shell bash

shell
  • the shell assigns its exit code to the $? environment variable. The $? variable is how we usually test whether a script has succeeded or not in its execution.

syntax

set -e

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Causes the script to exit immediately if any command fails (returns a non-zero status).

set -x

​ 1. Debugging: When you’re trying to troubleshoot a script, set -x helps you see exactly what commands are being executed and in what order.

​ 2. Selective Debugging: You can use set -x and set +x around specific parts of your script to narrow down the problematic sections.

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#!/bin/bash

echo "This is a normal message"

set -x  # Start debugging from this point
name="John"
echo "Hello, $name"

set +x  # Stop debugging after this point
echo "Debugging mode is now off"

output:

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This is a normal message
+ name=John
+ echo 'Hello, John'
Hello, John
Debugging mode is now off

shift 2

  • Removes the first two positional parameters from command line arguments
  • Shifts remaining parameters left by 2 positions
  • Useful when processing command line options/arguments in loops

{xx..xx}

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for node in node{1..9} node{20..29} ; do
  echo -n "$node: "
  kubectl get node "$node" -o jsonpath='{.spec.taints}' || echo "Not found"
  echo
done

point

  • (cd third-party && ...) 这样的语法，这是一个命令替换，它实质上是先临时改变到 third-party 目录执行里面的命令，然后脚本会继续在原先的目录中执行。

nohup

  • output of a script to a specified file

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    nohup your_script.sh > output.log 2>&1 &

    0: Standard input (stdin)

    1: Standard output (stdout)

    2: Standard error (stderr)

    >&: This is used to redirect the output.

    &: Puts the command in the background.

getopt

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local long_opts="role:,args:,help"
local args=getopt -o ra --long $long_opts -n "$0" -- "$@"
eval set -- "${args}"

This snippet is a common pattern in Bash scripting to parse command-line options using getopt. Here’s a line-by-line breakdown:

Explanation:

local long_opts="role:,args:,help"

Defines long options supported by the script:

  • role: means –role requires a value (e.g., –role=admin)
  • args: means –args requires a value
  • help means –help is a flag with no value

: means the option requires an argument; no colon means it’s a flag.

local args=getopt -o ra –long $long_opts -n “$0” – “$@”

  • -o ra: short options -r and -a are accepted
  • –long $long_opts: specify long-form options (see above)
  • -n “$0”: used for error messages (e.g., script name)
  • – “$@”: separates option parsing from positional arguments

This parses the options passed to the script and stores the normalized result in args.

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Example: ./myscript.sh --role=admin --args=val --help foo bar
Output of getopt:
--role admin --args val --help -- foo bar

eval set -- "${args}"

This re-sets the $1, $2, … positional parameters to the parsed list.

So later in the script, you can safely loop through options using:

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while true; do
  case "$1" in
    --role) ROLE=$2; shift 2 ;;
    --args) ARGS=$2; shift 2 ;;
    --help) show_help; exit 0 ;;
    --) shift; break ;;
    *) echo "Unknown option: $1"; exit 1 ;;
  esac
done

Summary

Line Purpose
long_opts=… Defines supported long options
getopt … Parses CLI arguments
eval set – … Makes parsed options available for normal while-case parsing