Sys

Local, immediate command execution.

A few rules about sys expression:

  • Everything after sys until the end of the line is interpreted to be a command
  • If the line ends with a backslash, next line is interpreted as part of the same command (same as in a shell script)
  • Variables are interpolated. E.g. sys echo Hello $person will replace '$person' by the variable's name before sending it to the OS for execution.
  • BDS immediately executes the OS command in the local machine and waits until the command finishes execution.
  • If the command exits with an error condition, then bds creates a checkpoint, and exits with a non-zero exit code.
  • No resource accounting is performed, the command is executed even if all CPUs are busy executing tasks.

E.g.

$ cat z.bds
print("Before\n")
sys echo Hello
print("After\n")

$ bds z.bds
Before
Hello
After
  • Everything after a sys keyword until the end of the line is considered part of the command to be executed. So multiple shell commands can be separated by semicolon
sys echo Hello ; echo Bye
  • Multi-line statements are allowed, by using a backslash at the end of the line (same as a shell script)
sys echo HeLLo \
    | tr [A-Z] [a-z] \
    | grep hell
  • sys returns the STDOUT of the command: File test_12.bds
#!/usr/bin/env bds

dir := sys ls *.bds | head -n 3
print("\nVariable dir is:\n$dir\n")

Executing, we get:

$ ./test_12.bds 
test_01.bds
test_02.bds
test_03.bds

Variable dir is:
test_01.bds
test_02.bds
test_03.bds

Note that (roughly) the first half of the output is printed by the command execution, while the second half is printed by the print statement (i.e. printing the variable dir).

  • Characters are passed literally to the interpreting shell. For example, when you write '\t' it is NOT converted to a tab character before sending it to the shell (no escaping is required):
$ cat z.bds
sys echo -e "Hello\tWorld" | awk '{print $1 "\n" $2}'

$ bds z.bds
$ bds z.bds
Hello
World