From 90fd4f95576864b413008ce035ca0bf79cc689b4 Mon Sep 17 00:00:00 2001 From: Paul Buetow Date: Sat, 13 Sep 2025 12:05:29 +0300 Subject: Update content for html --- gemfeed/2025-09-14-bash-golf-part-4.html | 662 +++++++++++++++++++++++++++++++ 1 file changed, 662 insertions(+) create mode 100644 gemfeed/2025-09-14-bash-golf-part-4.html (limited to 'gemfeed/2025-09-14-bash-golf-part-4.html') diff --git a/gemfeed/2025-09-14-bash-golf-part-4.html b/gemfeed/2025-09-14-bash-golf-part-4.html new file mode 100644 index 00000000..457b15d8 --- /dev/null +++ b/gemfeed/2025-09-14-bash-golf-part-4.html @@ -0,0 +1,662 @@ + + + + +Bash Golf Part 4 + + + + + +

+Home | Markdown | Gemini +

+

Bash Golf Part 4


+
+Published at 2025-09-13T12:04:03+03:00
+
+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.
+
+2021-11-29 Bash Golf Part 1
+2022-01-01 Bash Golf Part 2
+2023-12-10 Bash Golf Part 3
+2025-09-14 Bash Golf Part 4 (You are currently reading this)
+
+
+    '\       '\        '\        '\                   .  .        |>18>>
+      \        \         \         \              .         ' .   |
+     O>>      O>>       O>>       O>>         .                 'o |
+      \       .\. ..    .\. ..    .\. ..   .                      |
+      /\    .  /\     .  /\     .  /\    . .                      |
+     / /   .  / /  .'.  / /  .'.  / /  .'    .                    |
+jgs^^^^^^^`^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+                        Art by Joan Stark, mod. by Paul Buetow
+
+
+

Table of Contents


+
+
+

Split pipelines with tee + process substitution


+
+Sometimes you want to fan out one stream to multiple consumers and still continue the original pipeline. tee plus process substitution does exactly that:
+
+
+somecommand \
+    | tee >(command1) >(command2) \
+    | command3
+
+
+All of command1, command2, and command3 see the output of somecommand. Example:
+
+ +
printf 'a\nb\n' \
+    | tee >(sed 's/.*/X:&/; s/$/ :c1/') >(tr a-z A-Z | sed 's/$/ :c2/') \
+    | sed 's/$/ :c3/'
+
+
+Output:
+
+
+a :c3
+b :c3
+A :c2 :c3
+B :c2 :c3
+X:a :c1 :c3
+X:b :c1 :c3
+
+
+This relies on Bash process substitution (>(...)). Make sure your shell is Bash and not a POSIX /bin/sh.
+
+Example (fails under dash/POSIX sh):
+
+ +
/bin/sh -c 'echo hi | tee >(cat)'
+# /bin/sh: 1: Syntax error: "(" unexpected
+
+
+Combine with set -o pipefail if failures in side branches should fail the whole pipeline.
+
+Example:
+
+ +
set -o pipefail
+printf 'ok\n' | tee >(false) | cat >/dev/null
+echo $?   # 1 because a side branch failed
+
+
+Further reading:
+
+Splitting pipelines with tee
+
+

Heredocs for remote sessions (and their gotchas)


+
+Heredocs are great to send multiple commands over SSH in a readable way:
+
+ +
ssh "$SSH_USER@$SSH_HOST" <<EOF
+    # Go to the work directory
+    cd "$WORK_DIR"
+  
+    # Make a git pull
+    git pull
+  
+    # Export environment variables required for the service to run
+    export AUTH_TOKEN="$APP_AUTH_TOKEN"
+  
+    # Start the service
+    docker compose up -d --build
+EOF
+
+
+Tips:
+
+Quoting the delimiter changes interpolation. Use <<'EOF' to avoid local expansion and send the content literally.
+
+Example:
+
+ +
FOO=bar
+cat <<'EOF'
+$FOO is not expanded here
+EOF
+
+
+Prefer explicit quoting for variables (as above) to avoid surprises. Example (spaces preserved only when quoted):
+
+ +
WORK_DIR="/tmp/my work"
+ssh host <<EOF
+    cd $WORK_DIR      # may break if unquoted
+    cd "$WORK_DIR"   # safe
+EOF
+
+
+Consider set -euo pipefail at the top of the remote block for stricter error handling. Example:
+
+ +
ssh host <<'EOF'
+    set -euo pipefail
+    false   # causes immediate failure
+    echo never
+EOF
+
+
+Indent-friendly variant: use a dash to strip leading tabs in the body:
+
+ +
cat <<-EOF > script.sh
+	#!/usr/bin/env bash
+	echo "tab-indented content is dedented"
+EOF
+
+
+Further reading:
+
+Heredoc headaches and fixes
+
+

Namespacing and dynamic dispatch with ::


+
+You can emulate simple namespacing by encoding hierarchy in function names. One neat pattern is pseudo-inheritance via a tiny super helper that maps pkg::lang::action to a pkg::base::action default.
+
+ +
#!/usr/bin/env bash
+set -euo pipefail
+
+super() {
+    local -r fn=${FUNCNAME[1]}
+    # Split name on :: and dispatch to base implementation
+    local -a parts=( ${fn//::/ } )
+    "${parts[0]}::base::${parts[2]}" "$@"
+}
+
+foo::base::greet() { echo "base: $@"; }
+foo::german::greet()  { super "Guten Tag, $@!"; }
+foo::english::greet() { super "Good day,  $@!"; }
+
+for lang in german english; do
+    foo::$lang::greet Paul
+done
+
+
+Output:
+
+
+base: Guten Tag, Paul!
+base: Good day,  Paul!
+
+
+

Indirect references with namerefs


+
+declare -n creates a name reference — a variable that points to another variable. It’s cleaner than eval for indirection:
+
+ +
user_name=paul
+declare -n ref=user_name
+echo "$ref"       # paul
+ref=julia
+echo "$user_name" # julia
+
+
+Output:
+
+
+paul
+julia
+
+
+Namerefs are local to functions when declared with local -n. Requires Bash ≥4.3.
+
+You can also construct the target name dynamically:
+
+ +
make_var() {
+    local idx=$1; shift
+    local name="slot_$idx"
+    printf -v "$name" '%s' "$*"   # create variable slot_$idx
+}
+
+get_var() {
+    local idx=$1
+    local -n ref="slot_$idx"      # bind ref to slot_$idx
+    printf '%s\n' "$ref"
+}
+
+make_var 7 "seven"
+get_var 7
+
+
+Output:
+
+
+seven
+
+
+

Function declaration forms


+
+All of these work in Bash, but only the first one is POSIX-ish:
+
+ +
foo() { echo foo; }
+function foo { echo foo; }
+function foo() { echo foo; }
+
+
+Recommendation: prefer name() { ... } for portability and consistency.
+
+

Chaining function calls in conditionals


+
+Functions return a status like commands. You can short-circuit them in conditionals:
+
+ +
deploy_check() { test -f deploy.yaml; }
+smoke_test()   { curl -fsS http://localhost/healthz >/dev/null; }
+
+if deploy_check || smoke_test; then
+    echo "All good."
+else
+    echo "Something failed." >&2
+fi
+
+
+You can also compress it golf-style:
+
+ +
deploy_check || smoke_test && echo ok || echo fail >&2
+
+
+

Grep, sed, awk quickies


+
+Word match and context: grep -w word file; with context: grep -C3 foo file (same as -A3 -B3). Example:
+
+ +
cat > /tmp/ctx.txt <<EOF
+one
+foo
+two
+three
+bar
+EOF
+grep -C1 foo /tmp/ctx.txt
+
+
+Output:
+
+
+one
+foo
+two
+
+
+Skip a directory while recursing: grep -R --exclude-dir=foo 'bar' /path. Example:
+
+ +
mkdir -p /tmp/golf/foo /tmp/golf/src
+printf 'bar\n' > /tmp/golf/src/a.txt
+printf 'bar\n' > /tmp/golf/foo/skip.txt
+grep -R --exclude-dir=foo 'bar' /tmp/golf
+
+
+Output:
+
+
+/tmp/golf/src/a.txt:bar
+
+
+Insert lines with sed: sed -e '1isomething' -e '3isomething' file. Example:
+
+ +
printf 'A\nB\nC\n' > /tmp/s.txt
+sed -e '1iHEAD' -e '3iMID' /tmp/s.txt
+
+
+Output:
+
+
+HEAD
+A
+B
+MID
+C
+
+
+Drop last column with awk: awk 'NF{NF-=1};1' file. Example:
+
+ +
printf 'a b c\nx y z\n' > /tmp/t.txt
+cat /tmp/t.txt
+echo
+awk 'NF{NF-=1};1' /tmp/t.txt
+
+
+Output:
+
+
+a b c
+x y z
+
+a b
+x y
+
+
+

Safe xargs with NULs


+
+Avoid breaking on spaces/newlines by pairing find -print0 with xargs -0:
+
+ +
find . -type f -name '*.log' -print0 | xargs -0 rm -f
+
+
+Example with spaces and NULs only:
+
+ +
printf 'a\0b c\0' | xargs -0 -I{} printf '<%s>\n' {}
+
+
+Output:
+
+
+<a>
+<b c>
+
+
+

Efficient file-to-variable and arrays


+
+Read a whole file into a variable without spawning cat:
+
+ +
cfg=$(<config.ini)
+
+
+Read lines into an array safely with mapfile (aka readarray):
+
+ +
mapfile -t lines < <(grep -v '^#' config.ini)
+printf '%s\n' "${lines[@]}"
+
+
+Assign formatted strings without a subshell using printf -v:
+
+ +
printf -v msg 'Hello %s, id=%04d' "$USER" 42
+echo "$msg"
+
+
+Output:
+
+
+Hello paul, id=0042
+
+
+Read NUL-delimited data (pairs well with -print0):
+
+ +
mapfile -d '' -t files < <(find . -type f -print0)
+printf '%s\n' "${files[@]}"
+
+
+

Quick password generator


+
+Pure Bash with /dev/urandom:
+
+ +
LC_ALL=C tr -dc 'A-Za-z0-9_' </dev/urandom | head -c 16; echo
+
+
+Alternative using openssl:
+
+ +
openssl rand -base64 16 | tr -d '\n' | cut -c1-22
+
+
+

yes for automation


+
+yes streams a string repeatedly; handy for feeding interactive commands or quick load generation:
+
+ +
yes | rm -r large_directory        # auto-confirm
+yes n | dangerous-command          # auto-decline
+yes anything | head -n1            # prints one line: anything
+
+
+

Forcing true to fail (and vice versa)


+
+You can shadow builtins with functions:
+
+ +
true()  { return 1; }
+false() { return 0; }
+
+true  || echo 'true failed'
+false && echo 'false succeeded'
+
+# Bypass function with builtin/command
+builtin true # returns 0
+command true # returns 0
+
+
+To disable a builtin entirely: enable -n true (re-enable with enable true).
+
+Further reading:
+
+Force true to return false
+
+

Restricted Bash


+
+bash -r (or rbash) starts a restricted shell that limits potentially dangerous actions, for example:
+
+
+It’s a coarse sandbox for highly constrained shells; read man bash (RESTRICTED SHELL) for details and caveats.
+
+Example session:
+
+ +
rbash -c 'cd /'            # cd: restricted
+rbash -c 'PATH=/tmp'       # PATH: restricted
+rbash -c 'echo hi > out'   # redirection: restricted
+rbash -c '/bin/echo hi'    # commands with /: restricted
+rbash -c 'exec ls'         # exec: restricted
+
+
+

Useless use of cat (and when it’s ok)


+
+Avoid the extra process if a command already reads files or STDIN:
+
+ +
# Prefer
+grep -i foo file
+<file grep -i foo        # or feed via redirection
+
+# Over
+cat file | grep -i foo
+
+
+But for interactive composition, or when you truly need to concatenate multiple sources into a single stream, cat 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:
+
+ +
cat file1 file2 | grep -i foo
+
+
+From notes: “Good for interactivity; Useless use of cat” — use judgment.
+
+

Atomic locking with mkdir


+
+Portable advisory locks can be emulated with mkdir because it’s atomic:
+
+ +
lockdir=/tmp/myjob.lock
+if mkdir "$lockdir" 2>/dev/null; then
+    trap 'rmdir "$lockdir"' EXIT INT TERM
+    # critical section
+    do_work
+else
+    echo "Another instance is running" >&2
+    exit 1
+fi
+
+
+This works well on Linux. Remove the lock in trap so crashes don’t leave stale locks.
+
+

Smarter globs and faster find-exec


+
+
+ +
find . -name '*.log' -exec gzip -9 {} +
+
+
+Example for extglob (exclude two dirs from listing):
+
+ +
shopt -s extglob
+ls -d -- !(.git|node_modules) 2>/dev/null
+
+
+E-Mail your comments to paul@nospam.buetow.org :-)
+
+Other related posts are:
+
+2025-09-14 Bash Golf Part 4 (You are currently reading this)
+2023-12-10 Bash Golf Part 3
+2022-01-01 Bash Golf Part 2
+2021-11-29 Bash Golf Part 1
+2021-06-05 Gemtexter - One Bash script to rule it all
+2021-05-16 Personal Bash coding style guide
+
+Back to the main site
+

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