Bash – Using groovy, how do you pipe multiple shell commands

bashgroovyprocessshell

Using Groovy and it's java.lang.Process support, how do I pipe multiple shell commands together?

Consider this bash command (and assume your username is foo):

ps aux | grep ' foo' | awk '{print $1}'

This will print out usernames – one line for some processes related to your user account.

Using Groovy, the ProcessGroovyMethods documentation and code says I should be able to do this to achieve the same result:

def p = "ps aux".execute() | "grep ' foo'".execute() | "awk '{print $1}'".execute()
p.waitFor()
println p.text

However, I can't get any text output for anything other than this:

def p = "ps aux".execute()
p.waitFor()
println p.text

As soon as I start piping, the println does not print out any anything.

Thoughts?

Best Answer

This works for me :

def p = 'ps aux'.execute() | 'grep foo'.execute() | ['awk', '{ print $1 }'].execute()
p.waitFor()
println p.text

for an unknown reason, the parameters of awk can't be send with only one string (i don't know why! maybe bash is quoting something differently). If you dump with your command the error stream, you'll see error relative to the compilation of the awk script.

Edit : In fact,

"-string-".execute() delegate to Runtime.getRuntime().exec(-string-)
It's bash job to handle arguments containing spaces with ' or ". Runtime.exec or the OS are not aware of the quotes
Executing "grep ' foo'".execute() execute the command grep, with ' as the first parameters, and foo' as the second one : it's not valid. the same for awk

Related Solutions

Ruby – How to call shell commands from Ruby

This explanation is based on a commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.

First, note that when Ruby calls out to a shell, it typically calls /bin/sh, not Bash. Some Bash syntax is not supported by /bin/sh on all systems.

Here are ways to execute a shell script:

cmd = "echo 'hi'" # Sample string that can be used

Kernel#` , commonly called backticks – `cmd`

This is like many other languages, including Bash, PHP, and Perl.

Returns the result (i.e. standard output) of the shell command.

Docs: http://ruby-doc.org/core/Kernel.html#method-i-60
```
value = `echo 'hi'`
value = `#{cmd}`
```
Built-in syntax, %x( cmd )

Following the x character is a delimiter, which can be any character. If the delimiter is one of the characters (, [, {, or <, the literal consists of the characters up to the matching closing delimiter, taking account of nested delimiter pairs. For all other delimiters, the literal comprises the characters up to the next occurrence of the delimiter character. String interpolation #{ ... } is allowed.

Returns the result (i.e. standard output) of the shell command, just like the backticks.

Docs: https://docs.ruby-lang.org/en/master/syntax/literals_rdoc.html#label-Percent+Strings
```
value = %x( echo 'hi' )
value = %x[ #{cmd} ]
```
Kernel#system

Executes the given command in a subshell.

Returns true if the command was found and run successfully, false otherwise.

Docs: http://ruby-doc.org/core/Kernel.html#method-i-system
```
wasGood = system( "echo 'hi'" )
wasGood = system( cmd )
```
Kernel#exec

Replaces the current process by running the given external command.

Returns none, the current process is replaced and never continues.

Docs: http://ruby-doc.org/core/Kernel.html#method-i-exec
```
exec( "echo 'hi'" )
exec( cmd ) # Note: this will never be reached because of the line above
```

Here's some extra advice: $?, which is the same as $CHILD_STATUS, accesses the status of the last system executed command if you use the backticks, system() or %x{}. You can then access the exitstatus and pid properties:

$?.exitstatus

For more reading see:

Bash – How to check if a directory exists in a Bash shell script

To check if a directory exists in a shell script, you can use the following:

if [ -d "$DIRECTORY" ]; then
  # Control will enter here if $DIRECTORY exists.
fi

Or to check if a directory doesn't exist:

if [ ! -d "$DIRECTORY" ]; then
  # Control will enter here if $DIRECTORY doesn't exist.
fi

However, as Jon Ericson points out, subsequent commands may not work as intended if you do not take into account that a symbolic link to a directory will also pass this check. E.g. running this:

ln -s "$ACTUAL_DIR" "$SYMLINK"
if [ -d "$SYMLINK" ]; then 
  rmdir "$SYMLINK" 
fi

Will produce the error message:

rmdir: failed to remove `symlink': Not a directory

So symbolic links may have to be treated differently, if subsequent commands expect directories:

if [ -d "$LINK_OR_DIR" ]; then 
  if [ -L "$LINK_OR_DIR" ]; then
    # It is a symlink!
    # Symbolic link specific commands go here.
    rm "$LINK_OR_DIR"
  else
    # It's a directory!
    # Directory command goes here.
    rmdir "$LINK_OR_DIR"
  fi
fi

Take particular note of the double-quotes used to wrap the variables. The reason for this is explained by 8jean in another answer.

If the variables contain spaces or other unusual characters it will probably cause the script to fail.

Best Answer

Related Solutions

Ruby – How to call shell commands from Ruby

Bash – How to check if a directory exists in a Bash shell script

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