Shell Scripting

• Shebang is line 1 only; the kernel reads it on exec
• #!/usr/bin/env bash picks bash from PATH
• #!/bin/bash is a fixed path

• readonly blocks reassignment; export shows it to child processes
• Untyped strings; global unless local in a function

• Why env bash? It finds bash on PATH, so it works where bash is not in /bin
• ./s.sh vs source? ./ runs a child shell; source runs in the current one
• Why does x = 5 fail? The spaces make x a command with two arguments

• Unquoted $var is word-split and glob-expanded
• f='my file.txt'; rm $f gets two args
• Always write "$var"
• "..." keeps $var, $(cmd); \ escapes only $ backtick " \
• '...' is fully literal

• [[ ]]: left side needs no quotes; == is pattern match
• [ ]: unquoted empty var gives a syntax error

• rm $dir/* with a space or empty $dir? Splits into extra args; empty becomes rm /*
• Single vs double quotes? Single is literal; double expands $ and $(cmd)
• "$@" vs "$*"? Separate words vs one string

• if cmd; then ...; elif ...; else ...; fi tests exit status
• 0 = true, nonzero = false
• [ ] and [[ ]] are just commands returning a status

• -f file, -d dir, -e exists
• -z empty string, -n non-empty
• -eq -lt -gt integers; == strings

for f in *.log; do echo "$f"; done
for ((i=0;i<5;i++)); do echo "$i"; done
while read -r line; do echo "$line"; done < file
until cmd; do sleep 1; done

break leaves the loop, continue skips to the next pass

• [ ] vs [[ ]]? POSIX command vs bash keyword: no splitting, &&, =~
• Why while read -r? for over $(cat f) splits on spaces; -r keeps backslashes
• if grep -q x f tests? grep's exit status: 0 when a match is found

• Define name() { ...; }, call it like a command
• Inside: $1 $2 ... and $#
• local x=1 keeps x out of the global scope

• return n and exit n set a status 0-255
• return leaves the function; exit ends the script
• $? = last status, overwritten by the next command
• exit 0 = success, anything else = failure
• Functions return a status; data goes out via stdout: result=$(fn)
• Test success with if fn; then

• Put set -euo pipefail at the top of the script
• set -e skips if/while conditions
• Also skips any &&/|| command except the last: false && x lives, true && false exits
• Without pipefail, false | true returns 0 and hides the failure

• Fields: min hour dom mon dow command
• crontab -e edits, crontab -l lists

#!/usr/bin/env bash
set -euo pipefail
v="${1:-x}"
now=$(date +%F)
n=$(( $# + 1 ))

default, substitution, math

• Run: chmod +x s.sh, then ./s.sh
• Always quote: "$var"

• */5 * * * * every 5 minutes
• 0 2 * * 1 02:00 Mondays
• crontab -e edit, -l list