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What a shell does
A shell is both a command interpreter and a programming language. At a terminal, it reads commands and runs programs; in a script, it combines commands with variables, conditions, loops, functions, and input/output rules. The GNU Bash Reference Manual, Edition 5.3, updated May 18, 2025, describes these roles and the parts of shell programming: Bash Reference Manual.
A script is a file containing commands for the shell to read. Instead of retyping a routine sequence, you can save it, make it executable, and run it again. The shell also handles text before a program receives it: it recognizes syntax, expands variables and patterns, applies redirections, then executes commands. Understanding that order explains many beginner surprises.
Write and run your first Bash script
Create a file named hello.sh with this content:
#!/usr/bin/env bash
printf 'Hello, %s!n' "${1:-world}"The first line is the shebang: it asks the operating system to run the file with Bash found through /usr/bin/env. The second line prints the first argument, or world if none was supplied. Save the file, then run it explicitly with Bash:
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bash hello.sh
bash hello.sh AdaAlternatively, make it executable and invoke it by path:
chmod +x hello.sh
./hello.sh AdaUse ./ to run a script in the current directory; the current directory is not necessarily searched when you type only a filename. The shebang’s interpreter must exist on the target system. Running bash hello.sh explicitly avoids relying on executable permissions or the shebang, but still requires Bash.
Commands, arguments, and shell parsing
A command is commonly a program name followed by arguments. For example, printf '%sn' 'two words' runs printf with a format argument and one data argument. The shell does not simply pass each line untouched to a program: it reads input, recognizes words and operators under quoting rules, parses commands, performs expansions, applies redirections, and executes. The program receives the resulting arguments, not the original line of text.
Whitespace usually separates words. Thus printf '%sn' two words passes two data arguments, whereas quoting 'two words' keeps that phrase as one argument. Characters such as *, $, |, and > have special shell meanings in relevant contexts.
Quoting: keep data from becoming syntax
Quoting controls which characters the shell treats specially. Single quotes preserve their contents literally, while double quotes preserve most characters but still allow selected expansions such as variables and command substitutions. The Bash manual explains quoting in detail: Quoting.
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'$HOME *.txt'is literal text: neither the variable nor the wildcard expands."$HOME"expands the variable, but keeps its result as one word, even if it contains spaces."*.txt"keeps the asterisk literal rather than matching filenames.
Quote variable expansions by default, especially when passing filenames or user-provided text to commands. For example:
name='Ada Lovelace'
printf 'User: %sn' "$name"Without quotes, an expansion can be split into multiple words and wildcard characters in its result can expand against filenames. Use unquoted expansions only when you deliberately need the shell’s splitting or pattern matching behavior.
Variables, parameters, and expansions
Assign a variable with no spaces around the equals sign. Refer to its value with a dollar sign:
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printf 'Writing to %sn' "$output_file"Spaces around = would make the line a command and its arguments rather than an assignment. Positional parameters provide the script’s arguments: $1 is the first, $2 the second, and $# their count. Prefer "$@" to pass all arguments on individually while preserving spaces:
printf 'Argument: %sn' "$@"Bash’s ${name:-default} expansion supplies a default when a parameter is unset or empty, as in the first example. Bash also offers other parameter expansions; mark Bash-specific syntax when portability matters.
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Exit status and handling failures
Each command returns an exit status: conventionally, zero means success and a nonzero value indicates an error or another unsuccessful result. The special parameter $? contains the most recent command’s status, so inspect it immediately if needed:
cp source.txt backup.txt
status=$?
if [ "$status" -ne 0 ]; then
printf 'Copy failed (status %s)n' "$status" >&2
exit "$status"
fiOften it is clearer to put a command directly in an if condition:
if cp source.txt backup.txt; then
printf 'Copy completen'
else
printf 'Copy failedn' >&2
exit 1
fiDo not assume a script succeeded merely because it printed output. Check the status when later steps depend on a command working. Bash has options such as set -e, but its behavior has exceptions and context-dependent details; explicit checks make failure handling easier to understand.
Conditions and loops
Use if to choose actions based on command success or a test. This Bash example checks whether a path names a regular file:
file='report.txt'
if [ -f "$file" ]; then
printf 'Found %sn' "$file"
else
printf 'Missing %sn' "$file" >&2
fiQuote the path expansion so a filename with spaces remains a single test argument. For a basic repeat over arguments, a for loop is useful:
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for file in "$@"; do
printf 'Received: %sn' "$file"
doneA while loop repeats while its condition succeeds. Shell loop syntax is concise, but be careful with data from files: reading lines with read has details around backslashes and whitespace that require deliberate options when preserving arbitrary input.
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Functions for reusable steps
Functions let you give a command sequence a name. In Bash:
say_hello() {
printf 'Hello, %s!n' "${1:-world}"
}
say_hello AdaFunction arguments use the same positional parameters, such as $1. A function’s exit status is usually the status of its last command unless you explicitly return another status. Keep functions focused and quote their parameter expansions as you would in the top-level script.
Redirection and pipelines
Redirection connects a command’s input or output to a file or another destination. For example, command >out.txt writes standard output to a file, replacing its previous contents; command >>out.txt appends instead. Standard error can be redirected separately with 2>errors.txt. A pipeline sends one command’s standard output to the next command’s standard input:
printf '%sn' *.txt | sortThis prints the shell-expanded matching filenames in sorted order. Redirection order matters; pipeline exit-status behavior can also depend on the shell. Bash offers set -o pipefail so a pipeline can report failure from a command before the last one, but that option is not POSIX shell syntax.
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Choose Bash or portable sh deliberately
Write for the shell named by the script’s shebang and required by the target environment. POSIX specifies important shell behavior, including pipelines, redirection, quoting, argument handling, variable expansion, and flow control. Bash aims to implement the POSIX Shell and Tools specification, but its default behavior is not identical to POSIX in every area; Bash’s POSIX mode narrows some differences. See the manual’s Bash POSIX mode discussion.
| Choice | When it fits | Portability consideration |
|---|---|---|
#!/bin/sh |
When the script should use POSIX shell constructs and the system’s sh. |
Do not use Bash-only syntax; sh may be another shell. |
#!/usr/bin/env bash |
When Bash is required and available through the user’s command search path. | Bash-specific constructs are not guaranteed in sh or other shells. |
For a script intended for multiple Unix-like systems, keep syntax within the POSIX shell language and verify the target’s available utilities too; shell portability does not make every external command identical across systems. For a Bash-only script, say so and use Bash consistently rather than relying on whichever shell happens to launch it.
Common beginner problems and fixes
- “Command not found” for the script: invoke a file in the current directory as
./script.sh, or provide its full path. Check that its interpreter exists. - Permission denied: run
chmod +x script.shbefore direct execution, or runbash script.shif Bash is the intended interpreter. - Arguments split unexpectedly: quote expansions such as
"$name"; use"$@"when forwarding all arguments separately. - Wildcard stayed literal: check whether it was quoted. A quoted
"*.txt"is data, not a filename pattern. - Assignment appears to run as a command: write
name='value'without spaces around the equals sign. - A later command runs after an earlier failure: test the earlier command’s status and choose explicitly whether to report, recover, or stop.
- Script works in Bash but not with
sh: check the shebang and remove Bash-only syntax, or run it with Bash as intended.
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Frequently Asked Questions
Does a shell script need a .sh extension?
No. The extension is a naming convention; execution depends on how you invoke the file, its contents, permissions, and interpreter.
Can I run a Bash script on every Unix-like system?
Only where Bash is available and the script’s assumptions are supported. For broader compatibility, target POSIX shell syntax and the utilities available on your target systems.
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