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<h1 style='display: inline' id='bash-golf-part-4'>Bash Golf Part 4</h1><br />
<br />
<span class='quote'>Published at 2025-09-13T12:04:03+03:00</span><br />
<br />
<span>This is the fourth blog post about my Bash Golf series. This series is random Bash tips, tricks, and weirdnesses I have encountered over time. </span><br />
<br />
<a class='textlink' href='./2021-11-29-bash-golf-part-1.html'>2021-11-29 Bash Golf Part 1</a><br />
<a class='textlink' href='./2022-01-01-bash-golf-part-2.html'>2022-01-01 Bash Golf Part 2</a><br />
<a class='textlink' href='./2023-12-10-bash-golf-part-3.html'>2023-12-10 Bash Golf Part 3</a><br />
<a class='textlink' href='./2025-09-14-bash-golf-part-4.html'>2025-09-14 Bash Golf Part 4 (You are currently reading this)</a><br />
<br />
<pre>
'\ '\ '\ '\ . . |>18>>
\ \ \ \ . ' . |
O>> O>> O>> O>> . 'o |
\ .\. .. .\. .. .\. .. . |
/\ . /\ . /\ . /\ . . |
/ / . / / .'. / / .'. / / .' . |
jgs^^^^^^^`^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Art by Joan Stark, mod. by Paul Buetow
</pre>
<br />
<h2 style='display: inline' id='table-of-contents'>Table of Contents</h2><br />
<br />
<ul>
<li><a href='#bash-golf-part-4'>Bash Golf Part 4</a></li>
<li>⇢ <a href='#split-pipelines-with-tee--process-substitution'>Split pipelines with tee + process substitution</a></li>
<li>⇢ <a href='#heredocs-for-remote-sessions-and-their-gotchas'>Heredocs for remote sessions (and their gotchas)</a></li>
<li>⇢ <a href='#namespacing-and-dynamic-dispatch-with-'>Namespacing and dynamic dispatch with <span class='inlinecode'>::</span></a></li>
<li>⇢ <a href='#indirect-references-with-namerefs'>Indirect references with namerefs</a></li>
<li>⇢ <a href='#function-declaration-forms'>Function declaration forms</a></li>
<li>⇢ <a href='#chaining-function-calls-in-conditionals'>Chaining function calls in conditionals</a></li>
<li>⇢ <a href='#grep-sed-awk-quickies'>Grep, sed, awk quickies</a></li>
<li>⇢ <a href='#safe-xargs-with-nuls'>Safe xargs with NULs</a></li>
<li>⇢ <a href='#efficient-file-to-variable-and-arrays'>Efficient file-to-variable and arrays</a></li>
<li>⇢ <a href='#quick-password-generator'>Quick password generator</a></li>
<li>⇢ <a href='#yes-for-automation'><span class='inlinecode'>yes</span> for automation</a></li>
<li>⇢ <a href='#forcing-true-to-fail-and-vice-versa'>Forcing <span class='inlinecode'>true</span> to fail (and vice versa)</a></li>
<li>⇢ <a href='#restricted-bash'>Restricted Bash</a></li>
<li>⇢ <a href='#useless-use-of-cat-and-when-its-ok'>Useless use of cat (and when it’s ok)</a></li>
<li>⇢ <a href='#atomic-locking-with-mkdir'>Atomic locking with <span class='inlinecode'>mkdir</span></a></li>
<li>⇢ <a href='#smarter-globs-and-faster-find-exec'>Smarter globs and faster find-exec</a></li>
</ul><br />
<h2 style='display: inline' id='split-pipelines-with-tee--process-substitution'>Split pipelines with tee + process substitution</h2><br />
<br />
<span>Sometimes you want to fan out one stream to multiple consumers and still continue the original pipeline. <span class='inlinecode'>tee</span> plus process substitution does exactly that:</span><br />
<br />
<pre>
somecommand \
| tee >(command1) >(command2) \
| command3
</pre>
<br />
<span>All of <span class='inlinecode'>command1</span>, <span class='inlinecode'>command2</span>, and <span class='inlinecode'>command3</span> see the output of <span class='inlinecode'>somecommand</span>. Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><b><u><font color="#000000">printf</font></u></b> <font color="#808080">'a</font>\n<font color="#808080">b</font>\n<font color="#808080">'</font> \
| tee >(sed <font color="#808080">'s/.*/X:&/; s/$/ :c1/'</font>) >(tr a-z A-Z | sed <font color="#808080">'s/$/ :c2/'</font>) \
| sed <font color="#808080">'s/$/ :c3/'</font>
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
a :c3
b :c3
A :c2 :c3
B :c2 :c3
X:a :c1 :c3
X:b :c1 :c3
</pre>
<br />
<span>This relies on Bash process substitution (<span class='inlinecode'>>(...)</span>). Make sure your shell is Bash and not a POSIX <span class='inlinecode'>/bin/sh</span>.</span><br />
<br />
<span>Example (fails under <span class='inlinecode'>dash</span>/POSIX sh):</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>/bin/sh -c <font color="#808080">'echo hi | tee >(cat)'</font>
<i><font color="silver"># /bin/sh: 1: Syntax error: "(" unexpected</font></i>
</pre>
<br />
<span>Combine with <span class='inlinecode'>set -o pipefail</span> if failures in side branches should fail the whole pipeline.</span><br />
<br />
<span>Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre><b><u><font color="#000000">set</font></u></b> -o pipefail
<b><u><font color="#000000">printf</font></u></b> <font color="#808080">'ok</font>\n<font color="#808080">'</font> | tee >(<b><u><font color="#000000">false</font></u></b>) | cat >/dev/null
echo $? <i><font color="silver"># 1 because a side branch failed</font></i>
</pre>
<br />
<span>Further reading:</span><br />
<br />
<a class='textlink' href='https://blogtitle.github.io/splitting-pipelines/'>Splitting pipelines with tee</a><br />
<br />
<h2 style='display: inline' id='heredocs-for-remote-sessions-and-their-gotchas'>Heredocs for remote sessions (and their gotchas)</h2><br />
<br />
<span>Heredocs are great to send multiple commands over SSH in a readable way:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre>ssh <font color="#808080">"$SSH_USER@$SSH_HOST"</font> <<EOF
<i><font color="silver"># Go to the work directory</font></i>
cd <font color="#808080">"$WORK_DIR"</font>
<i><font color="silver"># Make a git pull</font></i>
git pull
<i><font color="silver"># Export environment variables required for the service to run</font></i>
<b><u><font color="#000000">export</font></u></b> AUTH_TOKEN=<font color="#808080">"$APP_AUTH_TOKEN"</font>
<i><font color="silver"># Start the service</font></i>
docker compose up -d --build
EOF
</pre>
<br />
<span>Tips:</span><br />
<br />
<span>Quoting the delimiter changes interpolation. Use <span class='inlinecode'><<'EOF'</span> to avoid local expansion and send the content literally.</span><br />
<br />
<span>Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre>FOO=bar
cat <<<font color="#808080">'EOF'</font>
$FOO is not expanded here
EOF
</pre>
<br />
<span>Prefer explicit quoting for variables (as above) to avoid surprises. Example (spaces preserved only when quoted):</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>WORK_DIR=<font color="#808080">"/tmp/my work"</font>
ssh host <<EOF
cd $WORK_DIR <i><font color="silver"># may break if unquoted</font></i>
cd <font color="#808080">"$WORK_DIR"</font> <i><font color="silver"># safe</font></i>
EOF
</pre>
<br />
<span>Consider <span class='inlinecode'>set -euo pipefail</span> at the top of the remote block for stricter error handling. Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>ssh host <<<font color="#808080">'EOF'</font>
<b><u><font color="#000000">set</font></u></b> -euo pipefail
<b><u><font color="#000000">false</font></u></b> <i><font color="silver"># causes immediate failure</font></i>
echo never
EOF
</pre>
<br />
<span>Indent-friendly variant: use a dash to strip leading tabs in the body:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
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http://www.gnu.org/software/src-highlite -->
<pre>cat <<-EOF > script.sh
<i><font color="silver">#!/usr/bin/env bash</font></i>
echo <font color="#808080">"tab-indented content is dedented"</font>
EOF
</pre>
<br />
<span>Further reading:</span><br />
<br />
<a class='textlink' href='https://rednafi.com/misc/heredoc_headache/'>Heredoc headaches and fixes</a><br />
<br />
<h2 style='display: inline' id='namespacing-and-dynamic-dispatch-with-'>Namespacing and dynamic dispatch with <span class='inlinecode'>::</span></h2><br />
<br />
<span>You can emulate simple namespacing by encoding hierarchy in function names. One neat pattern is pseudo-inheritance via a tiny <span class='inlinecode'>super</span> helper that maps <span class='inlinecode'>pkg::lang::action</span> to a <span class='inlinecode'>pkg::base::action</span> default.</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
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http://www.gnu.org/software/src-highlite -->
<pre><i><font color="silver">#!/usr/bin/env bash</font></i>
<b><u><font color="#000000">set</font></u></b> -euo pipefail
super() {
<b><u><font color="#000000">local</font></u></b> -r fn=${FUNCNAME[1]}
<i><font color="silver"># Split name on :: and dispatch to base implementation</font></i>
<b><u><font color="#000000">local</font></u></b> -a parts=( ${fn//::/ } )
<font color="#808080">"${parts[0]}::base::${parts[2]}"</font> <font color="#808080">"$@"</font>
}
foo::base::greet() { echo <font color="#808080">"base: $@"</font>; }
foo::german::greet() { super <font color="#808080">"Guten Tag, $@!"</font>; }
foo::english::greet() { super <font color="#808080">"Good day, $@!"</font>; }
<b><u><font color="#000000">for</font></u></b> lang <b><u><font color="#000000">in</font></u></b> german english; <b><u><font color="#000000">do</font></u></b>
foo::$lang::greet Paul
<b><u><font color="#000000">done</font></u></b>
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
base: Guten Tag, Paul!
base: Good day, Paul!
</pre>
<br />
<h2 style='display: inline' id='indirect-references-with-namerefs'>Indirect references with namerefs</h2><br />
<br />
<span><span class='inlinecode'>declare -n</span> creates a name reference — a variable that points to another variable. It’s cleaner than <span class='inlinecode'>eval</span> for indirection:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>user_name=paul
<b><u><font color="#000000">declare</font></u></b> -n ref=user_name
echo <font color="#808080">"$ref"</font> <i><font color="silver"># paul</font></i>
ref=julia
echo <font color="#808080">"$user_name"</font> <i><font color="silver"># julia</font></i>
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
paul
julia
</pre>
<br />
<span>Namerefs are local to functions when declared with <span class='inlinecode'>local -n</span>. Requires Bash ≥4.3.</span><br />
<br />
<span>You can also construct the target name dynamically:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre>make_var() {
<b><u><font color="#000000">local</font></u></b> idx=$1; <b><u><font color="#000000">shift</font></u></b>
<b><u><font color="#000000">local</font></u></b> name=<font color="#808080">"slot_$idx"</font>
<b><u><font color="#000000">printf</font></u></b> -v <font color="#808080">"$name"</font> <font color="#808080">'%s'</font> <font color="#808080">"$*"</font> <i><font color="silver"># create variable slot_$idx</font></i>
}
get_var() {
<b><u><font color="#000000">local</font></u></b> idx=$1
<b><u><font color="#000000">local</font></u></b> -n ref=<font color="#808080">"slot_$idx"</font> <i><font color="silver"># bind ref to slot_$idx</font></i>
<b><u><font color="#000000">printf</font></u></b> <font color="#808080">'%s</font>\n<font color="#808080">'</font> <font color="#808080">"$ref"</font>
}
make_var <font color="#000000">7</font> <font color="#808080">"seven"</font>
get_var <font color="#000000">7</font>
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
seven
</pre>
<br />
<h2 style='display: inline' id='function-declaration-forms'>Function declaration forms</h2><br />
<br />
<span>All of these work in Bash, but only the first one is POSIX-ish:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>foo() { echo foo; }
function foo { echo foo; }
function foo() { echo foo; }
</pre>
<br />
<span>Recommendation: prefer <span class='inlinecode'>name() { ... }</span> for portability and consistency.</span><br />
<br />
<h2 style='display: inline' id='chaining-function-calls-in-conditionals'>Chaining function calls in conditionals</h2><br />
<br />
<span>Functions return a status like commands. You can short-circuit them in conditionals:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>deploy_check() { <b><u><font color="#000000">test</font></u></b> -f deploy.yaml; }
smoke_test() { curl -fsS http://localhost/healthz >/dev/null; }
<b><u><font color="#000000">if</font></u></b> deploy_check || smoke_test; <b><u><font color="#000000">then</font></u></b>
echo <font color="#808080">"All good."</font>
<b><u><font color="#000000">else</font></u></b>
echo <font color="#808080">"Something failed."</font> >&<font color="#000000">2</font>
<b><u><font color="#000000">fi</font></u></b>
</pre>
<br />
<span>You can also compress it golf-style:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>deploy_check || smoke_test && echo ok || echo fail >&<font color="#000000">2</font>
</pre>
<br />
<h2 style='display: inline' id='grep-sed-awk-quickies'>Grep, sed, awk quickies</h2><br />
<br />
<span>Word match and context: <span class='inlinecode'>grep -w word file</span>; with context: <span class='inlinecode'>grep -C3 foo file</span> (same as <span class='inlinecode'>-A3 -B3</span>). Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>cat > /tmp/ctx.txt <<EOF
one
foo
two
three
bar
EOF
grep -C<font color="#000000">1</font> foo /tmp/ctx.txt
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
one
foo
two
</pre>
<br />
<span>Skip a directory while recursing: <span class='inlinecode'>grep -R --exclude-dir=foo 'bar' /path</span>. Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>mkdir -p /tmp/golf/foo /tmp/golf/src
<b><u><font color="#000000">printf</font></u></b> <font color="#808080">'bar</font>\n<font color="#808080">'</font> > /tmp/golf/src/a.txt
<b><u><font color="#000000">printf</font></u></b> <font color="#808080">'bar</font>\n<font color="#808080">'</font> > /tmp/golf/foo/skip.txt
grep -R --exclude-dir=foo <font color="#808080">'bar'</font> /tmp/golf
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
/tmp/golf/src/a.txt:bar
</pre>
<br />
<span>Insert lines with sed: <span class='inlinecode'>sed -e '1isomething' -e '3isomething' file</span>. Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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http://www.gnu.org/software/src-highlite -->
<pre><b><u><font color="#000000">printf</font></u></b> <font color="#808080">'A</font>\n<font color="#808080">B</font>\n<font color="#808080">C</font>\n<font color="#808080">'</font> > /tmp/s.txt
sed -e <font color="#808080">'1iHEAD'</font> -e <font color="#808080">'3iMID'</font> /tmp/s.txt
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
HEAD
A
B
MID
C
</pre>
<br />
<span>Drop last column with awk: <span class='inlinecode'>awk 'NF{NF-=1};1' file</span>. Example:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><b><u><font color="#000000">printf</font></u></b> <font color="#808080">'a b c</font>\n<font color="#808080">x y z</font>\n<font color="#808080">'</font> > /tmp/t.txt
cat /tmp/t.txt
echo
awk <font color="#808080">'NF{NF-=1};1'</font> /tmp/t.txt
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
a b c
x y z
a b
x y
</pre>
<br />
<h2 style='display: inline' id='safe-xargs-with-nuls'>Safe xargs with NULs</h2><br />
<br />
<span>Avoid breaking on spaces/newlines by pairing <span class='inlinecode'>find -print0</span> with <span class='inlinecode'>xargs -0</span>:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>find . -type f -name <font color="#808080">'*.log'</font> -print<font color="#000000">0</font> | xargs -<font color="#000000">0</font> rm -f
</pre>
<br />
<span>Example with spaces and NULs only:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><b><u><font color="#000000">printf</font></u></b> <font color="#808080">'a</font>\0<font color="#808080">b c</font>\0<font color="#808080">'</font> | xargs -<font color="#000000">0</font> -I{} <b><u><font color="#000000">printf</font></u></b> <font color="#808080">'<%s></font>\n<font color="#808080">'</font> {}
</pre>
<br />
<span>Output:</span><br />
<span> </span><br />
<pre>
<a>
<b c>
</pre>
<br />
<h2 style='display: inline' id='efficient-file-to-variable-and-arrays'>Efficient file-to-variable and arrays</h2><br />
<br />
<span>Read a whole file into a variable without spawning <span class='inlinecode'>cat</span>:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>cfg=$(<config.ini)
</pre>
<br />
<span>Read lines into an array safely with <span class='inlinecode'>mapfile</span> (aka <span class='inlinecode'>readarray</span>):</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre>mapfile -t lines < <(grep -v <font color="#808080">'^#'</font> config.ini)
<b><u><font color="#000000">printf</font></u></b> <font color="#808080">'%s</font>\n<font color="#808080">'</font> <font color="#808080">"${lines[@]}"</font>
</pre>
<br />
<span>Assign formatted strings without a subshell using <span class='inlinecode'>printf -v</span>:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre><b><u><font color="#000000">printf</font></u></b> -v msg <font color="#808080">'Hello %s, id=%04d'</font> <font color="#808080">"$USER"</font> <font color="#000000">42</font>
echo <font color="#808080">"$msg"</font>
</pre>
<br />
<span>Output:</span><br />
<br />
<pre>
Hello paul, id=0042
</pre>
<br />
<span>Read NUL-delimited data (pairs well with <span class='inlinecode'>-print0</span>):</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
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<pre>mapfile -d <font color="#808080">''</font> -t files < <(find . -type f -print<font color="#000000">0</font>)
<b><u><font color="#000000">printf</font></u></b> <font color="#808080">'%s</font>\n<font color="#808080">'</font> <font color="#808080">"${files[@]}"</font>
</pre>
<br />
<h2 style='display: inline' id='quick-password-generator'>Quick password generator</h2><br />
<br />
<span>Pure Bash with <span class='inlinecode'>/dev/urandom</span>:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>LC_ALL=C tr -dc <font color="#808080">'A-Za-z0-9_'</font> </dev/urandom | head -c <font color="#000000">16</font>; echo
</pre>
<br />
<span>Alternative using <span class='inlinecode'>openssl</span>:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>openssl rand -base<font color="#000000">64</font> <font color="#000000">16</font> | tr -d <font color="#808080">'</font>\n<font color="#808080">'</font> | cut -c<font color="#000000">1</font>-<font color="#000000">22</font>
</pre>
<br />
<h2 style='display: inline' id='yes-for-automation'><span class='inlinecode'>yes</span> for automation</h2><br />
<br />
<span><span class='inlinecode'>yes</span> streams a string repeatedly; handy for feeding interactive commands or quick load generation:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
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<pre>yes | rm -r large_directory <i><font color="silver"># auto-confirm</font></i>
yes n | dangerous-command <i><font color="silver"># auto-decline</font></i>
yes anything | head -n<font color="#000000">1</font> <i><font color="silver"># prints one line: anything</font></i>
</pre>
<br />
<h2 style='display: inline' id='forcing-true-to-fail-and-vice-versa'>Forcing <span class='inlinecode'>true</span> to fail (and vice versa)</h2><br />
<br />
<span>You can shadow builtins with functions:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre>true() { <b><u><font color="#000000">return</font></u></b> <font color="#000000">1</font>; }
false() { <b><u><font color="#000000">return</font></u></b> <font color="#000000">0</font>; }
<b><u><font color="#000000">true</font></u></b> || echo <font color="#808080">'true failed'</font>
<b><u><font color="#000000">false</font></u></b> && echo <font color="#808080">'false succeeded'</font>
<i><font color="silver"># Bypass function with builtin/command</font></i>
<b><u><font color="#000000">builtin</font></u></b> <b><u><font color="#000000">true</font></u></b> <i><font color="silver"># returns 0</font></i>
<b><u><font color="#000000">command</font></u></b> <b><u><font color="#000000">true</font></u></b> <i><font color="silver"># returns 0</font></i>
</pre>
<br />
<span>To disable a builtin entirely: <span class='inlinecode'>enable -n true</span> (re-enable with <span class='inlinecode'>enable true</span>).</span><br />
<br />
<span>Further reading:</span><br />
<br />
<a class='textlink' href='https://blog.robertelder.org/force-true-command-to-return-false/'>Force true to return false</a><br />
<br />
<h2 style='display: inline' id='restricted-bash'>Restricted Bash</h2><br />
<br />
<span><span class='inlinecode'>bash -r</span> (or <span class='inlinecode'>rbash</span>) starts a restricted shell that limits potentially dangerous actions, for example:</span><br />
<br />
<ul>
<li>Changing directories (<span class='inlinecode'>cd</span>).</li>
<li>Modifying <span class='inlinecode'>PATH</span>, <span class='inlinecode'>SHELL</span>, <span class='inlinecode'>BASH_ENV</span>, or <span class='inlinecode'>ENV</span>.</li>
<li>Redirecting output.</li>
<li>Running commands with <span class='inlinecode'>/</span> in the name.</li>
<li>Using <span class='inlinecode'>exec</span>.</li>
</ul><br />
<span>It’s a coarse sandbox for highly constrained shells; read <span class='inlinecode'>man bash</span> (RESTRICTED SHELL) for details and caveats.</span><br />
<br />
<span>Example session:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>rbash -c <font color="#808080">'cd /'</font> <i><font color="silver"># cd: restricted</font></i>
rbash -c <font color="#808080">'PATH=/tmp'</font> <i><font color="silver"># PATH: restricted</font></i>
rbash -c <font color="#808080">'echo hi > out'</font> <i><font color="silver"># redirection: restricted</font></i>
rbash -c <font color="#808080">'/bin/echo hi'</font> <i><font color="silver"># commands with /: restricted</font></i>
rbash -c <font color="#808080">'exec ls'</font> <i><font color="silver"># exec: restricted</font></i>
</pre>
<br />
<h2 style='display: inline' id='useless-use-of-cat-and-when-its-ok'>Useless use of cat (and when it’s ok)</h2><br />
<br />
<span>Avoid the extra process if a command already reads files or <span class='inlinecode'>STDIN</span>:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><i><font color="silver"># Prefer</font></i>
grep -i foo file
<file grep -i foo <i><font color="silver"># or feed via redirection</font></i>
<i><font color="silver"># Over</font></i>
cat file | grep -i foo
</pre>
<br />
<span>But for interactive composition, or when you truly need to concatenate multiple sources into a single stream, <span class='inlinecode'>cat</span> is fine, as you may think, "First I need the content, then I do X." Changing the "useless use of cat" in retrospect is really a waste of time for one-time interactive use:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
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<pre>cat file1 file2 | grep -i foo
</pre>
<br />
<span>From notes: “Good for interactivity; Useless use of cat” — use judgment.</span><br />
<br />
<h2 style='display: inline' id='atomic-locking-with-mkdir'>Atomic locking with <span class='inlinecode'>mkdir</span></h2><br />
<br />
<span>Portable advisory locks can be emulated with <span class='inlinecode'>mkdir</span> because it’s atomic:</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
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http://www.gnu.org/software/src-highlite -->
<pre>lockdir=/tmp/myjob.lock
<b><u><font color="#000000">if</font></u></b> mkdir <font color="#808080">"$lockdir"</font> <font color="#000000">2</font>>/dev/null; <b><u><font color="#000000">then</font></u></b>
<b><u><font color="#000000">trap</font></u></b> <font color="#808080">'rmdir "$lockdir"'</font> EXIT INT TERM
<i><font color="silver"># critical section</font></i>
do_work
<b><u><font color="#000000">else</font></u></b>
echo <font color="#808080">"Another instance is running"</font> >&<font color="#000000">2</font>
<b><u><font color="#000000">exit</font></u></b> <font color="#000000">1</font>
<b><u><font color="#000000">fi</font></u></b>
</pre>
<br />
<span>This works well on Linux. Remove the lock in <span class='inlinecode'>trap</span> so crashes don’t leave stale locks.</span><br />
<br />
<h2 style='display: inline' id='smarter-globs-and-faster-find-exec'>Smarter globs and faster find-exec</h2><br />
<br />
<ul>
<li>Enable extended globs when useful: <span class='inlinecode'>shopt -s extglob</span>; then patterns like <span class='inlinecode'>!(tmp|cache)</span> work.</li>
<li>Use <span class='inlinecode'>-exec ... {} +</span> to batch many paths in fewer process invocations:</li>
</ul><br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre>find . -name <font color="#808080">'*.log'</font> -exec gzip -<font color="#000000">9</font> {} +
</pre>
<br />
<span>Example for extglob (exclude two dirs from listing):</span><br />
<br />
<!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><b><u><font color="#000000">shopt</font></u></b> -s extglob
ls -d -- !(.git|node_modules) <font color="#000000">2</font>>/dev/null
</pre>
<br />
<span>E-Mail your comments to <span class='inlinecode'>paul@nospam.buetow.org</span> :-)</span><br />
<br />
<span>Other related posts are:</span><br />
<br />
<a class='textlink' href='./2025-09-14-bash-golf-part-4.html'>2025-09-14 Bash Golf Part 4 (You are currently reading this)</a><br />
<a class='textlink' href='./2023-12-10-bash-golf-part-3.html'>2023-12-10 Bash Golf Part 3</a><br />
<a class='textlink' href='./2022-01-01-bash-golf-part-2.html'>2022-01-01 Bash Golf Part 2</a><br />
<a class='textlink' href='./2021-11-29-bash-golf-part-1.html'>2021-11-29 Bash Golf Part 1</a><br />
<a class='textlink' href='./2021-06-05-gemtexter-one-bash-script-to-rule-it-all.html'>2021-06-05 Gemtexter - One Bash script to rule it all</a><br />
<a class='textlink' href='./2021-05-16-personal-bash-coding-style-guide.html'>2021-05-16 Personal Bash coding style guide</a><br />
<br />
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