How can I execute a command stored in a variable?

by

Unix shells operate a series of transformations on each line of input before executing them. For most shells it looks something like this (taken from the Bash man page):

  • initial word splitting
  • brace expansion
  • tilde expansion
  • parameter, variable and arithmetic expansion
  • command substitution
  • secondary word splitting
  • path expansion (aka globbing)
  • quote removal

Using $cmd directly gets it replaced by your command during the parameter expansion phase, and it then undergoes all following transformations.

Using eval "$cmd" does nothing until the quote removal phase, where $cmd is returned as is, and passed as a parameter to eval, whose function is to run the whole chain again before executing.

So basically, they’re the same in most cases and differ when your command makes use of the transformation steps up to parameter expansion. For example, using brace expansion:

$ cmd="echo foo{bar,baz}"

$ $cmd
foo{bar,baz}

$ eval "$cmd"
foobar foobaz

More Related Contents:

  1. How to select lines between two marker patterns which may occur multiple times with awk/sed
  2. Find and replace in file and overwrite file doesn’t work, it empties the file
  3. How to use multiple arguments for awk with a shebang (i.e. #!)?
  4. What are the uses of the exec command in shell scripts? [closed]
  5. How can I reverse the order of lines in a file?
  6. How to make grep only match if the entire line matches?
  7. How to remove carriage return from a variable in shell script
  8. How/When does Execute Shell mark a build as failure in Jenkins?
  9. Compare two files line by line and generate the difference in another file
  10. Alternative to `sed -i` on Solaris
  11. Floating-point arithmetic in UNIX shell script
  12. Run java jar file on a server as background process
  13. How do you tell if a string contains another string in POSIX sh?
  14. How does the #! shebang work?
  15. Concise and portable “join” on the Unix command-line
  16. Unix find command, what are the {} and \; for?
  17. Compare a string using sh shell
  18. Shell one liner to prepend to a file
  19. How is % (percent sign) special in crontab?
  20. What does the line “#!/bin/sh” mean in a UNIX shell script?
  21. Trim leading and trailing spaces from a string in awk
  22. Can I call a function of a shell script from another shell script?
  23. Remove part of path on Unix
  24. What’s the difference between ${varname} and $varname in a shell scripts
  25. Which terminal command to get just IP address and nothing else?
  26. How do I determine if a web page exists with shell scripting?
  27. How to add text at the end of each line in unix
  28. Call a Unix Script from Excel Vba
  29. How to print 5 consecutive lines after a pattern in file using awk [duplicate]
  30. Unix shell script find out which directory the script file resides?

Leave a Comment