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formatting fixes for parameter expansion - my favorite article.
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Sahal Ansari
Florian Köhler
parent
991569619d
commit
738b13c314
@ -52,47 +52,47 @@ For a more technical view what a parameter is and which types exist,
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Looking for a specific syntax you saw, without knowing the name?
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- [Simple usage](#simple_usage)
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- [Simple usage](#simple-usage)
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- `$PARAMETER`
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- `${PARAMETER}`
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- [Indirection](#indirection)
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- `${!PARAMETER}`
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- [Case modification](#case_modification)
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- [Case modification](#case-modification)
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- `${PARAMETER^}`
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- `${PARAMETER^^}`
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- `${PARAMETER,}`
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- `${PARAMETER,,}`
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- `${PARAMETER~}`
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- `${PARAMETER~~}`
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- [Variable name expansion](#variable_name_expansion)
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- [Variable name expansion](#variable-name-expansion)
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- `${!PREFIX*}`
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- `${!PREFIX@}`
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- [Substring removal](#substring_removal) (also for **filename
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- [Substring removal](#substring-removal) (also for **filename
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manipulation**!)
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- `${PARAMETER#PATTERN}`
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- `${PARAMETER##PATTERN}`
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- `${PARAMETER%PATTERN}`
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- `${PARAMETER%%PATTERN}`
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- [Search and replace](#search_and_replace)
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- [Search and replace](#search-and-replace)
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- `${PARAMETER/PATTERN/STRING}`
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- `${PARAMETER//PATTERN/STRING}`
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- `${PARAMETER/PATTERN}`
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- `${PARAMETER//PATTERN}`
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- [String length](#string_length)
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- [String length](#string-length)
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- `${#PARAMETER}`
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- [Substring expansion](#substring_expansion)
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- [Substring expansion](#substring-expansion)
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- `${PARAMETER:OFFSET}`
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- `${PARAMETER:OFFSET:LENGTH}`
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- [Use a default value](#use_a_default_value)
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- [Use a default value](#use-a-default-value)
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- `${PARAMETER:-WORD}`
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- `${PARAMETER-WORD}`
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- [Assign a default value](#assign_a_default_value)
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- [Assign a default value](#assign-a-default-value)
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- `${PARAMETER:=WORD}`
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- `${PARAMETER=WORD}`
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- [Use an alternate value](#use_an_alternate_value)
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- [Use an alternate value](#use-an-alternate-value)
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- `${PARAMETER:+WORD}`
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- `${PARAMETER+WORD}`
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- [Display error if null or unset](#display_error_if_null_or_unset)
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- [Display error if null or unset](#display-error-if-null-or-unset)
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- `${PARAMETER:?WORD}`
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- `${PARAMETER?WORD}`
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@ -226,14 +226,11 @@ The `^` operator modifies the first character to uppercase, the `,`
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operator to lowercase. When using the double-form (`^^` and `,,`), all
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characters are converted.
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<wrap center round info 60%>
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The (**currently undocumented**) operators `~` and `~~` reverse the case
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of the given text (in `PARAMETER`).`~` reverses the case of first letter
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of words in the variable while `~~` reverses case for all. Thanks to
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`Bushmills` and `geirha` on the Freenode IRC channel for this finding.
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</wrap>
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!!! INFO
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The (**currently undocumented**) operators `~` and `~~` reverse the case
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of the given text (in `PARAMETER`).`~` reverses the case of first letter
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of words in the variable while `~~` reverses case for all. Thanks to
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`Bushmills` and `geirha` on the Freenode IRC channel for this finding.
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<u>**Example: Rename all `*.txt` filenames to lowercase**</u>
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@ -270,9 +267,8 @@ Assume: `array=(This is some Text)`
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- => `This Is Some Text`
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- `echo "${array[@]^^}"`
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- => `THIS IS SOME TEXT`
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* ''echo "${array[2]^^}"''
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* => ''SOME''
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- `echo "${array[2]^^}"`
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- => `SOME`
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## Variable name expansion
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@ -351,16 +347,16 @@ filename**. Just look at the following list with examples:
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- **Get name without extension**
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- `${FILENAME%.*}`
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- => `bash_hackers.txt`
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- => <code>bash_hackers<del>.txt</del></code>
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- **Get extension**
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- `${FILENAME##*.}`
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- => `bash_hackers.txt`
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- => <code><del>bash_hackers.</del>txt</code>
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- **Get directory name**
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- `${PATHNAME%/*}`
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- => `/home/bash/bash_hackers.txt`
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- => <code>/home/bash<del>/bash_hackers.txt</del></code>
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- **Get filename**
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- `${PATHNAME##*/}`
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- => `/home/bash/bash_hackers.txt`
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- => <code><del>/home/bash/</del>bash_hackers.txt</code>
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These are the syntaxes for filenames with a single extension. Depending
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on your needs, you might need to adjust shortest/longest match.
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@ -413,7 +409,7 @@ example string:
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${MYSTRING//conservative/happy}
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=>
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`Be liberal in what you accept, and conservativehappy in what you send`
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<code>Be liberal in what you accept, and <del>conservative</del>happy in what you send</code>
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Since there is only one "conservative" in that example, it really
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doesn't matter which of the two forms we use.
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@ -425,14 +421,14 @@ but let's substitute it with "by".
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${MYSTRING/in/by}
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=> `Be liberal inby what you accept, and conservative in what you send`
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<code>Be liberal <del>in</del>by what you accept, and conservative by what you send</code>
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<u>**Second form: Substitute all occurrences**</u>
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${MYSTRING//in/by}
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=>
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`Be liberal inby what you accept, and conservative inby what you send`
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<code>Be liberal <del>in</del>by what you accept, and conservative <del>in</del>by what you send</code>
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<u>**Anchoring**</u> Additionally you can "anchor" an
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expression: A `#` (hashmark) will indicate that your expression is
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@ -536,7 +532,7 @@ that the offset 0 is the first character:
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echo ${MYSTRING:35}
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=>
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`<del>Be liberal in what you accept, and </del>conservative in what you send`
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<code><del>Be liberal in what you accept, and </del>conservative in what you send</code>
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### Using Offset and Length
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@ -545,7 +541,7 @@ In the second form we also give a length value:
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echo ${MYSTRING:35:12}
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=>
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`<del>Be liberal in what you accept, and </del>conservative<del> in what you send</del>`
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<code><del>Be liberal in what you accept, and </del>conservative<del> in what you send</del></code>
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### Negative Offset Value
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@ -570,7 +566,7 @@ then:
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echo "${MYSTRING:11:-17}"
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=>
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`<del>Be liberal </del>in what you accept, and conservative<del> in what you send</del>`
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<code><del>Be liberal </del>in what you accept, and conservative<del> in what you send</del></code>
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This works since Bash 4.2-alpha, see also
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[bashchanges](../scripting/bashchanges.md).
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@ -823,9 +819,11 @@ Removing the first 6 characters from a text string:
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parameters plus the adjacent expansion are concatenated into a
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single argument. As a workaround, each expansion needs to be quoted
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separately. Unfortunately, this bug took a very long time to
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notice.`~ $ set -- a b c; x=foo; printf '<%s> ' "$@$x" "$*""$x" "$@""$x"
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notice.
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```
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~ $ set -- a b c; x=foo; printf '<%s> ' "$@$x" "$*""$x" "$@""$x"
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<a b cfoo> <a b cfoo> <a> <b> <cfoo>
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`
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```
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- Almost all shells disagree about the treatment of an unquoted `$@`,
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`${arr[@]}`, `$*`, and `${arr[*]}` when
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@ -836,7 +834,9 @@ Removing the first 6 characters from a text string:
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are few good reasons to leave `IFS` set to null for more than the
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duration of a command or two, and even fewer to expand `$@` and `$*`
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unquoted, this should be a rare issue. **Always quote
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them**!`touch x 'y z'
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them**!
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```
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touch x 'y z'
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for sh in bb {{d,b}a,{m,}k,z}sh; do
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echo "$sh"
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"$sh" -s a 'b c' d \* </dev/fd/0
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@ -848,7 +848,9 @@ Removing the first 6 characters from a text string:
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printf "<%s> " $@
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echo
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EOF
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``bb
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```
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```
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bb
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<ab cd*>
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<ab cd*>
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dash
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@ -866,7 +868,8 @@ Removing the first 6 characters from a text string:
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zsh
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<a> <b c> <d> <x> <y z>
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<a> <b c> <d> <x> <y z>
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`When `IFS` is set to a non-null value, or unset, all shells behave
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```
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When `IFS` is set to a non-null value, or unset, all shells behave
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the same - first expanding into separate args, then applying
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pathname expansion and word-splitting to the results, except for
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zsh, which doesn't do pathname expansion in its default mode.
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@ -875,7 +878,9 @@ Removing the first 6 characters from a text string:
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the behavior of inserting delimiter characters from IFS in `$*`, and
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the way adjacent arguments are concatenated, when IFS is modified in
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the middle of expansion through
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side-effects.`for sh in bb {{d,b}a,po,{m,}k,z}sh; do
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side-effects.
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```
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for sh in bb {{d,b}a,po,{m,}k,z}sh; do
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printf '%-4s: ' "$sh"
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"$sh" </dev/fd/0
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done <<\EOF
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@ -885,14 +890,17 @@ Removing the first 6 characters from a text string:
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printf '<%s> ' ${*}${IFS=}${*}${IFS:=-}"${*}"
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echo
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EOF
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``bb : <a b cabc> <a-b-c>
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```
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```
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bb : <a b cabc> <a-b-c>
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dash: <a b cabc> <a-b-c>
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bash: <a> <b> <ca> <b> <c-a b c>
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posh: <a> <b> <ca b c> <a-b-c>
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mksh: <a> <b> <ca b c> <a-b-c>
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ksh : <a> <b> <ca> <b> <c> <a b c>
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zsh : <a> <b> <ca> <b> <c> <a-b-c>
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`ksh93 and mksh can additionally achieve this side effect (and
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```
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ksh93 and mksh can additionally achieve this side effect (and
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others) via the `${ cmds;}` expansion. I haven't yet tested every
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possible side-effect that can affect expansion halfway through
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expansion that way.
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@ -920,7 +928,8 @@ Removing the first 6 characters from a text string:
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if the first part is out of range, the second won't be evaluated.
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ksh93 and mksh always evaluate the subscript parts even if the
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parameter is unset.
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` $ bash -c 'n="y[\$(printf yo >&2)1]" m="y[\$(printf jo >&2)1]"; x=(); echo "${x[@]:n,6:m}"' # No output
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```
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$ bash -c 'n="y[\$(printf yo >&2)1]" m="y[\$(printf jo >&2)1]"; x=(); echo "${x[@]:n,6:m}"' # No output
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$ bash -c 'n="y[\$(printf yo >&2)1]" m="y[\$(printf jo >&2)1]"; x=([5]=hi); echo "${x[@]:n,6:m}"'
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yo
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$ bash -c 'n="y[\$(printf yo >&2)1]" m="y[\$(printf jo >&2)1]"; x=([6]=hi); echo "${x[@]:n,6:m}"'
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@ -929,7 +938,7 @@ Removing the first 6 characters from a text string:
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yojo
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$ bash -c 'n="y[\$(printf yo >&2)1]" m="y[\$(printf jo >&2)1]"; x=12345; echo "${x:n,6:m}"'
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yo
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`
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```
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### Quote Nesting
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@ -937,26 +946,30 @@ Removing the first 6 characters from a text string:
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expansion that expands to multiple words, and nesting such
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expansions, not all combinations of nested quoting are possible.
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# Bash
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```
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# Bash
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$ typeset -a a=(meh bleh blerg) b
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$ IFS=e
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$ printf "<%s> " "${b[@]-"${a[@]}" "${a[@]}"}"; echo # The entire PE is quoted so Bash considers the inner quotes redundant.
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<meh> <bleh> <blerg meh> <bleh> <blerg>
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<meh> <bleh> <blerg meh> <bleh> <blerg>
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$ printf "<%s> " "${b[@]-${a[@]} ${a[@]}}"; echo # The outer quotes cause the inner expansions to be considered quoted.
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<meh> <bleh> <blerg meh> <bleh> <blerg>
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<meh> <bleh> <blerg meh> <bleh> <blerg>
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$ b=(meep beep)
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$ printf "<%s> " "${b[@]-"${a[@]}" "${a[@]}"}" "${b[@]-${a[@]} ${a[@]}}"; echo # Again no surprises. Outer quotes quote everything recursively.
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<meep> <beep> <meep> <beep>
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<meep> <beep> <meep> <beep>
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```
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Now lets see what can happen if we leave the outside unquoted.
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# Bash
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```
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# Bash
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$ typeset -a a=(meh bleh blerg) b
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$ IFS=e
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$ printf "<%s> " ${b[@]-"${a[@]}" "${a[@]}"}; echo # Inner quotes make inner expansions quoted.
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<meh> <bleh> <blerg meh> <bleh> <blerg>
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<meh> <bleh> <blerg meh> <bleh> <blerg>
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$ printf "<%s> " ${b[@]-${a[@]} ${a[@]}}; echo' # No quotes at all wordsplits / globs, like you'd expect.
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<m> <h> <bl> <h> <bl> <rg m> <h> <bl> <h> <bl> <rg>
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<m> <h> <bl> <h> <bl> <rg m> <h> <bl> <h> <bl> <rg>
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```
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This all might be intuitive, and is the most common implementation, but
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this design sucks for a number of reasons. For one, it means Bash makes
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