How to concatenate string variables in Bash

Question

In PHP  strings are concatenated together as follows    foo    Hello    foo      World     Here   foo becomes  Hello World    How is this accomplished in Bash

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Here is the one through AWK     foo  Hello    foo   awk -v var  foo  BEGIN print var  World       echo  foo Hello World

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foo  Hello   foo   foo World     nbsp  nbsp  nbsp  nbsp  nbsp

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If you want to append something like an underscore  use escape      FILEPATH  opt myfile   This does not work                         echo  FILEPATH  DATEX   This works fine                              echo  FILEPATH    DATEX

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If what you are trying to do is to split a string into several lines  you can use a backslash     a  hello   gt  world    echo  a helloworld   With one space in between     a  hello    gt  world    echo  a hello world   This one also adds only one space in between     a  hello    gt       world    echo  a hello world

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bla hello laber kthx echo    bla ohai  laber bye    Will output  helloohaikthxbye   This is useful when       blaohai leads to a variable not found error  Or if you have spaces or other special characters in your strings     foo   properly escapes anything you put into it

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You can do this too     var  myscript     echo  var  myscript     var   var  sh    echo  var  myscript sh

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Note that this won t work  foo HELLO bar WORLD foobar PREFIX  foo  bar   as it seems to drop  foo and leaves you with       PREFIX WORLD   but this will work   foobar PREFIX   foo    bar    and leave you with the correct output       PREFIX HELLO WORLD

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Safer way   a  AAAAAAAAAAAA  b  BBBBBBBBBBBB  c  CCCCCCCCCCCC  d  DD DD  s    a   b   c   d   echo   s  AAAAAAAAAAAABBBBBBBBBBBBCCCCCCCCCCCCDD DD   Strings containing spaces can become part of command  use   XXX  and    XXX   to avoid these errors   Plus take a look at other answer about

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a  Hello   a  a  World   echo  a   This is how you concatenate two strings

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Bash first  As this question stand specifically for Bash  my first part of the answer would present different ways of doing this properly       Append to variable  The syntax    may be used in different ways   Append to string var        Because I am frugal  I will only use two variables foo and a and then re-use the same in the whole answer   -   a 2 a  4 echo  a 24   Using the Stack nbsp Overflow question syntax   foo  Hello  foo    World  echo  foo Hello World   works fine   Append to an integer   var         variable a is a string  but also an integer  echo  a 24   a  12   echo  a 36   Append to an array var         Our a is also an array of only one element   echo   a     36  a   18   echo   a     36 18 echo   a 0   36 echo   a 1   18   Note that between parentheses  there is a space separated array  If you want to store a string containing spaces in your array  you have to enclose them   a   one word  hello world     bash      event not found   Hmm   this is not a bug  but a feature    To prevent bash to try to develop     you could   a   one word  hello world    hello world     hello world 041    declare -p a declare -a a    0   36   1   18   2   one   3   word   4   hello world    5   h ello world    6   hello world       printf  Re-construct variable using the builtin command  The printf builtin command gives a powerful way of drawing string format  As this is a Bash builtin  there is a option for sending formatted string to a variable instead of printing on stdout   echo   a     36 18 one word hello world  hello world  hello world    There are seven strings in this array  So we could build a formatted string containing exactly seven positional arguments   printf -v a   s    s     s    s     s     s     s      a      echo  a 36   18    one   word    hello world     hello world     hello world     Or we could use one argument format string which will be repeated as many argument submitted     Note that our a is still an array  Only first element is changed   declare -p a declare -a a    0   36   18       one         word          hello world            hello world           hello world        1   18   2   one   3   word   4   hel lo world    5   hello world    6   hello world       Under bash  when you access a variable name without specifying index  you always address first element only   So to retrieve our seven field array  we only need to re-set 1st element   a 36 declare -p a declare -a a    0   36   1   18   2   one   3   word   4   hello world    5   he llo world    6   hello world       One argument format string with many argument passed to   printf -v a 0    lt  s gt  n     a      echo   a   lt 36 gt   lt 18 gt   lt one gt   lt word gt   lt hello world  gt   lt hello world  gt   lt hello world  gt    Using the Stack nbsp Overflow question syntax   foo  Hello  printf -v foo   s World   foo echo  foo Hello World   Nota  The use of double-quotes may be useful for manipulating strings that contain spaces  tabulations and or newlines  printf -v foo   s World    foo    Shell now  Under POSIX shell  you could not use bashisms  so there is no builtin printf   Basically  But you could simply do   foo  Hello  foo   foo World  echo  foo Hello World   Formatted  using forked printf  If you want to use more sophisticated constructions you have to use a fork  new child process that make the job and return the result via stdout    foo  Hello  foo   printf   s World    foo   echo  foo Hello World   Historically  you could use backticks for retrieving result of a fork   foo  Hello  foo  printf   s World    foo   echo  foo Hello World   But this is not easy for nesting   foo  Today is    foo   printf   s  s    foo     date    echo  foo Today is  Sun Aug 4 11 58 23 CEST 2013   with backticks  you have to escape inner forks with backslashes   foo  Today is    foo  printf   s  s    foo     date     echo  foo Today is  Sun Aug 4 11 59 10 CEST 2013

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I do it this way when convenient  Use an inline command   echo  The current time is  date   echo  Current User   echo  USER

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var1  hello  var2  world  var3  var1    var2  echo  var3

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There are voiced concerns about performance  but no data is offered  Let me suggest a simple test    NOTE  date on macOS does not offer nanoseconds  so this must be done on Linux    I have created append test sh on GitHub with the contents      bin bash -e  output        ptime  ctime      ctime   date   s  N       delta   bc  lt  lt  lt   ctime -  ptime        printf   2s   16s chars  time   s  delta   s n   n    bc  lt  lt  lt  10  2  n      ctime  delta     method1        echo  Method  a   a a       for n in  1  32   do a   a a   output  done    method2        echo  Method  a    a       for n in  1  32   do a    a    output  done    ctime 0  a  0123456789   time method 1   Test 1       append test sh 1 Method  a   a a   1                20 chars  time  1513640431 861671143  delta  1513640431 861671143  2                40 chars  time  1513640431 865036344  delta   003365201  3                80 chars  time  1513640431 868200952  delta   003164608  4               160 chars  time  1513640431 871273553  delta   003072601  5               320 chars  time  1513640431 874358253  delta   003084700  6               640 chars  time  1513640431 877454625  delta   003096372  7              1280 chars  time  1513640431 880551786  delta   003097161  8              2560 chars  time  1513640431 883652169  delta   003100383  9              5120 chars  time  1513640431 886777451  delta   003125282 10             10240 chars  time  1513640431 890066444  delta   003288993 11             20480 chars  time  1513640431 893488326  delta   003421882 12             40960 chars  time  1513640431 897273327  delta   003785001 13             81920 chars  time  1513640431 901740563  delta   004467236 14            163840 chars  time  1513640431 907592388  delta   005851825 15            327680 chars  time  1513640431 916233664  delta   008641276 16            655360 chars  time  1513640431 930577599  delta   014343935 17           1310720 chars  time  1513640431 954343112  delta   023765513 18           2621440 chars  time  1513640431 999438581  delta   045095469 19           5242880 chars  time  1513640432 086792464  delta   087353883 20          10485760 chars  time  1513640432 278492932  delta   191700468 21          20971520 chars  time  1513640432 672274631  delta   393781699 22          41943040 chars  time  1513640433 456406517  delta   784131886 23          83886080 chars  time  1513640435 012385162  delta  1 555978645 24         167772160 chars  time  1513640438 103865613  delta  3 091480451 25         335544320 chars  time  1513640444 267009677  delta  6 163144064   append test sh  fork  Cannot allocate memory   Test 2       append test sh 2 Method  a    a   1                20 chars  time  1513640473 460480052  delta  1513640473 460480052  2                40 chars  time  1513640473 463738638  delta   003258586  3                80 chars  time  1513640473 466868613  delta   003129975  4               160 chars  time  1513640473 469948300  delta   003079687  5               320 chars  time  1513640473 473001255  delta   003052955  6               640 chars  time  1513640473 476086165  delta   003084910  7              1280 chars  time  1513640473 479196664  delta   003110499  8              2560 chars  time  1513640473 482355769  delta   003159105  9              5120 chars  time  1513640473 485495401  delta   003139632 10             10240 chars  time  1513640473 488655040  delta   003159639 11             20480 chars  time  1513640473 491946159  delta   003291119 12             40960 chars  time  1513640473 495354094  delta   003407935 13             81920 chars  time  1513640473 499138230  delta   003784136 14            163840 chars  time  1513640473 503646917  delta   004508687 15            327680 chars  time  1513640473 509647651  delta   006000734 16            655360 chars  time  1513640473 518517787  delta   008870136 17           1310720 chars  time  1513640473 533228130  delta   014710343 18           2621440 chars  time  1513640473 560111613  delta   026883483 19           5242880 chars  time  1513640473 606959569  delta   046847956 20          10485760 chars  time  1513640473 699051712  delta   092092143 21          20971520 chars  time  1513640473 898097661  delta   199045949 22          41943040 chars  time  1513640474 299620758  delta   401523097 23          83886080 chars  time  1513640475 092311556  delta   792690798 24         167772160 chars  time  1513640476 660698221  delta  1 568386665 25         335544320 chars  time  1513640479 776806227  delta  3 116108006   append test sh  fork  Cannot allocate memory   The errors indicate that my Bash got up to 335 54432 MB before it crashed  You could change the code from doubling the data to appending a constant to get a more granular graph and failure point  But I think this should give you enough information to decide whether you care  Personally  below 100 nbsp MB I don t  Your mileage may vary

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In my opinion  the simplest way to concatenate two strings is to write a function that does it for you  then use that function   function concat          prefix  1     suffix  2      echo    prefix   suffix      foo  Super  bar  man   concat  foo  bar     Superman  alien   concat  foo  bar   echo  alien          Superman

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Despite of the special operator      for concatenation  there is a simpler way to go   foo  Hello  foo  foo  World  echo  foo   Double quotes take an extra calculation time for interpretation of variables inside  Avoid it if possible

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Even if the    operator is now permitted  it has been introduced in Bash 3 1 in 2004   Any script using this operator on older Bash versions will fail with a  command not found  error if you are lucky  or a  syntax error near unexpected token    For those who cares about backward compatibility  stick with the older standard Bash concatenation methods  like those mentioned in the chosen answer   foo  Hello  foo   foo World  echo  foo  gt  Hello World

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I kind of like making a quick function       bin sh -f function combo         echo       echo   combo  foo  bar     Yet another way to skin a cat  This time with functions  D

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You can concatenate without the quotes  Here is an example    Variable1 Open  Variable2 Systems  Variable3  Variable1 Variable2  echo  Variable3   This last statement would print  OpenSystems   without quotes    This is an example of a Bash script   v1 hello v2 world v3   v1        v2  echo  v3              Output  hello world echo   v3             Output  hello       world

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I don t know about PHP yet  but this works in Linux Bash  If you don t want to affect it to a variable  you could try this   read pp      Assumes I will affect Hello to pp  pp    printf  pp  printf   World   printf       echo  pp    gt Hello World    You could place another variable instead of  Hello  or      You could concatenate more strings as well

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Yet another approach      gt  H  Hello    gt  U   H  universe    gt  echo  U Hello universe       and yet yet another one    gt  H  Hello    gt  U  H universe    gt  echo  U Hello universe

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a hip   b hop   ab  a b   echo  ab hiphop   echo  a b hiphop

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The simplest way with quotation marks   B Bar b bar var   B   b  a  echo  Hello    var

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I prefer to use curly brackets     for expanding variable in string   foo  Hello  foo    foo  World  echo  foo  gt  Hello World     Curly brackets will fit to Continuous string usage   foo  Hello  foo    foo World  echo  foo  gt  HelloWorld   Otherwise using foo     fooWorld  will not work

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I wanted to build a string from a list  Couldn t find an answer for that so I post it here  Here is what I did   list  1 2 3 4 5  string     for elm in    list       do     string    string    elm   done  echo   string    and then I get the following output   1 2 3 4 5

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Bash also supports a    operator as shown in this code  A  quot X Y quot  A   quot  Z quot  echo  quot  A quot   output  X Y Z

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If it is as your example of adding   World  to the original string  then it can be      bin bash  foo  Hello  foo  foo  World  echo  foo   The output   Hello World

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There s one particular case where you should take care   user daniel cat  gt  output file  lt  lt  EOF   user san EOF   Will output  daniel san  and not danielsan  as you might have wanted  In this case you should do instead   user daniel cat  gt  output file  lt  lt  EOF   user san EOF

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foo  Hello  foo    foo  World  echo    foo    gt  Hello World   In general to concatenate two variables you can just write them one after another   a  Hello  b  World  c    a    b   echo    c    gt  Hello World

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Here is a concise summary of what most answers are talking about   Let s say we have two variables and  1 is set to  one    set one two a hello b world   The table below explains the different contexts where we can combine the values of a and b to create a new variable  c   Context                                 Expression              Result  value of c  -------------------------------------- ----------------------- --------------------- Two variables                           c  a b                  helloworld A variable and a literal                c   a  world            hello world A variable and a literal                c  1world               oneworld A variable and a literal                c  a world              hello world A variable  a literal  with a space     c   a   world           hello world A more complex expression               c    a  one   b  2      hello one world 2 Using    operator  Bash 3 1 or later    c  a  c   b             helloworld Append literal with                     c  a  c    world        hello world   A few notes    enclosing the RHS of an assignment in double quotes is generally a good practice  though it is quite optional in many cases    is better from a performance standpoint if a big string is being constructed in small increments  especially in a loop use    around variable names to disambiguate their expansion  as in row 2 in the table above   As seen on rows 3 and 4  there is no need for    unless a variable is being concatenated with a string that starts with a character that is a valid first character in shell variable name  that is alphabet or underscore      See also    BashFAQ 013 - How can I concatenate two variables  When do we need curly braces around shell variables

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The way I d solve the problem is just   a b   For example   a  Hello  b   World  c  a b echo   c    which produces  Hello World   If you try to concatenate a string with another string  for example   a  Hello  c   a World    then echo   c  will produce  Hello World   with an extra space    aWorld   doesn t work  as you may imagine  but    a World   produces  HelloWorld

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