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Shell Scripting.md
## Conditional Statements ``` if test; then ... elif test; then ... else ... fi ``` `test` can be: * A bare command (no additional brackets) in which case the `if` will switch on the exit result of the command. * A simple expression in which case double square brackets are used `[[ expression ]]`. The expression can use standard comparison operators (`==, !=, >, >=`) etc. * Integer arithmetic in which case double round brackets are use `(( expression ))`. **This works on integers only**. Be careful with spaces. In general put spaces around everything since the double brackets are actually commands and need to be obviously separated from the expressions. ## Process Substitution Allows you to treat the stdout of a command as a file input to another command. In Z Shell there are two variants: * `<()` creates a FIFO to hold the sub-command's output (which is removed on completion). This also works in Bash. * `=()` creates a temporary file (in `/tmp` which is removed on completion) which is much more reliable — if a FIFO doesn't work, this might. This is Z Shell only. The `<()` expression returns the path to the FIFO that holds the sub-command's output via command substitution. Likewise, `=()` returns the path to the temporary file. You can see this using: ``` echo <(echo foo) echo =(echo foo) ``` Example — `diff` a local file against the output of another command: ``` diff -day --color localfile <(othercommand) ``` FYI — the parameters for the `diff` command here are: * -d — try harder to find minimal diff set * -a — treat files as text (not generally needed, but makes the parameter list an easy to remember mnemonic) * -y — side-by-side output * --color — colorize the diffs (red for deletions, green for additions) ## Non-capturing groups with grep Grep doesn't support capture groups, but you can fake it with a combination of Perl look-behind/ahead groups and `--only-matching` to exclude things. Basically we are using groups to exclude stuff we don't want rather than capture the stuff we do want: ``` grep --only-matching --perl-regexp "(?<=excluded start stuff)captured stuff(?=excluded end stuff)" ``` This works pretty well so long as the groups are fixed length — you can't use wild-cards in the look-behind/ahead groups, but you can in the non-group text. ## Processing a chunk of text from stdin Something I regularly need to do is format a chunk of unformatted JSON. This can be done using: ``` read json; echo ${json} | jq . ``` Run the command, then paste the JSON into the terminal. The `read var; echo ${var} | some_command` technique is handy for processing chunks of text without creating temporary files.
## Conditional Statements ``` if test; then ... elif test; then ... else ... fi ``` `test` can be: * A bare command (no additional brackets) in which case the `if` will switch on the exit result of the command. * A simple expression in which case double square brackets are used `[[ expression ]]`. The expression can use standard comparison operators (`==, !=, >, >=`) etc. * Integer arithmetic in which case double round brackets are use `(( expression ))`. **This works on integers only**. Be careful with spaces. In general put spaces around everything since the double brackets are actually commands and need to be obviously separated from the expressions. ## Process Substitution Allows you to treat the stdout of a command as a file input to another command. In Z Shell there are two variants: * `<()` creates a FIFO to hold the sub-command's output (which is removed on completion). This also works in Bash. * `=()` creates a temporary file (in `/tmp` which is removed on completion) which is much more reliable — if a FIFO doesn't work, this might. This is Z Shell only. The `<()` expression returns the path to the FIFO that holds the sub-command's output via command substitution. Likewise, `=()` returns the path to the temporary file. You can see this using: ``` echo <(echo foo) echo =(echo foo) ``` Example — `diff` a local file against the output of another command: ``` diff -day --color localfile <(othercommand) ``` FYI — the parameters for the `diff` command here are: * -d — try harder to find minimal diff set * -a — treat files as text (not generally needed, but makes the parameter list an easy to remember mnemonic) * -y — side-by-side output * --color — colorize the diffs (red for deletions, green for additions) ## Non-capturing groups with grep Grep doesn't support capture groups, but you can fake it with a combination of Perl look-behind/ahead groups and `--only-matching` to exclude things. Basically we are using groups to exclude stuff we don't want rather than capture the stuff we do want: ``` grep --only-matching --perl-regexp "(?<=excluded start stuff)captured stuff(?=excluded end stuff)" ``` This works pretty well so long as the groups are fixed length — you can't use wild-cards in the look-behind/ahead groups, but you can in the non-group text.
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4.21.2