Official docs
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The bible (short version)
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The bible (full version)
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Parameter expansion
${var:-word} |
Substitute word if var is unset or null |
${var:=word} |
Substitute and assign word to var if var is unset/null |
${var:?word} |
Write word to stderr and exit if var is unset/null |
${var:+word} |
Substitute word if var is set or non null |
${var:offset[:len]} |
Expands to up to len characters of var starting at the character specified by offset. |
${!var} |
Indirection: expands the the value of the value of var |
${!prefix*} |
Expands to the names of variables whose names begin with prefix separated by IFS[0] (as single word) |
${!prefix@} |
Same as above but as multiple words |
${!name[*]} |
Expands to the list of array indices (keys) assigned in name |
${#var} |
Length of var, or length of array for ${#arr[*]} |
${var#word} |
Removes shortest prefix word from var. |
${var##word} |
Same as above but with longest prefix |
${parameter%word} |
Removes shortest suffix word from var. |
${parameter%%word} |
Same as above but with longest suffix |
${var/pattern/string} |
Substitute first instance of pattern in var with string |
${var//pattern/string} |
Same as above, but all instances |
${var/#pattern/string} |
Same as above, but pattern must be at beginning of var |
${var/%pattern/string} |
Same as above, but pattern must be at the end of var |
${var^pattern} |
Uppercases the first character of var if it matches pattern. Empty pattern matches all. |
${var^^pattern} |
Uppercases every match of pattern in var |
${var,pattern} |
Same as ${var^pattern} but lowercase |
${var,,pattern} |
Same as ${var^^pattern} but lowercase |
${var@operator} |
Various operations, see manual. Includes case modification, length, automatic quoting and escaping, variable export, reading attributes. |
3.5.3 Shell Parameter Expansion
word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. pattern matches according to pattern matching.
Conditional Expressions
Checks for return code of command
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[ expression ] External tool ("test") that runs with parameters expression and returns 0 for true, 1 for false.
Word splitting, parameter expansion and filename glob expansions occur before the test command runs: always quote variables to prevent word expansion and filename expansion unless desired.
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[[ conditional ]] Internal bash builtin that runs the conditional check. Expansions are performed except word splitting and filename expansion: quoting variables is not necessary.
Supports < and > as comparison operators, && and || as and/or chaining operators, =~ as regex matching operator
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Order of expansions
1 - Word splitting |
2 - Braces { } |
3 - Tildes ~ |
4 - Parameters / variables ${ } |
5 - Arithmetic $(( )) |
6 - Commands $( ) |
7 - Word splitting (again) |
8 - Pathnames ./* |
9 - Process substitution <( ) |
Only brace expansion, word splitting, pathname expansion, "$@" and "${name[@]}" can change the number of words of the expansion; other expansions expand a single word to a single word.
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Command expansion
$(command) |
Replaces the command substitution with the standard output of the command |
`command` |
Same as above but backslash retains its literal meaning unless escaped |
If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results.
Process Substitution
<(process) |
Substitutes with the standard output fd of process |
>(process) |
Substitutes with the standard input fd of process |
Similar to piping but acts with files (named pipes), allowing them to work with commands that do not accept standard input unlike pipes, and allows working with multiple inputs/outputs without ... | tee
Arithmetic Expansion
$((expression)) |
Evaluates expression as an arithmetic expression |
Operator precedence, associativity, and values are the same as in C. Shell variables can be used raw without $. Evaluation is done in fixed-width integers. Prefixes indicate bases: "0" for octal, "0x" for hex, "n#" for base n.
Pattern Matching
\ |
Escapes next character |
* |
Matches any string incl null string. |
? |
Matches any single char |
[...] |
As regex |
?(pattern-list) |
Matches 0 or 1 instance of one of the patterns separated by | |
*(pattern-list) |
Matches 0+ instances of one of the patterns separated by | |
+(pattern-list) |
Matches 1+ instances of one of the patterns separated by | |
@(pattern-list) |
Matches exactly 1 instance of one of the patterns separated by | |
!(pattern-list) |
Matches anything except one of the patterns separated by | |
** |
When globstar is enabled, matches all files, dirs, subdirs and files in subdirs |
"Pattern list" patterns require the extglob option to be enabled.
Works in case statements, == and != inside [[ ]], case modification, pattern match variable expansion and filename expansion if the "pattern characters" are not quoted.
I/O redirection
command [n]< file |
Opens file for reading and redirects fd n (default 0, stdin) of command to file |
command [n]> file |
Opens file for writing and redirects fd n (default 1, stdout) of command to file |
&>file |
Redirects both stdin and stderr to file. Same as >word 2>&1
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&>>file |
Same as above but appends |
<<word
text
word |
Substitute standard input with text (can be multiple lines) |
<<-word
text
word |
As above but strips tabs from text and final word |
<<"word"
text
word |
As <<word but do not perform parameter expansion, command substitution, and arithmetic expansion in text |
<<<word |
Supplies word as a single line to fd n (default stdin) |
[n]<&word |
File descriptor n is made to be a copy of fd word for input |
[n]>&word |
Same as above but for output |
[n]<&digit- |
Moves fd digit to fd n (default stdin) and closes digit |
[n]>&digit- |
Same but for output (default stdout) |
[n]<>word |
Opens word for reading and writing on fd n |
A fd number can be prepended to most forms in order to redirect to another fd instead of the default stdin/out/err.
Grouping Commands
(list) |
Executes the commands in a subshell |
{ list; } |
Executes the commands in the current environment |
The exit status of both of these constructs is the exit status of list. See pipefail option.
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