Bash

Avoid common Bash mistakes — quoting traps, word splitting, and subshell gotchas.

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Last updated: April 23, 2026

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Quick Reference

Topic	File
Arrays and loops	`arrays.md`
Parameter expansion	`expansion.md`
Error handling patterns	`errors.md`
Testing and conditionals	`testing.md`

Quoting Traps

Always quote variables—
```
"$var"
```
not
```
$var
```
, spaces break unquoted
```
"${arr[@]}"
```
preserves elements—
```
${arr[*]}
```
joins into single string
Single quotes are literal—
```
'$var'
```
doesn't expand
Quote command substitution—
```
"$(command)"
```
not
```
$(command)
```

Word Splitting and Globbing

Unquoted
```
$var
```
splits on whitespace—
```
file="my file.txt"; cat $file
```
fails
Unquoted
```
*
```
expands to files—quote or escape if literal:
```
"*"
```
or
```
\*
```
```
set -f
```
disables globbing—or quote everything properly

Test Brackets

```
[[ ]]
```
preferred over
```
[ ]
```
—no word splitting, supports
```
&&
```
,
```
||
```
, regex
```
[[ $var == pattern* ]]
```
—glob patterns without quotes on right side
```
[[ $var =~ regex ]]
```
—regex match, don't quote the regex
```
-z
```
is empty,
```
-n
```
is non-empty—
```
[[ -z "$var" ]]
```
tests if empty

Subshell Traps

Pipes create subshells—

cat file | while read; do ((count++)); done

—count lost

Use

while read < file

or process substitution—

while read; do ...; done < <(command)

```
( )
```
is subshell,
```
{ }
```
is same shell—variables in
```
( )
```
don't persist

Exit Handling

```
set -e
```
exits on error—but not in
```
if
```
,
```
||
```
,
```
&&
```
conditions
```
set -u
```
errors on undefined vars—catches typos
```
set -o pipefail
```
—pipeline fails if any command fails, not just last
```
trap cleanup EXIT
```
—runs on any exit, even errors

Arrays

Declare:
```
arr=(one two three)
```
—or
```
arr=()
```
then
```
arr+=(item)
```
Length:
```
${#arr[@]}
```
—not
```
${#arr}
```
All elements:
```
"${arr[@]}"
```
—always quote
Indices:
```
${!arr[@]}
```
—useful for sparse arrays

Parameter Expansion

Default value:
```
${var:-default}
```
—use default if unset/empty
Assign default:
```
${var:=default}
```
—also assigns to var
Error if unset:
```
${var:?error message}
```
—exits with message
Substring:
```
${var:0:5}
```
—first 5 chars
Remove prefix:
```
${var#pattern}
```
—
```
##
```
for greedy

Arithmetic

```
$(( ))
```
for math—
```
result=$((a + b))
```
```
(( ))
```
for conditions—
```
if (( count > 5 )); then
```
No
```
$
```
needed inside
```
$(( ))
```
—
```
$((count + 1))
```
not
```
$(($count + 1))
```

Common Mistakes

[ $var = "value" ]

fails if var empty—use

[ "$var" = "value" ]

[[ ]]

```
if [ -f $file ]
```
with spaces—always quote:
```
if [[ -f "$file" ]]
```
```
local
```
in functions—without it, variables are global
```
read
```
without
```
-r
```
—backslashes interpreted as escapes
```
echo
```
portability—use
```
printf
```
for reliable formatting

Bash

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Quick Reference

Quoting Traps

Word Splitting and Globbing

Test Brackets

Subshell Traps

Exit Handling

Arrays

Parameter Expansion

Arithmetic

Common Mistakes

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