -c  from string import ascii letters as l  print l   100     testrunner        for test in test while loop test fold loop test fold xargs test awk loop test grep loop  do         echo   test           time for    i 1  i lt      1 -100    1    i       do   test   done  gt  dev null  2 gt  amp 1   sed      d          echo     done    testrunner 100   Here are the results   test while loop real    0m5 821s user    0m5 322s sys     0m0 526s  test fold loop real    0m6 051s user    0m5 260s sys     0m0 822s  test fold xargs real    7m13 444s user    0m24 531s sys     6m44 704s  test awk loop real    0m6 507s user    0m5 858s sys     0m0 788s  test grep loop real    0m6 179s user    0m5 409s sys     0m0 921s

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var  returns the length of var    var pos N  returns N characters from pos onwards  Examples     words  abc    echo   words 0 1  a   echo   words 1 1  b   echo   words 2 1  c   so it is easy to iterate   another way     grep -o    lt  lt  lt   abc  a b c   or    grep -o    lt  lt  lt   abc    while read letter   do echo  my letter is  letter    done   my letter is a my letter is b my letter is c

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To iterate ASCII characters on a POSIX-compliant shell  you can avoid external tools by using the Parameter Expansions     bin sh  str  quot Hello World  quot   while      str  -gt 0    do     next   str        echo  quot   str  next  quot      str  next done  or str  quot Hello World  quot   while   -n  quot  str quot     do     next   str        echo  quot   str  next  quot      str  next done

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I share my solution   read word  for char in   grep -o    lt  lt  lt   word     do     echo  char done

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I ve only tested this with ascii strings  but you could do something like   while test -n   words   do    c   words 0 1        Get the first character    echo character is    c      words   words 1      trim the first character done

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sed works with unicode IFS    n  for z in   sed  s    amp  n g   lt  printf          do  echo hello   quot  z quot  done  outputs hello    hello    hello

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Another approach  if you don t care about whitespace being ignored   for char in   sed -E s         1   g  lt  lt  lt   your string    do       Handle  char here done

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The C style loop in  chepner s answer is in the shell function update terminal cwd  and the grep -o   solution is clever  but I was surprised not to see a solution using seq   Here s mine   read word for i in   seq 1    word    do   echo    word i-1 1   done

[bash] How to perform a for loop on each character in a string in Bash?

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