Pass commands as input to another command (su, ssh, sh, etc)

by

Adding to tripleee‘s answer:

It is important to remember that the section of the script formatted as a here-document for another shell is executed in a different shell with its own environment (and maybe even on a different machine).

If that block of your script contains parameter expansion, command substitution, and/or arithmetic expansion, then you must use the here-document facility of the shell slightly differently, depending on where you want those expansions to be performed.

1. All expansions must be performed within the scope of the parent shell.

Then the delimiter of the here document must be unquoted.

command <<DELIMITER
...
DELIMITER

Example:

#!/bin/bash

a=0
mylogin=$(whoami)
sudo sh <<END
    a=1
    mylogin=$(whoami)
    echo a=$a
    echo mylogin=$mylogin
END
echo a=$a
echo mylogin=$mylogin

Output:

a=0
mylogin=leon
a=0
mylogin=leon

2. All expansions must be performed within the scope of the child shell.

Then the delimiter of the here document must be quoted.

command <<'DELIMITER'
...
DELIMITER

Example:

#!/bin/bash

a=0
mylogin=$(whoami)
sudo sh <<'END'
    a=1
    mylogin=$(whoami)
    echo a=$a
    echo mylogin=$mylogin
END
echo a=$a
echo mylogin=$mylogin

Output:

a=1
mylogin=root
a=0
mylogin=leon

3. Some expansions must be performed in the child shell, some – in the parent.

Then the delimiter of the here document must be unquoted and you must escape those expansion expressions that must be performed in the child shell.

Example:

#!/bin/bash

a=0
mylogin=$(whoami)
sudo sh <<END
    a=1
    mylogin=\$(whoami)
    echo a=$a
    echo mylogin=\$mylogin
END
echo a=$a
echo mylogin=$mylogin

Output:

a=0
mylogin=root
a=0
mylogin=leon

More Related Contents:

  1. Difference between sh and Bash
  2. How to run the sftp command with a password from Bash script?
  3. How can I check if a directory exists in a Bash shell script?
  4. Why is testing “$?” to see if a command succeeded or not, an anti-pattern?
  5. Reading quoted/escaped arguments correctly from a string
  6. Why do you need ./ (dot-slash) before executable or script name to run it in bash?
  7. What is the cleanest way to ssh and run multiple commands in Bash?
  8. Bash tool to get nth line from a file
  9. How can I remove the extension of a filename in a shell script?
  10. How can I declare and use Boolean variables in a shell script?
  11. How do I run a shell script without using “sh” or “bash” commands?
  12. Assignment of variables with space after the (=) sign?
  13. How to get a password from a shell script without echoing
  14. How to suppress Terminated message after killing in bash?
  15. How does Ctrl-C terminate a child process?
  16. Delete files older than 10 days using shell script in Unix [duplicate]
  17. Copy folder recursively, excluding some folders
  18. Variable interpolation in the shell
  19. How do I pause my shell script for a second before continuing?
  20. Is there a way to make bash job control quiet?
  21. What does ” 2>&1 ” mean?
  22. Bash regex =~ operator
  23. How do I check if a directory exists or not in a Bash shell script?
  24. Validate date format in a shell script
  25. Convert Unix timestamp to a date string
  26. Using Bash to display a progress indicator (spinner) [duplicate]
  27. Extract version number from a string
  28. How to properly nest Bash backticks
  29. How to include nohup inside a bash script?
  30. Subtract two variables in Bash

Leave a Comment