In bash, both () and $() create a subshell.
What's the difference between each other? What's their typical usages?
() just creates a compound command, running the commands inside the parentheses. $() does the same, but also substitutes the output.
From the docs:
(list)
list is executed in a subshell environment ... Variable assignments and builtin
commands that affect the shell's environment do not remain in effect after the command completes. The return status is
the exit status of list.
Command Substitution
Command substitution allows the output of a command to replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command,
with any trailing newlines deleted.
Related
So I generally create job files with a list of commands in it. Then I execute it like so
cat jobFile | while read a; do $a; done
Which always works in bash. However, I've just started working in Mac which apparently uses zsh. And this command fails with "no such file" etc. I've tested the job file by running few lines from it manually, so it should be fine.
I've found questions on zsh read inbut they tend to be reading in from variables e.g. $a=('a' 'b' 'c') or echo $a
Thank you for your answers!
In bash, unquoted parameter expansions always undergo word-splitting, so if a="foo bar", then $a expands to two words, foo and bar. As a command, this means running the command foo with an argument bar.
In zsh, parameter expansions to not undergo word-splitting by default, which means the same expansion $a would produce a single word foo bar, treated as the name of the command to execute.
In either case, relying on parameter expansion to "parse" a shell command is fragile; in addition to word-splitting, the expansion is subject to pathname expansion (globbing), and you are limited to simple commands and their arguments. No pipes, lists (&&, ||), or redirections allowed, as everything will be treated as the command name and a sequence of arguments.
What you want in both shells is to simply treat your job file as a shell script, which can be executed in the current shell using the . command:
. jobFile
Why are you executing it in such a cumbersome way? Assuming jobFile is a file holding a sequence of bash commands, you can simply run it as
bash jobFile
If it contains a sequence of zsh commands, you can likewise run it as
zsh jobFile
If you follow this approach, I would however reflect in the name of the job file, what shell it is intended for, i.e.
bash jobFile.bash
zsh jobFile.zsh
and, if you write a job file so that it is supposed to be compatible with either shell, I would name it jobFile.sh.
I often see key=value bash-script to pass variables to a bash script. Here is an example:
$ echo $0
-bash
$ cat foo.sh
#!/usr/bin/env bash
echo "key1: $key1"
$ key1=value1 ./foo.sh
key1: value1
I have checked Bash Reference Manual. But I can't a description related to this usage.
Section 3.7.4 "Environment"
The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in Shell Parameters. These assignment statements affect only the environment seen by that command.
Apparently the online version of the documentation fails to describe the syntax of a simple command completely. It reads:
3.2.1 Simple Commands
A simple command is the kind of command encountered most often. It’s just a sequence of words separated by blanks, terminated by one of the shell’s control operators (see Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.
There is some information about the variable assignments that may precede a simple command on the section Simple Commands Expansion:
3.7.1 Simple Command Expansion
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.
The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.
...
The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.
The manual page bundled with the shell seems to be better at this. It says (the emphasis is mine):
Simple Commands
A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control operator.
You can always read the documentation of bash (and of any other CLI program installed on your computer) using man.
Type man bash on your terminal and use the usual keys (<spacebar>, b, /, q etc.) to navigate in the document. Behind the scenes, man uses your default pager program (which is, most probably, less) to put the information on screen.
I saw an expression a=($(cat)) which I am not able to understand from it's working mechanism perspective.
Functionally it takes input from the standard input and assigns it to variable a (which forms an array).
My understanding is , when shell executes the inner parenthesese it executes the cat command which brings the standard input, and when you type a few lines on the standard input and press CTRL+D it returns the lines to the outer parenthesese which then assign the lines to an array a.
My question is why this expression gives error when I remove the $ and write it as a=((cat)).
It is because $(..) is a command substitution syntax to run commands on. The cat in your example run in a sub-shell under this construct. Without it the command cat and ( are interpreted literally which the shell does not like
From the bash(1) - Linux man page
Command Substitution
Command substitution allows the output of a command to replace the command name. There are two forms:
$(command) (or) command
Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted.
The arithmetic operator in bash is $((..)) which is not the syntax you are using in your example
I was trying to run a bash shell script in ESXi shell.
Think ESXi is posix compatible, posix compiler threw much syntax errors.
please let me know if anyone has the answer
The manual lists the changed behavior when running in POSIX mode. You should adapt your script accordingly.
The following list is what’s changed when POSIX mode is in effect:
When a command in the hash table no longer exists, Bash will re-search $PATH to find the new location. This is also available with ‘shopt -s checkhash’.
The message printed by the job control code and builtins when a job exits with a non-zero status is ‘Done(status)’.
The message printed by the job control code and builtins when a job is stopped is ‘Stopped(signame)’, where signame is, for example, SIGTSTP.
The bg builtin uses the required format to describe each job placed in the background, which does not include an indication of whether the job is the current or previous job.
Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.
The POSIX PS1 and PS2 expansions of ‘!’ to the history number and ‘!!’ to ‘!’ are enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option.
The POSIX startup files are executed ($ENV) rather than the normal Bash files.
Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.
The default history file is ~/.sh_history (this is the default value of $HISTFILE).
The output of ‘kill -l’ prints all the signal names on a single line, separated by spaces, without the ‘SIG’ prefix.
The kill builtin does not accept signal names with a ‘SIG’ prefix.
Non-interactive shells exit if filename in . filename is not found.
Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.
Non-interactive shells exit if there is a syntax error in a script read with the . or source builtins, or in a string processed by the eval builtin.
Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive.
Redirection operators do not perform word splitting on the word in the redirection.
Function names must be valid shell names. That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells.
POSIX special builtins are found before shell functions during command lookup.
The time reserved word may be used by itself as a command. When used in this way, it displays timing statistics for the shell and its completed children. The TIMEFORMAT variable controls the format of the timing information.
When parsing and expanding a ${...} expansion that appears within double quotes, single quotes are no longer special and cannot be used to quote a closing brace or other special character, unless the operator is one of those defined to perform pattern removal. In this case, they do not have to appear as matched pairs.
The parser does not recognize time as a reserved word if the next token begins with a ‘-’.
If a POSIX special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the POSIX standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on.
A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable.
A non-interactive shell exists with an error status if a variable assignment error occurs in an assignment statement preceding a special builtin, but not with any other simple command.
A non-interactive shell exits with an error status if the iteration variable in a for statement or the selection variable in a select statement is a readonly variable.
Process substitution is not available.
Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.
Assignment statements preceding shell function calls persist in the shell environment after the function returns, as if a POSIX special builtin command had been executed.
The export and readonly builtin commands display their output in the format required by POSIX.
The trap builtin displays signal names without the leading SIG.
The trap builtin doesn’t check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is, unless that argument consists solely of digits and is a valid signal number. If users want to reset the handler for a given signal to the original disposition, they should use ‘-’ as the first argument.
The . and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.
Subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When not in POSIX mode, Bash clears the -e option in such subshells.
Alias expansion is always enabled, even in non-interactive shells.
When the alias builtin displays alias definitions, it does not display them with a leading ‘alias ’ unless the -p option is supplied.
When the set builtin is invoked without options, it does not display shell function names and definitions.
When the set builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.
When the cd builtin is invoked in logical mode, and the pathname constructed from $PWD and the directory name supplied as an argument does not refer to an existing directory, cd will fail instead of falling back to physical mode.
The pwd builtin verifies that the value it prints is the same as the current directory, even if it is not asked to check the file system with the -P option.
When listing the history, the fc builtin does not include an indication of whether or not a history entry has been modified.
The default editor used by fc is ed.
The type and command builtins will not report a non-executable file as having been found, though the shell will attempt to execute such a file if it is the only so-named file found in $PATH.
The vi editing mode will invoke the vi editor directly when the ‘v’ command is run, instead of checking $VISUAL and $EDITOR.
When the xpg_echo option is enabled, Bash does not attempt to interpret any arguments to echo as options. Each argument is displayed, after escape characters are converted.
The ulimit builtin uses a block size of 512 bytes for the -c and -f options.
The arrival of SIGCHLD when a trap is set on SIGCHLD does not interrupt the wait builtin and cause it to return immediately. The trap command is run once for each child that exits.
I have a bash script where I trap errors using the trap command, and I would like to be able to print the name of the last command (the one that failed)
#!/bin/bash
function error
{
# echo program name
}
trap error ERR
# Some commands ...
/bin/false foo # For testing
I'm not sure what to put in the error function. I tried echo $_ but that only works if the command has no arguments. I also tried with !! but that gives me "!!: command not found". At an interactive prompt (also bash) I get:
$ /bin/false foo
$ !!
/bin/false foo
which seems to be pretty much what I want. Why the difference?
What is the easiest way to get the name of the previous command inside a script?
Try echo $BASH_COMMAND in your trap function.
From man bash:
BASH_COMMAND
The command currently being executed or about to be executed,
unless the shell is executing a command as the result of a trap,
in which case it is the command executing at the time of the
trap.
You need to set
set -o history
to quote bash manual page:
When the -o history option to the set builtin is enabled, the shell provides access to the command history, the list of commands previously typed. The value of the HISTSIZE variable is used
as the number of commands to save in a history list. The text of the last HISTSIZE commands (default 500) is saved. The shell stores each command in the history list prior to parameter and
variable expansion (see EXPANSION above) but after history expansion is performed, subject to the values of the shell variables HISTIGNORE and HISTCONTROL.
In general, read the HISTORY and HISTORY EXPANSION sections in bash man page.