eval command in Bash and its typical uses

Question

After reading the bash man pages and with respect to this post   I am still having trouble understanding what exactly the eval command does and which would be its typical uses  For example if we do   bash  set -- one two three    sets  1  2  3 bash  echo  1 one bash  n 1 bash  echo    n           First attempt to echo  1 using brackets fails bash     n   bad substitution bash  echo    n           Second attempt to echo  1 using parentheses fails bash  1  command not found bash  eval echo     n     Third attempt to echo  1 using  eval  succeeds one   What exactly is happening here and how do the dollar sign and the backslash tie into the problem

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Simply think of eval as  evaluating your expression one additional time before execution   eval echo     n  becomes echo  1 after the first round of evaluation  Three changes to notice    The    became    The backslash is needed  otherwise it tries to evaluate    n   which means a variable named   n   which is not allowed   n was evaluated to 1 The eval disappeared   In the second round  it is basically echo  1 which can be directly executed   So eval  lt some command gt  will first evaluate  lt some command gt   by evaluate here I mean substitute variables  replace escaped characters with the correct ones etc    and then run the resultant expression once again   eval is used when you want to dynamically create variables  or to read outputs from programs specifically designed to be read like this  See http   mywiki wooledge org BashFAQ 048 for examples  The link also contains some typical ways in which eval is used  and the risks associated with it

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You asked about typical uses   One common complaint about shell scripting is that you  allegedly  can t pass by reference to get values back out of functions   But actually  via  eval   you can pass by reference   The callee can pass back a list of variable assignments to be evaluated by the caller   It is pass by reference because the caller can allowed to specify the name s  of the result variable s  - see example below   Error results can be passed back standard names like errno and errstr     Here is an example of passing by reference in bash      bin bash isint         re    -   0-9             1     re        args 1  name of result variable  2  first addend  3  second addend  iadd         if isint   2   amp  amp  isint   3    then         echo   1      2    3    errno 0          return 0     else         echo  errstr   Error  non-integer argument to iadd        errno 329          return 1     fi    var 1 echo   1  var  var   eval   iadd var A B  if     errno -ne 0     then     echo  errstr  errstr      echo  errno  errno  fi echo   2  var  var  unchanged after error    eval   iadd var  var 1  if     errno -ne 0     then     echo  errstr  errstr      echo  errno  errno  fi   echo   3  var  var  successfully changed     The output looks like this    1  var 1 errstr Error  non-integer argument to iadd var A B errno 329  2  var 1  unchanged after error   3  var 2  successfully changed    There is almost unlimited band width in that text output   And there are more possibilities if the multiple output lines are used  e g   the first line could be used for variable assignments  the second for continuous  stream of thought   but that s beyond the scope of this post

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eval takes a string as its argument  and evaluates it as if you d typed that string on a command line   If you pass several arguments  they are first joined with spaces between them       n  is a syntax error in bash  Inside the braces  you can only have a variable name  with some possible prefix and suffixes  but you can t have arbitrary bash syntax and in particular you can t use variable expansion  There is a way of saying    the value of the variable whose name is in this variable     though   echo    n  one          runs the command specified inside the parentheses in a subshell  i e  in a separate process that inherits all settings such as variable values from the current shell   and gathers its output  So echo    n  runs  n as a shell command  and displays its output  Since  n evaluates to 1     n  attempts to run the command 1  which does not exist   eval echo     n  runs the parameters passed to eval  After expansion  the parameters are echo and   1   So eval echo     n  runs the command echo   1    Note that most of the time  you must use double quotes around variable substitutions and command substitutions  i e  anytime there s a       foo      foo    Always put double quotes around variable and command substitutions  unless you know you need to leave them off  Without the double quotes  the shell performs field splitting  i e  it splits value of the variable or the output from the command into separate words  and then treats each word as a wildcard pattern  For example     ls file1 file2 otherfile   set --  f       echo   1  f      echo  1 file1 file2 file1 file2 otherfile   n 1   eval echo     n  file1 file2 file1 file2 otherfile  eval echo       n    f      echo     n   f      eval is not used very often  In some shells  the most common use is to obtain the value of a variable whose name is not known until runtime  In bash  this is not necessary thanks to the    VAR  syntax  eval is still useful when you need to construct a longer command containing operators  reserved words  etc

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Update  Some people say one should -never- use eval  I disagree  I think the risk arises when corrupt input can be passed to eval  However there are many common situations where that is not a risk  and therefore it is worth knowing how to use eval in any case  This stackoverflow answer explains the risks of eval and alternatives to eval  Ultimately it is up to the user to determine if when eval is safe and efficient to use     The bash eval statement allows you to execute lines of code calculated or acquired  by your bash script   Perhaps the most straightforward example would be a bash program that opens another bash script as a text file  reads each line of text  and uses eval to execute them in order  That s essentially the same behavior as the bash source statement  which is what one would use  unless it was necessary to perform some kind of transformation  e g  filtering or substitution  on the content of the imported script   I rarely have needed eval  but I have found it useful to read or write variables whose names were contained in strings assigned to other variables  For example  to perform actions on sets of variables  while keeping the code footprint small and avoiding redundancy   eval is conceptually simple  However  the strict syntax of the bash language  and the bash interpreter s parsing order can be nuanced and make eval appear cryptic and difficult to use or understand  Here are the essentials    The argument passed to eval is a string expression that is calculated at runtime  eval will execute the final parsed result of its argument as an actual line of code in your script  Syntax and parsing order are stringent  If the result isn t an executable line of bash code  in scope of your script  the program will crash on the eval statement as it tries to execute garbage  When testing you can replace the eval statement with echo and look at what is displayed  If it is legitimate code in the current context  running it through eval will work      The following examples may help clarify how eval works     Example 1    eval statement in front of  normal  code is a NOP    eval a b   eval echo  a b   In the above example  the first eval statements has no purpose and can be eliminated  eval is pointless in the first line because there is no dynamic aspect to the code  i e  it already parsed into the final lines of bash code  thus it would be identical as a normal statement of code in the bash script   The 2nd eval is pointless too  because  although there is a parsing step converting  a to its literal string equivalent  there is no indirection  e g  no referencing via string value of an actual bash noun or bash-held script variable   so it would behave identically as a line of code without the eval prefix       Example 2    Perform var assignment using var names passed as string values     key  mykey    val  myval    eval  key  val   echo  mykey myval   If you were to echo  key  val  the output would be   mykey myval   That  being the final result of string parsing  is what will be executed by eval  hence the result of the echo statement at the end         Example 3   Adding more indirection to Example 2    keyA  keyB    valA  valB    keyB  that    valB  amazing    eval eval    keyA    valA   echo  that amazing   The above is a bit more complicated than the previous example  relying more heavily on the parsing-order and peculiarities of bash   The eval line would roughly get parsed internally in the following order  note the following statements are pseudocode  not real code  just to attempt to show how the statement would get broken down into steps internally to arrive at the final result     eval eval    keyA    valA    substitution of  keyA and  valA by interpreter  eval eval   keyB   valB      convert       name-strings to real vars by eval  eval  keyB  valB             substitution of  keyB and  valB by interpreter  eval that amazing            execute string literal  that amazing  by eval   If the assumed parsing order doesn t explain what eval is doing enough  the third example may describe the parsing in more detail to help clarify what is going on       Example 4    Discover whether vars  whose names are contained in strings  themselves contain string values   a  User-provided  b  Another user-provided optional value  c     myvarname a  a  myvarname b  b  myvarname c  c   for varname in  myvarname a   myvarname b   myvarname c   do     eval varval    varname     if   -z   varval     then         read -p   varname     varname     fi done   In the first iteration   varname  myvarname a    Bash parses the argument to eval  and eval sees literally this at runtime   eval varval    myvarname a   The following pseudocode attempts to illustrate how bash interprets the above line of real code  to arrive at the final value executed by eval   the following lines descriptive  not exact bash code    1  eval varval          varname         This substitution resolved in eval statement 2                       myvarname a      myvarname a previously resolved by for-loop 3                      a                    to this value 4  eval  varval  a                      This requires one more parsing step 5  eval varval  User-provided           Final result of parsing  eval executes this    Once all the parsing is done  the result is what is executed  and its effect is obvious  demonstrating there is nothing particularly mysterious about eval itself  and the complexity is in the parsing of its argument   varval  User-provided    The remaining code in the example above simply tests to see if the value assigned to  varval is null  and  if so  prompts the user to provide a value

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In my experience  a  typical  use of eval is for running commands that generate shell commands to set environment variables   Perhaps you have a system that uses a collection of environment variables  and you have a script or program that determines which ones should be set and their values  Whenever you run a script or program  it runs in a forked process  so anything it does directly to environment variables is lost when it exits  But that script or program can send the export commands to stdout   Without eval  you would need to redirect stdout to a temp file  source the temp file  and then delete it  With eval  you can just   eval    script-or-program     Note the quotes are important   Take this  contrived  example     activate sh echo  I got activated      test py print  export foo bar baz womp   print    activate sh      eval   python test py  bash  export       not a valid identifier bash  export   activate sh   not a valid identifier   eval    python test py   I got activated

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In the question   who   grep   tty   sed s  dev       outputs errors claiming that files a and tty do not exist  I understood this to mean that tty is not being interpreted before execution of grep  but instead that bash passed tty as a parameter to grep  which interpreted it as a file name   There is also a situation of nested redirection  which should be handled by matched parentheses which should specify a child process  but bash is primitively a word separator  creating parameters to be sent to a program  therefore parentheses are not matched first  but interpreted as seen   I got specific with grep  and specified the file as a parameter instead of using a pipe  I also simplified the base command  passing output from a command as a file  so that i o piping would not be nested   grep   tty   sed s  dev      lt  who    works well   who   grep   echo pts 3    is not really desired  but eliminates the nested pipe and also works well   In conclusion  bash does not seem to like nested pipping  It is important to understand that bash is not a new-wave program written in a recursive manner  Instead  bash is an old 1 2 3 program  which has been appended with features  For purposes of assuring backward compatibility  the initial manner of interpretation has never been modified  If bash was rewritten to first match parentheses  how many bugs would be introduced into how many bash programs  Many programmers love to be cryptic

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The eval statement tells the shell to take eval   s arguments as command and run them through the command-line  It is useful in a situation like below   In your script if you are defining a command into a variable and later on you want to use that command then you should use eval    home user1  gt  a  ls   more   home user1  gt   a bash  command not found  ls   more  home user1  gt    Above command didn t work as ls tried to list file with name pipe     and more  But these files are not there  home user1  gt  eval  a file txt mailids remote cmd sh sample txt tmp  home user1  gt

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I originally intentionally never learned how to use eval  because most people will recommend to stay away from it like the plague  However I recently discovered a use case that made me facepalm for not recognizing it sooner   If you have cron jobs that you want to run interactively to test  you might view the contents of the file with cat  and copy and paste the cron job to run it  Unfortunately  this involves touching the mouse  which is a sin in my book   Lets say you have a cron job at  etc cron d repeatme with the contents     10         root program arg1 arg2  You cant execute this as a script with all the junk in front of it  but we can use cut to get rid of all the junk  wrap it in a subshell  and execute the string with eval  eval    cut -d     -f 6-  etc cron d repeatme   The cut command only prints out the 6th field of the file  delimited by spaces  Eval then executes that command   I used a cron job here as an example  but the concept is to format text from stdout  and then evaluate that text   The use of eval in this case is not insecure  because we know exactly what we will be evaluating before hand

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I ve recently had to use eval to force multiple brace expansions to be evaluated in the order I needed  Bash does multiple brace expansions from left to right  so  xargs -I  cat    11  15   8  5  jpg   expands to  xargs -I  cat   11 8 jpg   11 7 jpg   11 6 jpg   11 5 jpg   12 8 jpg   12 7 jpg   12 6 jpg   12 5 jpg   13 8 jpg   13 7 jpg   13 6 jpg   13 5 jpg   14 8 jpg   14 7 jpg   14 6 jpg   14 5 jpg   15 8 jpg   15 7 jpg   15 6 jpg   15 5 jpg   but I needed the second brace expansion done first  yielding  xargs -I  cat   11 8 jpg   12 8 jpg   13 8 jpg   14 8 jpg   15 8 jpg   11 7 jpg   12 7 jpg   13 7 jpg   14 7 jpg   15 7 jpg   11 6 jpg   12 6 jpg   13 6 jpg   14 6 jpg   15 6 jpg   11 5 jpg   12 5 jpg   13 5 jpg   14 5 jpg   15 5 jpg   The best I could come up with to do that was  xargs -I  cat   eval echo     11  15    8  5  jpg    This works because the single quotes protect the first set of braces from expansion during the parsing of the eval command line  leaving them to be expanded by the subshell invoked by eval   There may be some cunning scheme involving nested brace expansions that allows this to happen in one step  but if there is I m too old and stupid to see it

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I like the  evaluating your expression one additional time before execution  answer  and would like to clarify with another example   var    par1 par2    echo  var   prints nicely  par1 par2   function cntpars       echo     gt  Count        echo     gt  Pars         echo     gt  par1    1    echo     gt  par2    2     if         1  amp  amp   1    par1 par2      then     echo     gt  PASS    else     echo     gt  FAIL      return 1   fi      Option 1  Will Pass echo  eval   cntpars   var    eval  cntpars  var     Option 2  Will Fail  with curious results echo  cntpars   var  cntpars  var   The Curious results in Option 2 are that we would have passed 2 parameters as follows    First Parameter   value Second Parameter  content    How is that for counter intuitive  The additional eval will fix that   Adapted from https   stackoverflow com a 40646371 744133

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