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elaborate on shell interpolation and examples

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Mahmood Ali 2019-05-20 10:06:15 -04:00
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website/source/docs/commands/alloc/exec.html.md.erb
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@ -3,7 +3,7 @@ layout: "docs"
page_title: "Commands: alloc exec"
sidebar_current: "docs-commands-alloc-exec"
description: >
Stream the exec of a task.
Runs a command in a container.
---
# Command: alloc exec
@ -15,7 +15,7 @@ The `alloc exec` command runs a command in a running allocation.
```
nomad alloc exec [options] <allocation> <command> [<args>...]
```
```
The nomad exec command can be use to run commands inside a running task/allocation.
@ -23,11 +23,11 @@ The nomad exec command can be use to run commands inside a running task/allocati
Use cases are for inspecting container state, debugging a failed application
without needing ssh access into the node that's running the allocation.
This command streams the execution of command in the given task in the allocation. If the
This command executes the command in the given task in the allocation. If the
allocation is only running a single task, the task name can be omitted.
Optionally, the `-job` option may be used in which case a random allocation from
the given job will be chosen.
#
## General Options
<%= partial "docs/commands/_general_options" %>
@ -53,21 +53,57 @@ transparent.
## Examples
```
$ # interactive debugging
$ nomad alloc exec eb17e557 /bin/sh
/ # ps -ef
...
To start an interactive debugging session in a particular alloc, invoke exec
command with your desired shell available inside the task:
```
$ nomad alloc exec eb17e557 /bin/bash
root@eb17e557:/data# # now run any debugging commands inside container
root@eb17e557:/data# # ps -ef
```
To run a command and stream results without starting an interactive shell, you
can pass the command and its arguments to exec directly:
```
$ # run commands without starting an interactive session
$ nomad alloc exec eb17e557 cat /etc/resolv.conf
...
$ # run commands on an arbitrary alloc of a job
$ nomad alloc exec -job example printenv NOMAD_CPU_LIMIT
...
```
When passing command arguments to be evaluated in task, you may need to ensure
that your host shell doesn't interpolate values before invoking `exec` command.
For example, the following command would return the environment variable on
operator shell rather than task containers:
```
$ nomad alloc exec eb17e557 echo $NOMAD_ALLOC_ID # wrong
```
Here, we must start a shell in task to interpolate `$NOMAD_ALLOC_ID`, and quote
command or use the [heredoc syntax][heredoc]
```
$ # by quoting argument
$ nomad alloc exec eb17e557 /bin/sh -c 'echo $NOMAD_ALLOC_ID'
eb17e557-443e-4c51-c049-5bba7a9850bc
$ # by using heredoc and passing command in stdin
$ nomad alloc exec eb17e557 /bin/sh <<'EOF'
> echo $NOMAD_ALLOC_ID
> EOF
eb17e557-443e-4c51-c049-5bba7a9850bc
```
This technique applies when aiming to run a shell pipeline without streaming
intermediate command output across the network:
```
$ # e.g. find top appearing lines in some output
$ nomad alloc exec eb17e557 /bin/sh -c 'cat /output | sort | uniq -c | sort -rn | head -n 5'
```
[heredoc]: http://tldp.org/LDP/abs/html/here-docs.html
## Using Job ID instead of Allocation ID
Setting the `-job` flag causes a random allocation of the specified job to be