execline: the backtick command
Keywords: execline command backtick

execline
Software
skarnet.org

# The `backtick` program

`backtick` runs a program and stores its output in an environment variable, then executes another program.

## Interface

In an execlineb script:

         backtick [ -i | -I | -x | -D default ] [ -N | -n ] [ -E | -e ] variable { prog1... } prog2...

- `backtick` reads prog1... in a block and unquotes it.
- It runs prog1... as a child process and saves its output in memory. This output must not contain a null character.
- `backtick` execs into prog2..., with variable added to the environment with prog1...'s output as a value.

## Options

- `-N` : store prog1...'s output as is, including the last newline, if any.
- `-n` : chomp an ending newline off prog1...'s output. This is the default.
- `-e` : no autoimport. This is the default.
- `-E` : autoimport. Before exec'ing into prog2..., perform a substitution so that `$?` is replaced with prog1...'s exitcode (if applicable, see below), and `${variable}` is replaced with prog1...'s output, instead of keeping the results in the environment. The variables will not appear in the environment; if they preexisted in `backtick`'s environment, they will be removed.

The other options tell backtick what to do if prog1...'s output is not suitable as the contents of an environment variable (i.e. it contains a null character) or if prog1... crashes or exits nonzero:

- `-i` : backtick exits with an approximation of prog1's exit code, or 124 if prog1 wrote a null character. This is the default.
- `-I`: the value of variable is set to whatever the start of prog1...'s output is, up to the first null character, or to whatever prog1... wrote before crashing; chomping is applied if required; then execution proceeds.
- `-x` : variable is removed from the environment, and execution proceeds.
- `-D default` : the value of variable is set to default, and execution proceeds.

If the command line can keep running after prog1 crashes or exits nonzero, i.e. if one of the `-I`, `-x` -or `-D` has been given, then `backtick` also adds the `?` variable to the environment. If prog1 exited normally, the variable contains its exit code; otherwise, 256 plus the number of the signal that killed it.