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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><link href=https://flokoe.github.io/bash-hackers-wiki/scripting/nonportable/ rel=canonical><link href=../newbie_traps/ rel=prev><link href=../obsolete/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.44"><title>Portability talk - The Bash Hackers Wiki</title><link rel=stylesheet href=../../assets/stylesheets/main.0253249f.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CRoboto+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"Roboto Mono"}</style><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#portability-talk class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="The Bash Hackers Wiki" class="md-header__button md-logo" aria-label="The Bash Hackers Wiki" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> The Bash Hackers Wiki </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Portability talk </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-s
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if ((MATH)); then
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...
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# portable equivalent command
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if [ "$((MATH))" -ne 0 ]; then
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...
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</code></pre></div> <p>Quotes around the arithmetic expansion <code>$((MATH))</code> should not be necessary as per POSIX, but Bash and AT&T-KSH perform word-splitting on aritrhmetic expansions, so the most portable is <em>with quotes</em>.</p> <h3 id=echo-command>echo command<a class=headerlink href=#echo-command title="Permanent link">¶</a></h3> <p>The overall problem with <code>echo</code> is, that there are 2 (maybe more) mainstream flavours around. The only case where you may safely use an <code>echo</code> on all systems is: Echoing non-variable arguments that don't start with a <code>-</code> (dash) and don't contain a <code>\</code> (backslash).</p> <p>Why? (list of known behaviours)</p> <ul> <li>may or may not automatically interpret backslash escpape codes in the strings</li> <li>may or may not automatically interpret switches (like <code>-n</code>)</li> <li>may or may not ignore "end of options" tag (<code>--</code>)</li> <li><code>echo -n</code> and <code>echo -e</code> are neither portable nor standard (<strong>even within the same shell</strong>, depending on the version or environment variables or the build options, especially KSH93 and Bash)</li> </ul> <p>For these, and possibly other, reasons, POSIX (SUS) standardized the existance of <a href=../../commands/builtin/printf/ >the <code>printf</code> command</a>.</p> <h3 id=parameter-expansions>Parameter expansions<a class=headerlink href=#parameter-expansions title="Permanent link">¶</a></h3> <ul> <li><code>${var:x:x}</code> is KSH93/Bash specific</li> <li><code>${var/../..}</code> and <code>${var//../..}</code> are KSH93/Bash specific</li> <li><code>var=$*</code> and <code>var=$@</code> are not handled the same in all shells if the first char of IFS is not " " (space). <code>var="$*"</code> should work (except the Bourne shell always joins the expansions with space)</li> </ul> <h3 id=special-variables>Special variables<a class=headerlink href=#special-variables title="Permanent link">¶</a></h3> <h4 id=pwd>PWD<a class=headerlink href=#pwd title="Permanent link">¶</a></h4> <p><a href=../../syntax/shellvars/#PWD>PWD</a> is POSIX but not Bourne. Most shells are <em>not POSIX</em> in that they don't ignore the value of the <code>PWD</code> environment variable. Workaround to fix the value of <code>PWD</code> at the start of your script:</p> <div class=highlight><pre><span></span><code>pwd -P > dev/null
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</code></pre></div> <h4 id=random>RANDOM<a class=headerlink href=#random title="Permanent link">¶</a></h4> <p><a href=../../syntax/shellvars/#RANDOM>RANDOM</a> is Bash/KSH/ZSH specific variable that will give you a random number up to 32767 (2^15-1). Among many other available external options, you can use awk to generate a random number. There are multiple implementations of awk and which version your system uses will depend. Most modern systems will call 'gawk' (i.e. GNU awk) or 'nawk'. 'oawk' (i.e. Original/Old awk) does not have the rand() or srand() functions, so is best avoided.</p> <div class=highlight><pre><span></span><code># 'gawk' can produce random numbers using srand(). In this example, 10 integers between 1 and 500:
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randpm=$(gawk -v min=1 -v max=500 -v nNum=10 'BEGIN { srand(systime() + PROCINFO["pid"]); for (i = 0; i < nNum; ++i) {print int(min + rand() * (max - min)} }')
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# 'nawk' and 'mawk' does the same, but needs a seed to be provided for its rand() function. In this example we use $(date)
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randpm=$(mawk -v min=1 -v max=500 -v nNum=10 -v seed="$(date +%Y%M%d%H%M%S)" 'BEGIN { srand(seed); for (i = 0; i < nNum; ++i) {print int(min + rand() * (max - min)} }')
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</code></pre></div> <p><em>Yes, I'm not an <code>awk</code> expert, so please correct it, rather than complaining about possible stupid code!</em></p> <div class=highlight><pre><span></span><code># Well, seeing how this //is// BASH-hackers.org I kinda missed the bash way of doing the above ;-)
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# print a number between 0 and 500 :-)
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printf $(( 500 * RANDOM / 32767 ))
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# Or print 30 random numbers between 0 and 10 ;)
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X=0; while (( X++ < 30 )); do echo $(( 10 * RANDOM / 32767 )); done
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</code></pre></div> <h4 id=seconds>SECONDS<a class=headerlink href=#seconds title="Permanent link">¶</a></h4> <p><a href=../../syntax/shellvars/#SECONDS>SECONDS</a> is KSH/ZSH/Bash specific. Avoid it. Find another method.</p> <h3 id=check-for-a-command-in-path>Check for a command in PATH<a class=headerlink href=#check-for-a-command-in-path title="Permanent link">¶</a></h3> <p>The <a href=../../syntax/shellvars/#PATH>PATH</a> variable is a colon-delimited list of directory names, so it's basically possible to run a loop and check every <code>PATH</code> component for the command you're looking for and for executability.</p> <p>However, this method doesn't look nice. There are other ways of doing this, using commands that are <em>not directly</em> related to this task.</p> <h4 id=hash>hash<a class=headerlink href=#hash title="Permanent link">¶</a></h4> <p>The <code>hash</code> command is used to make the shell store the full pathname of a command in a lookup-table (to avoid re-scanning the <code>PATH</code> on every command execution attempt). Since it has to do a <code>PATH</code> search, it can be used for this check.</p> <p>For example, to check if the command <code>ls</code> is available in a location accessible by <code>PATH</code>:</p> <div class=highlight><pre><span></span><code>if hash ls >/dev/null 2>&1; then
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echo "ls is available"
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fi
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</code></pre></div> <p>Somewhat of a mass-check:</p> <div class=highlight><pre><span></span><code>for name in ls grep sed awk; do
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if ! hash "$name" >/dev/null 2>&1; then
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echo "FAIL: Missing command '$name'"
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exit 1
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fi
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done
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</code></pre></div> <p>Here (bash 3), <code>hash</code> also respects builtin commands. I don't know if this works everywhere, but it seems logical.</p> <h4 id=command>command<a class=headerlink href=#command title="Permanent link">¶</a></h4> <p>The <code>command</code> command is used to explicitly call an external command, rather than a builtin with the same name. For exactly this reason, it has to do a <code>PATH</code> search, and can be used for this check.</p> <p>For example, to check if the command <code>sed</code> is available in a location accessible by <code>PATH</code>:</p> <div class=highlight><pre><span></span><code>if command -v sed >/dev/null 2>&1; then
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echo "sed is available"
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fi
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</code></pre></div> <div class=footnote> <hr> <ol> <li id=fn:1> <p>"portable" doesn't necessarily mean it's POSIX, it can also mean it's "widely used and accepted", and thus maybe more portable than POSIX(r <a class=footnote-backref href=#fnref:1 title="Jump back to footnote 1 in the text">↩</a></p> </li> </ol> </div> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 7, 2024</span> </span> <span class=md-source-file__fact> <span class=md-icon title=Created> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M14.47 15.08 11 13V7h1.5v5.25l3.08 1.83c-.41.28-.79.62-1.11 1m-1.39 4.84c-.36.05-.71.08-1.08.08-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8c0 .37-.03.72-.08 1.08.69.1 1.33.32 1.92.64.1-.56.16-1.13.16-1.72 0-5.5-4.5-10-10-10S2 6.5 2 12s4.47 10 10 10c.59 0 1.16-.06 1.72-.16-.32-.59-.54-1.23-.64-1.92M18 15v3h-3v2h3v3h2v-3h3v-2h-3v-3z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 7, 2024</span> </span> </aside> <h2 id=__comments>Comments</h2> <script src=https://giscus.app/client.js data-repo=flokoe/bash-hackers-wiki data-repo-id=R_kgDOJ3Nr6Q data-category="Giscus Page Comments" data-category-id=DIC_kwDOJ3Nr6c4CXq9t data-mapping=pathname data-strict=1 data-reactions-enabled=1 data-emit-metadata=0 data-input-position=top data-theme=preferred_color_scheme data-lang=en data-loading=lazy crossorigin=anonymous async>
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