{"id":291,"date":"2026-10-13T09:32:26","date_gmt":"2026-10-13T09:32:26","guid":{"rendered":"http:\/\/www.scmgalaxy.com\/tutorials\/2010\/12\/10\/shell-script-parameters\/"},"modified":"2026-10-13T09:32:26","modified_gmt":"2026-10-13T09:32:26","slug":"shell-script-parameters","status":"publish","type":"post","link":"https:\/\/www.devopsschool.com\/blog\/shell-script-parameters\/","title":{"rendered":"Bash Shell Script Parameters: A Practical Reference for Positional and Special Variables"},"content":{"rendered":"\n

Originally published on 2010-12-10; fully revised and updated.<\/em><\/p>\n\n\n\n

This reference is for DevOps engineers writing Bash automation. You will learn how positional and special parameters work, how argument expansion differs between $*<\/code> and $@<\/code>, and how to handle script inputs safely. For historical context, see the original post<\/a>.<\/p>\n\n\n\n

\"Shell<\/figure>\n\n\n\n

Bash parameter types at a glance<\/h2>\n\n\n\n

In Bash, a parameter is any entity that stores a value. For day-to-day scripting, the most important categories are positional parameters<\/strong> (arguments passed to a script) and special parameters<\/strong> (shell-provided values such as exit code and process IDs).<\/p>\n\n\n\n

  • Positional parameters<\/strong>: $0<\/code>, $1<\/code> … $9<\/code>, and ${10}<\/code> onward.<\/li>
  • Special parameters<\/strong>: $#<\/code>, $@<\/code>, $*<\/code>, $?<\/code>, $$<\/code>, $!<\/code>, $-<\/code>, $_<\/code>.<\/li>
  • Important<\/strong>: for argument numbers above 9, use braces (for example, ${10}<\/code>), not $10<\/code>.<\/li><\/ul>\n\n\n\n

    Quick reference for common Bash parameters<\/h2>\n\n\n\n
    Parameter<\/th>What it represents<\/th>Typical use<\/th><\/tr><\/thead>
    $0<\/code><\/td>Script name as invoked (may include a path)<\/td>Usage\/help messages<\/td><\/tr>
    $1<\/code> … $9<\/code>, ${10}<\/code><\/td>Positional arguments<\/td>Read user input values<\/td><\/tr>
    $#<\/code><\/td>Argument count<\/td>Validate required inputs<\/td><\/tr>
    $*<\/code><\/td>All positional parameters<\/td>Aggregate argument handling<\/td><\/tr>
    $@<\/code><\/td>All positional parameters<\/td>Forward arguments safely<\/td><\/tr>
    $?<\/code><\/td>Exit status of last command<\/td>Error handling branches<\/td><\/tr>
    $$<\/code><\/td>Current shell process ID<\/td>Logging and process correlation<\/td><\/tr>
    $!<\/code><\/td>PID of most recent background job<\/td>Wait\/monitor background work<\/td><\/tr>
    $-<\/code><\/td>Current shell option flags<\/td>Debugging shell mode state<\/td><\/tr>
    $_<\/code><\/td>Last argument of previous command (context-dependent)<\/td>Occasional debugging<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    How parameters flow through a script<\/h2>\n\n\n\n

    A typical argument-handling flow validates count first, then processes inputs, and finally checks command status or background job IDs as needed.<\/p>\n\n\n\n

    \nflowchart TD\n    A[Run script with arguments] --> B[Shell assigns script name and positional values]\n    B --> C[Read argument count]\n    C --> D{Enough arguments}\n    D -- no --> E[Print usage and exit]\n    D -- yes --> F[Process positional parameters]\n    F --> G[Optional set -- to replace values]\n    G --> H[Run commands and check exit status]\n    H --> I[Optionally read process ids]\n<\/pre>\n\n\n\n
    Example 1: Positional parameters for arithmetic input<\/h2>\n\n\n\n
    This Bash script accepts two integer arguments, validates them, and performs arithmetic. Bash arithmetic uses integer math here, so division is integer division.<\/p>\n\n\n
    #!\/usr\/bin\/env bash<\/span>\nif<\/span> [ \"$#\"<\/span> -lt 2<\/span> ]; then\n  echo<\/span> \"Usage: $0 num1 num2\"<\/span>\n  exit<\/span> 1<\/span>\nfi\n\na=$1<\/span>\nb=$2<\/span>\n\nif<\/span> [ \"$b\"<\/span> -eq 0<\/span> ]; then\n  echo<\/span> \"Error: num2 must not be zero for division\"<\/span>\n  exit<\/span> 1<\/span>\nfi\n\nprintf '$1=%s\\n'<\/span> \"$a\"<\/span>\nprintf '$2=%s\\n'<\/span> \"$b\"<\/span>\necho<\/span> \"Addition=$((a + b))\"<\/span>\necho<\/span> \"Subtraction=$((a - b))\"<\/span>\necho<\/span> \"Multiplication=$((a * b))\"<\/span>\necho<\/span> \"Division=$((a \/ b))\"<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n
    chmod +x arithmetic.sh\n.\/arithmetic.sh 12 10\n\n$1=12\n$2=10\nAddition=22\nSubtraction=2\nMultiplication=120\nDivision=1<\/code><\/span><\/pre>\n\n\n
    Example 2: Set, replace, and clear positional parameters with set —<\/h2>\n\n\n\n
    The set<\/code> builtin can overwrite positional parameters inside the current shell context. Using set --<\/code> with no following values clears them.<\/p>\n\n\n
    #!\/usr\/bin\/env bash<\/span>\nprintf 'Initial: $1=%s $2=%s $3=%s\\n'<\/span> \"$1\"<\/span> \"$2\"<\/span> \"$3\"<\/span>\n\nset -- First Second Third\nprintf 'After set words: $1=%s $2=%s $3=%s\\n'<\/span> \"$1\"<\/span> \"$2\"<\/span> \"$3\"<\/span>\n\nset -- -f -s -t\nprintf 'After set -- with hyphen values: $1=%s $2=%s $3=%s\\n'<\/span> \"$1\"<\/span> \"$2\"<\/span> \"$3\"<\/span>\n\nset --\nprintf 'After clear: $1=%s $2=%s $3=%s\\n'<\/span> \"$1\"<\/span> \"$2\"<\/span> \"$3\"<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n
    .\/positional.sh 12<\/span> 10<\/span>\nInitial<\/span>: $1<\/span>=12<\/span> $2<\/span>=10<\/span> $3<\/span>=\nAfter set<\/span> words: $1=First $2=Second $3=Third\nAfter set<\/span> -- with hyphen values: $1=-f $2=-s $3=-t\nAfter clear: $1= $2= $3=<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
    Example 3: The difference between “$*” and “$@”<\/h2>\n\n\n\n
    The most important difference appears in quoted form. Quoted \"$*\"<\/code> becomes a single word, while quoted \"$@\"<\/code> preserves each argument as a separate word.<\/p>\n\n\n\n
    Form<\/th>Behavior<\/th>When to use<\/th><\/tr><\/thead>
    \"$*\"<\/code><\/td>One string containing all arguments, joined by first character of IFS<\/td>When you intentionally need one combined value<\/td><\/tr>
    \"$@\"<\/code><\/td>Each argument remains a separate word<\/td>Preferred for forwarding arguments to another command<\/td><\/tr>
    $*<\/code> (unquoted)<\/td>Subject to word splitting and pathname expansion<\/td>Usually avoid in robust scripts<\/td><\/tr>
    $@<\/code> (unquoted)<\/td>Also subject to splitting\/expansion<\/td>Usually avoid in robust scripts<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n
    #!\/usr\/bin\/env bash<\/span>\nIFS='-'<\/span>\ncnt=1<\/span>\n\necho<\/span> 'Values of \"$*\":'<\/span>\nfor<\/span> arg in \"$*\"<\/span>; do<\/span>\n  echo<\/span> \"Arg #$cnt = $arg\"<\/span>\n  ((cnt+=1<\/span>))\ndone\n\ncnt=1<\/span>\necho<\/span> 'Values of \"$@\":'<\/span>\nfor<\/span> arg in \"$@\"<\/span>; do<\/span>\n  echo<\/span> \"Arg #$cnt = $arg\"<\/span>\n  ((cnt+=1<\/span>))\ndone<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n
    .\/expan.sh \"This is\"<\/span> 2<\/span> 3<\/span>\nValues of \"$*\"<\/span>:\nArg #1 = This is-2-3<\/span>\nValues of \"$@\"<\/span>:\nArg #1 = This is<\/span>\nArg #2 = 2<\/span>\nArg #3 = 3<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
    Process and status parameters: $$, $!, $?, $-, $_<\/h2>\n\n\n\n
    These special parameters are useful for process control and troubleshooting in automation scripts.<\/p>\n\n\n\n
    • $$<\/code>: PID of the current shell process.<\/li>
    • $!<\/code>: PID of the most recently started background process.<\/li>
    • $?<\/code>: exit code of the previous command.<\/li>
    • $-<\/code>: currently enabled shell option flags.<\/li>
    • $_<\/code>: last argument of the previous command; useful sometimes, but context-sensitive.<\/li><\/ul>\n\n\n
      #!\/usr\/bin\/env bash<\/span>\necho<\/span> \"Shell PID: $$\"<\/span>\n\nsleep 30<\/span> &\nbg_pid=$!\necho<\/span> \"Most recent background PID: $bg_pid\"<\/span>\n\nwait \"$bg_pid\"<\/span>\necho<\/span> \"Exit status of wait: $?\"<\/span>\n\necho<\/span> \"Shell option flags: $-\"<\/span>\necho<\/span> \"alpha beta gamma\"<\/span>\necho<\/span> \"Previous command last argument via $_: $_\"<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
      Common pitfalls and best practices<\/h2>\n\n\n\n
      • Validate argument count early with $#<\/code> and fail fast with a clear usage message.<\/li>
      • Use \"$@\"<\/code> when forwarding arguments; avoid unquoted $*<\/code> and $@<\/code> in production scripts.<\/li>
      • Use ${10}<\/code>, ${11}<\/code>, and so on for argument numbers above 9.<\/li>
      • Guard arithmetic operations (especially division) with input checks.<\/li>
      • Use set --<\/code> intentionally; it replaces the current positional parameters in the active shell context.<\/li>
      • Treat $_<\/code> as convenience, not core control logic, because behavior can vary by context.<\/li><\/ul>\n\n\n\n
        If you consistently combine argument validation, careful quoting, and exit-status checks, your shell scripts become much safer for CI\/CD and operations automation.<\/p>\n","protected":false},"excerpt":{"rendered":"
        Learn how Bash passes arguments to scripts and how special parameters like $#, $@, $*, $$, and $? behave. This updated reference includes workflow diagrams, comparison tables, and production-friendly examples.<\/p>\n","protected":false},"author":1,"featured_media":4336,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_joinchat":[],"footnotes":""},"categories":[454],"tags":[137,4034,4033,4036,4035,4031,278,138,4038,3046,4037,4032,4040,4041,4043,4039,4042],"class_list":["post-291","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-shell-script","tag-bash","tag-bash-positional-parameter","tag-bash-shell","tag-bash-shell-script-parameters","tag-bash-special-parameters","tag-parameters","tag-script","tag-shell","tag-shell-parameter-values","tag-shell-script","tag-shell-script-arguments-example","tag-shell-script-parameters","tag-shell-script-parameters-guide","tag-shell-script-parameters-instructions","tag-shell-script-parameters-reference","tag-shell-script-pass-parameter","tag-understand-shell-script-parameters"],"_links":{"self":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts\/291","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/comments?post=291"}],"version-history":[{"count":3,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts\/291\/revisions"}],"predecessor-version":[{"id":79127,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts\/291\/revisions\/79127"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/media\/4336"}],"wp:attachment":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/media?parent=291"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/categories?post=291"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/tags?post=291"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}