How to convert a string to lower case in Bash

Question

Is there a way in bash to convert a string into a lower case string   For example  if I have   a  Hi all    I want to convert it to    hi all

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In spite of how old this question is and similar to this answer by technosaurus. I had a hard time finding a solution that was portable across most platforms (That I Use) as well as older versions of bash. I have also been frustrated with arrays, functions and use of prints, echos and temporary files to retrieve trivial variables. This works very well for me so far I thought I would share. My main testing environments are:

GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

GNU bash, version 3.2.57(1)-release (sparc-sun-solaris2.10)

lcs="abcdefghijklmnopqrstuvwxyz"
ucs="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
input="Change Me To All Capitals"
for (( i=0; i<"${#input}"; i++ )) ; do :
    for (( j=0; j<"${#lcs}"; j++ )) ; do :
        if [[ "${input:$i:1}" == "${lcs:$j:1}" ]] ; then
            input="${input/${input:$i:1}/${ucs:$j:1}}" 
        fi
    done
done

Simple C-style for loop to iterate through the strings. For the line below if you have not seen anything like this before this is where I learned this. In this case the line checks if the char ${input:$i:1} (lower case) exists in input and if so replaces it with the given char ${ucs:$j:1} (upper case) and stores it back into input.

input="${input/${input:$i:1}/${ucs:$j:1}}"

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tr   a    tr  A-Z   a-z   lt  lt  lt    a      AWK     print tolower  0      sed   y ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz

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To store the transformed string into a variable  Following worked for me -  SOURCE NAME to  TARGET NAME   TARGET NAME   echo  SOURCE NAME   tr    upper       lower

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Regular expression  I would like to take credit for the command I wish to share but the truth is I obtained it for my own use from http   commandlinefu com  It has the advantage that if you cd to any directory within your own home folder that is it will change all files and folders to lower case recursively please use with caution  It is a brilliant command line fix and especially useful for those multitudes of albums you have stored on your drive   find   -depth -exec rename  s                 1   L 2           You can specify a directory in place of the dot    after the find which denotes current directory or full path   I hope this solution proves useful the one thing this command does not do is replace spaces with underscores - oh well another time perhaps

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For a standard shell  without bashisms  using only builtins   uppers ABCDEFGHIJKLMNOPQRSTUVWXYZ lowers abcdefghijklmnopqrstuvwxyz  lc     usage  lc  SOME STRING  - gt   some string      i 0     while     i -lt    1     do         CUR   1  i 1          case  uppers in               CUR  CUR   uppers  CUR   OUTPUT    OUTPUT   lowers    CUR  1                   OUTPUT    OUTPUT  CUR            esac         i    i 1       done     echo    OUTPUT       And for upper case   uc     usage  uc  some string  - gt   SOME STRING      i 0     while     i -lt    1     do         CUR   1  i 1          case  lowers in               CUR  CUR   lowers  CUR   OUTPUT    OUTPUT   uppers    CUR  1                   OUTPUT    OUTPUT  CUR            esac         i    i 1       done     echo    OUTPUT

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Using GNU sed   sed  s     L amp      Example     foo  Some STRIng     foo   echo   foo    sed  s     L amp       echo   foo  some string

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Many answers using external programs  which is not really using Bash   If you know you will have Bash4 available you should really just use the   VAR    notation  it is easy and cool   For Bash before 4  My Mac still uses Bash 3 2 for example   I used the corrected version of  ghostdog74  s answer to create a more portable version   One you can call lowercase  my STRING  and get a lowercase version  I read comments about setting the result to a var  but that is not really portable in Bash  since we can t return strings  Printing it is the best solution  Easy to capture with something like var    lowercase  str      How this works  The way this works is by getting the ASCII integer representation of each char with printf and then adding 32 if upper-to- gt lower  or subtracting 32 if lower-to- gt upper  Then use printf again to convert the number back to a char  From  A  -to- gt   a  we have a difference of 32 chars   Using printf to explain     printf   d n    a  97   printf   d n    A  65   97 - 65   32  And this is the working version with examples  Please note the comments in the code  as they explain a lot of stuff      bin bash    lowerupper sh    Prints the lowercase version of a char lowercaseChar        case   1  in          A-Z               n   printf   d     1               n    n 32               printf     printf   o    n                                         printf   s    1                     esac      Prints the lowercase version of a sequence of strings lowercase         word          for  i 0 i lt    word  i      do         ch    word  i 1           lowercaseChar   ch      done      Prints the uppercase version of a char uppercaseChar        case   1  in          a-z               n   printf   d     1               n    n-32               printf     printf   o    n                                         printf   s    1                     esac      Prints the uppercase version of a sequence of strings uppercase         word          for  i 0 i lt    word  i      do         ch    word  i 1           uppercaseChar   ch      done      The functions will not add a new line  so use echo or   append it if you want a new line after printing    Printing stuff directly lowercase  I AM the Walrus     n  uppercase  I AM the Walrus     n   echo  ----------     Printing a var str  A StRing WITH mixed sTUFF   lowercase   str    n  uppercase   str    n   echo  ----------     Not quoting the var should also work     since we use      inside the functions lowercase  str   n  uppercase  str   n   echo  ----------     Assigning to a var myLowerVar    lowercase  str   myUpperVar    uppercase  str   echo  myLowerVar   myLowerVar  echo  myUpperVar   myUpperVar   echo  ----------     You can even do stuff like if     option 2       lowercase  OPTION 2        then     echo  Fine  All the same   else     echo  Ops  Not the same   fi  exit 0   And the results after running this       lowerupper sh  i am the walrus  I AM THE WALRUS  ---------- a string with mixed stuff  A STRING WITH MIXED STUFF  ---------- a string with mixed stuff  A STRING WITH MIXED STUFF  ---------- myLowerVar  a string with mixed stuff  myUpperVar  A STRING WITH MIXED STUFF  ---------- Fine  All the same    This should only work for ASCII characters though    For me it is fine  since I know I will only pass ASCII chars to it  I am using this for some case-insensitive CLI options  for example

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You can try this  s  Hello World     echo  s    Hello World   a   s    echo  a    hello world   b   s    echo  b    HELLO WORLD      ref   http   wiki workassis com shell-script-convert-text-to-lowercase-and-uppercase

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Converting case is done for alphabets only  So  this should work neatly   I am focusing on converting alphabets between a-z from upper case to lower case  Any other characters should just be printed in stdout as it is     Converts the all text in path to file filename within a-z range to A-Z  For converting lower case to upper case  cat path to file filename   tr  a-z   A-Z    For converting from upper case to lower case  cat path to file filename   tr  A-Z   a-z    For example   filename   my name is xyz   gets converted to   MY NAME IS XYZ   Example 2   echo  my name is 123 karthik    tr  a-z   A-Z    Output    MY NAME IS 123 KARTHIK   Example 3   echo  my name is 123  amp  amp   amp  amp          amp  kAR2 thik    tr  a-z   A-Z    Output    MY NAME IS 123  amp  amp   amp  amp    0    amp  KAR2 THIK

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Simple way  echo  Hi all    awk    print tolower  0

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I know this is an oldish post but I made this answer for another site so I thought I d post it up here  UPPER - gt  lower  use python  b  echo  quot print   a  lower   quot    python   Or Ruby  b  echo  quot print   a  downcase quot    ruby   Or Perl  b  perl -e  quot print lc   a    quot    Or PHP  b  php -r  quot print strtolower   a    quot    Or Awk  b  echo  quot  a quot    awk    print tolower  1       Or Sed  b  echo  quot  a quot    sed  s    L amp  g    Or Bash 4  b   a     Or NodeJS  b  node -p  quot   quot  a  quot  toLowerCase   quot    You could also use dd  b  echo  quot  a quot    dd  conv lcase 2 gt   dev null   lower - gt  UPPER  use python  b  echo  quot print   a  upper   quot    python   Or Ruby  b  echo  quot print   a  upcase quot    ruby   Or Perl  b  perl -e  quot print uc   a    quot    Or PHP  b  php -r  quot print strtoupper   a    quot    Or Awk  b  echo  quot  a quot    awk    print toupper  1       Or Sed  b  echo  quot  a quot    sed  s    U amp  g    Or Bash 4  b   a     Or NodeJS  b  node -p  quot   quot  a  quot  toUpperCase   quot    You could also use dd  b  echo  quot  a quot    dd  conv ucase 2 gt   dev null   Also when you say  shell  I m assuming you mean bash but if you can use zsh it s as easy as b  a l  for lower case and b  a u  for upper case

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This is a far faster variation of JaredTS486 s approach that uses native Bash capabilities  including Bash versions  lt 4 0  to optimize his approach   I ve timed 1 000 iterations of this approach for a small string  25 characters  and a larger string  445 characters   both for lowercase and uppercase conversions   Since the test strings are predominantly lowercase  conversions to lowercase are generally faster than to uppercase   I ve compared my approach with several other answers on this page that are compatible with Bash 3 2   My approach is far more performant than most approaches documented here  and is even faster than tr in several cases   Here are the timing results for 1 000 iterations of 25 characters    0 46s for my approach to lowercase  0 96s for uppercase 1 16s for Orwellophile s approach to lowercase  1 59s for uppercase 3 67s for tr to lowercase  3 81s for uppercase 11 12s for ghostdog74 s approach to lowercase  31 41s for uppercase 26 25s for technosaurus  approach to lowercase  26 21s for uppercase 25 06s for JaredTS486 s approach to lowercase  27 04s for uppercase   Timing results for 1 000 iterations of 445 characters  consisting of the poem  The Robin  by Witter Bynner     2s for my approach to lowercase  12s for uppercase 4s for tr to lowercase  4s for uppercase 20s for Orwellophile s approach to lowercase  29s for uppercase 75s for ghostdog74 s approach to lowercase  669s for uppercase   It s interesting to note how dramatic the performance difference is between a test with predominant matches vs  a test with predominant misses 467s for technosaurus  approach to lowercase  449s for uppercase 660s for JaredTS486 s approach to lowercase  660s for uppercase   It s interesting to note that this approach generated continuous page faults  memory swapping  in Bash   Solution      bin bash set -e set -u  declare LCS  abcdefghijklmnopqrstuvwxyz  declare UCS  ABCDEFGHIJKLMNOPQRSTUVWXYZ   function lcase       local TARGET    1-     local UCHAR      local UOFFSET       while       TARGET        A-Z        do     UCHAR    BASH REMATCH 1        UOFFSET    UCS    UCHAR         TARGET    TARGET    UCHAR    LCS    UOFFSET  1      done    echo -n    TARGET      function ucase       local TARGET    1-     local LCHAR      local LOFFSET       while       TARGET        a-z        do     LCHAR    BASH REMATCH 1        LOFFSET    LCS    LCHAR         TARGET    TARGET    LCHAR    UCS    LOFFSET  1      done    echo -n    TARGET       The approach is simple  while the input string has any remaining uppercase letters present  find the next one  and replace all instances of that letter with its lowercase variant   Repeat until all uppercase letters are replaced   Some performance characteristics of my solution    Uses only shell builtin utilities  which avoids the overhead of invoking external binary utilities in a new process Avoids sub-shells  which incur performance penalties Uses shell mechanisms that are compiled and optimized for performance  such as global string replacement within variables  variable suffix trimming  and regex searching and matching   These mechanisms are far faster than iterating manually through strings Loops only the number of times required by the count of unique matching characters to be converted   For example  converting a string that has three different uppercase characters to lowercase requires only 3 loop iterations   For the preconfigured ASCII alphabet  the maximum number of loop iterations is 26 UCS and LCS can be augmented with additional characters

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Bash 5 1 provides a straight forward way to do this with the L parameter transformation    var L   So for example you can say    v  quot heLLo quot    echo  quot   v L  quot  hello   You can also do uppercase with U    v  quot hello quot    echo  quot   v U  quot  HELLO  And uppercase the first letter with u    v  quot hello quot    echo  quot   v u  quot  Hello

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In Bash 4   To lowercase    string  A FEW WORDS    echo    string    a FEW WORDS   echo    string     a few words   echo    string   AEIUO    a FeW WoRDS    string  A Few Words    declare -l string   string  string  echo   string  a few words   To uppercase    string  a few words    echo    string    A few words   echo    string     A FEW WORDS   echo    string   aeiou    A fEw wOrds    string  A Few Words    declare -u string   string  string  echo   string  A FEW WORDS   Toggle  undocumented  but optionally configurable at compile time     string  A Few Words    echo    string     a fEW wORDS   string  A FEW WORDS    echo    string    a FEW WORDS   string  a few words    echo    string    A few words   Capitalize  undocumented  but optionally configurable at compile time     string  a few words    declare -c string   string  string   echo   string  A few words   Title case     string  a few words    string   string    string    string         echo   string  A Few Words    declare -c string   string  a few words    echo    string      A Few Words    string  a FeW WOrdS    string   string      string   string     echo   string  A few words   To turn off a declare attribute  use    For example  declare  c string  This affects subsequent assignments and not the current value   The declare options change the attribute of the variable  but not the contents  The reassignments in my examples update the contents to show the changes   Edit   Added  toggle first character by word     var    as suggested by ghostdog74   Edit  Corrected tilde behavior to match Bash 4 3

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In zsh   echo  a u   Gotta love zsh

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The are various ways   POSIX standard  tr    echo   a    tr    upper       lower    hi all   AWK    echo   a    awk   print tolower  0    hi all   Non-POSIX  You may run into portability issues with the following examples   Bash 4 0    echo    a     hi all   sed    echo   a    sed -e  s         L 1   hi all   this also works    sed -e  s         L 1    lt  lt  lt    a  hi all   Perl    echo   a    perl -ne  print lc  hi all   Bash  lc        case   1  in          A-Z           n   printf   d     1           n    n 32           printf     printf   o    n                                 printf   s    1                 esac   word  I Love Bash  for  i 0 i lt    word  i     do     ch    word  i 1       lc   ch  done   Note  YMMV on this one  Doesn t work for me  GNU bash version 4 2 46 and 4 0 33  and same behaviour 2 05b 0 but nocasematch is not implemented   even with using shopt -u nocasematch   Unsetting that nocasematch causes     fooBaR      FOObar     to match OK BUT inside case weirdly  b-z  are incorrectly matched by  A-Z   Bash is confused by the double-negative   unsetting nocasematch     -

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For Bash versions earlier than 4 0  this version should be fastest  as it doesn t fork exec any commands    function string monolithic tolower      local   word  1    local   len      word     local   char    local   octal    local   decimal    local   result     for    i 0  i lt   len  i         do         char     word  i 1        case     char  in           A-Z                printf -v   decimal   d       char              printf -v   octal   03o         decimal   0x20                printf -v   char      octal                      esac         result      char     done    REPLY     result      technosaurus s answer had potential too  although it did run properly for mee

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If using v4  this is baked-in   If not  here is a simple  widely applicable solution   Other answers  and comments  on this thread were quite helpful in creating the code below     Like echo  but converts to lowercase echolcase          tr   upper     lower    lt  lt  lt              Takes one arg by reference  var name  and makes it lowercase lcase           eval    1       echo    1               tr   upper     lower           Notes    Doing  a  Hi All  and then  lcase a will do the same thing as  a    echolcase  Hi All    In the lcase function  using    1             instead of    1  allows this to work even when the string has quotes

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echo  Hi All    tr    upper       lower

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In bash 4 you can use typeset  Example   A  HELLO WORLD  typeset -l A  A

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Pre Bash 4 0  Bash Lower the Case of a string and assign to variable  VARIABLE   echo   VARIABLE    tr    upper       lower       echo   VARIABLE

[string] How to convert a string to lower case in Bash?

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