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Examples

I've recently found examples of using pairs of enclosing braces {}, with nothing in between the opening and closing braces, as arguments to commands and even to their options:

cat foo | xargs -I{} echo {}

find . -size 0 -exec rm -i {} ;

No Documentation

My problem is that I cannot find a documentation in the GNU Bash Manual that describes the usage of {} in such context as in the examples above.

I do not think it is a parameter expansion, because a dollar sign must precede the enclosing braces in a parameter expansion as in ${}.

It cannot be a brace expansion either, because it takes the form of {x..y[..incr]}, where x and y are not optional.

It also cannot be a command grouping either, because {} is used as arguments.

Questions

1. What does a pair of enclosing braces {} even mean, in general, as an argument to any command that accepts it?




2. Where can I find a documentation that describes the usage of {} as arguments?

These curly braces are left alone by bash; they belong to find and xargs, respectively, and are described in their man-pages.

man find

   -exec command ;

          Execute command; true if 0 status is returned.  All following arguments to find are

          taken to be arguments to the command until an argument consisting of `;' is encoun‐

          tered.  The string `{}' is replaced by the current file name being processed every‐

          where it occurs in the arguments to the command, not just in arguments where it  is

          alone,  as  in some versions of find.  Both of these constructions might need to be

          escaped (with a `') or quoted to protect them from expansion by  the  shell.   See

          the  EXAMPLES  section  for examples of the use of the -exec option.  The specified

          command is run once for each matched file.  The command is executed in the starting

          directory.    There  are unavoidable security problems surrounding use of the -exec

          action; you should use the -execdir option instead.



   -exec command {} +

          This variant of the -exec action runs the specified command on the selected  files,

          but  the command line is built by appending each selected file name at the end; the

          total number of invocations of the command will be much less  than  the  number  of

          matched  files.   The  command line is built in much the same way that xargs builds

          its command lines.  Only one instance of `{}' is allowed within the  command.   The

          command  is  executed in the starting directory.  If find encounters an error, this

          can sometimes cause an immediate exit, so some pending commands may not be  run  at

          all.  This variant of -exec always returns true.



   -execdir command ;



   -execdir command {} +

          Like  -exec,  but the specified command is run from the subdirectory containing the

          matched file, which is not normally the directory in which you started find.   This

          a  much more secure method for invoking commands, as it avoids race conditions dur‐

          ing resolution of the paths to the matched files.  As with the  -exec  action,  the

          `+'  form  of  -execdir  will build a command line to process more than one matched

          file, but any given invocation of command will only list files that  exist  in  the

          same  subdirectory.   If you use this option, you must ensure that your $PATH envi‐

          ronment variable does not reference `.'; otherwise, an attacker can  run  any  com‐

          mands  they like by leaving an appropriately-named file in a directory in which you

          will run -execdir.  The same applies to having entries in $PATH which are empty  or

          which  are  not  absolute  directory  names.  If find encounters an error, this can

          sometimes cause an immediate exit, so some pending commands may not be run at  all.

          The  result  of the action depends on whether the + or the ; variant is being used;

          -execdir command {} + always returns true, while -execdir command {} ; returns true

          only if command returns 0.

man xargs

   -I replace-str

          Replace occurrences of replace-str in the initial-arguments with  names  read  from

          standard  input.   Also,  unquoted blanks do not terminate input items; instead the

          separator is the newline character.  Implies -x and -L 1.



   -i[replace-str], --replace[=replace-str]

          This option is a synonym for -Ireplace-str if replace-str is specified.  If the re‐

          place-str argument is missing, the effect is the same as -I{}.  This option is dep‐

          recated; use -I instead.