I have the following code in my ~/.bashrc:
date=$(which date)
date() {
if [[ $1 == -R || $1 == --rfc-822 ]]; then
# Output RFC-822 compliant date string.
# e.g. Wed, 16 Dec 2009 15:18:11 +0100
$date | sed "s/[^ ][^ ]*$/$($date +%z)/"
else
$date "$#"
fi
}
This works fine, as far as I can tell. Is there a reason to avoid having a variable and a function with the same name?
It's alright apart from being confusing. Besides, they are not the same:
$ date=/bin/ls
$ type date
date is hashed (/bin/date)
$ type $date
/bin/ls is /bin/ls
$ moo=foo
$ type $moo
-bash: type: foo: not found
$ function date() { true; }
$ type date
date is a function
date ()
{
true*emphasized text*
}
$ which true
/bin/true
$ type true
true is a shell builtin
Whenever you type a command, bash looks in three different places to find that command. The priority is as follows:
shell builtins (help)
shell aliases (help alias)
shell functions (help function)
hashed binaries files from $PATH ('leftmost' folders scanned first)
Variables are prefixed with a dollar sign, which makes them different from all of the above. To compare to your example: $date and date are not the same thing. So It's not really possible to have the same name for a variable and a function because they have different "namespaces".
You may find this somewhat confusing, but many scripts define "method variables" at the top of the file. e.g.
SED=/bin/sed
AWK=/usr/bin/awk
GREP/usr/local/gnu/bin/grep
The common thing to do is type the variable names in capitals. This is useful for two purposes (apart from being less confusing):
There is no $PATH
Checking that all "dependencies" are runnable
You can't really check like this:
if [ "`which binary`" ]; then echo it\'s ok to continue.. ;fi
Because which will give you an error if binary has not yet been hashed (found in a path folder).
Since you always have to use $ to dereference a variable in Bash, you're free to use any name you like.
Beware of overriding a global, though.
See also:
http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_02.html
An alternative to using a variable: use bash's command keyword (see the manual or run help command from a prompt):
date() {
case $1 in
-R|--rfc-2822) command date ... ;;
*) command date "$#" ;;
esac
}
Related
Bash has the command substitution syntax $(f), which allows to capture
the STDOUT of a command f. If the command is an executable, this is fine
– the creation of a new process is necessary anyway. But if the command is
a shell-function, using this syntax creates an overhead of about 25ms for
each subshell on my system. This is enough to add up to noticable delays
when used in inner loops, especially in interactive contexts such as
command completions or $PS1.
A common optimization is to use global variables instead
[1] for returning values,
but it comes at a cost to readability: The intent becomes less clear, and
output capturing suddenly is inconsistent between shell functions and
executables. I am adding a comparison of options and their weaknesses below.
In order to get a consistent, reliable syntax, I was wondering if bash has
any feature that allows to capture shell-function and executable output
alike, while avoiding subshells for shell-functions.
Ideally, a solution would also contain a more efficient alternative to executing multiple commands in a subshell, which allows more cleanly isolating concerns, e.g.
person=$(
db_handler=$(database_connect) # avoids leaking the variable
query $db_handler lastname # outside it's required
echo ", " # scope.
query $db_handler firstname
database_close $db_handler
)
Such a construct allows the reader of the code to ignore everything inside $(), if the details of how $person is formatted aren't interesting to them.
Comparison of Options
1. With command substitution
person="$(get lastname), $(get firstname)"
Slow, but readable and consistent: It doesn't matter to the reader at first
glance whether get is a shell function or an executable.
2. With same global variable for all functions
get lastname
person="$R, "
get firstname
person+="$R"
Obscures what $person is supposed to contain. Alternatively,
get lastname
local lastname="$R"
get firstname
local firstname="$R"
person="$lastname, $firstname"
but that's very verbose.
3. With different global variable for each function
get_lastname
get_firstname
person="$lastname $firstname"
More readable assignment, but
If some function is invoked twice, we're back to (2).
The side-effect of setting the variable is not obvious.
It is easy to use the wrong variable by accident.
4. With global variable, whose name is passed as argument
get LN lastname
get FN firstname
person="$LN, $FN"
More readable, allows multiple return values easily.
Still inconsistent with capturing output from executables.
Note: Assignment to dynamic variable names should be done with declare
rather than eval:
$VARNAME="$LOCALVALUE" # doesn't work.
declare -g "$VARNAME=$LOCALVALUE" # will work.
eval "$VARNAME='$LOCALVALUE'" # doesn't work for *arbitrary* values.
eval "$VARNAME=$(printf %q "$LOCALVALUE")"
# doesn't avoid a subshell afterall.
[1] http://rus.har.mn/blog/2010-07-05/subshells/
If you want it to be efficient the shell functions can't return their result via stdout. If they did, there'd be no way to get it but by running the function in a subshell and capturing the output via an internal pipe, and these operations are kind of expensive (a few ms on a modern system).
When I was focusing on shell scripts and I needed to max their performance I used a convention where function foo would return its result via a variable foo. This you can do even in a POSIX shell and it has the nice property that it won't overwrite your locals because if foo is a function, you've already kind of reserved the name.
Then I had this bx_r getter function that runs a shell function and saves its output into either a variable whose name is given by the first argument or it outputs the output to stdout if the first argument is a word that's an illegal variable name (without a newline if the word is exactly an empty word, i.e., '').
I've modified it so it can be used uniformly with either commands or functions.
You can't use the type builtin to differentiate between the two here because
type returns its result via stdout => you'd need to capture that result and that would impose the forking penalty again.
So what I do when I'm about to run function foo is I check if there's a corresponding variable foo (this can catch a local variable but you'll avoid the chances of this if you limit yourself to properly namespaced shell function names). If there is, I assume that's where function foo returns its result, otherwise I run it in a $(), capturing its stdout.
Here's the code with some testing code:
bx_varlike_eh()
{
case $1 in
([!A-Za-z_0-9]*) false;;
(*) true;;
esac
}
bx_r() #{{{ Varname=$1; shift; Invoke $# and save it to $Varname if a legal varname or print it
{
# `bx_r '' some_command` prints without a newline
# `bx_r - some_command` (or any non-variable-character-containing word instead of -)
# prints with a newline
local bx_r__varname="$1"; shift 1
local bx_r
if ! bx_varlike_eh "$1" || eval "[ \"\${$1+set}\" != set ]"; then
#https://unix.stackexchange.com/a/465715/23692
bx_r=$( "$#" ) || return #$1 not varlike or unset => must be a regular command, so capture
else
#if $1 is a variable name, assume $1 is a function that saves its output there
"$#" || return
eval "bx_r=\$$1" #put it in bx_r
fi
case "$bx_r__varname" in
('') printf '%s' "$bx_r";;
([!A-Za-z_0-9]*) printf '%s\n' "$bx_r";;
(*) eval "$bx_r__varname=\$bx_r";;
esac
} #}}}
#TEST
for sh in sh bash; do
time $sh -c '
. ./bx_r.sh
bx_getnext=; bx_getnext() { bx_getnext=$((bx_getnext+1)); }
bx_r - bx_getnext
bx_r - bx_getnext
i=0; while [ $i -lt 10000 ]; do
bx_r ans bx_getnext
i=$((i+1)); done; echo ans=$ans
'
echo ====
$sh -c '
. ./bx_r.sh
bx_r - date
bx_r - /bin/date
bx_r ans /bin/date
echo ans=$ans
'
echo ====
time $sh -c '
. ./bx_r.sh
bx_echoget() { echo 42; }
i=0; while [ $i -lt 10000 ]; do
ans=$(bx_echoget)
i=$((i+1)); done; echo ans=$ans
'
done
exit
#MY TEST OUTPUT
1
2
ans=10002
0.14user 0.00system 0:00.14elapsed 99%CPU (0avgtext+0avgdata 1644maxresident)k
0inputs+0outputs (0major+76minor)pagefaults 0swaps
====
Thu Sep 5 17:12:01 CEST 2019
Thu Sep 5 17:12:01 CEST 2019
ans=Thu Sep 5 17:12:01 CEST 2019
====
ans=42
1.95user 1.14system 0:02.81elapsed 110%CPU (0avgtext+0avgdata 1656maxresident)k
0inputs+1256outputs (0major+350075minor)pagefaults 0swaps
1
2
ans=10002
0.92user 0.03system 0:00.96elapsed 99%CPU (0avgtext+0avgdata 3284maxresident)k
0inputs+0outputs (0major+159minor)pagefaults 0swaps
====
Thu Sep 5 17:12:05 CEST 2019
Thu Sep 5 17:12:05 CEST 2019
ans=Thu Sep 5 17:12:05 CEST 2019
====
ans=42
5.20user 2.40system 0:06.96elapsed 109%CPU (0avgtext+0avgdata 3220maxresident)k
0inputs+1248outputs (0major+949297minor)pagefaults 0swaps
As you can see, you can get uniform call syntax with this, while speeding up
the execution of small shell functions by up to about 14 times due to eliminating the need for captures ($()).
Use a bash nameref.
With bash v4 you can use variable namerefs:
get() {
declare -n _get__res
_get_res="$1"
case "$2" in
firstname) _get_res="Kamil"; ;;
lastname) _get_res="Cuk"; ;;
esac
}
get LN lastname
get FN firstname
person="$LN, $FN"
Namerefs can still clash with variables from outer scope. Use long names for the namerefs, like here I used underscore, function name, two underscores and then variable name.
A hitchhiker, waned by the time a function is taking to complete, wishes to find where a function is located, so that he can observe the function for himself by editting the file location. He does not wish to print the function body to the shell, simply get the path of the script file containing the function. Our hitchhiker only knows the name of his function, which is answer_life.
Imagine he has a function within a file universal-questions.sh, defined like this, the path of which is not known to our hitchhiker:
function answer_life() {
sleep $(date --date='7500000 years' +%s)
echo "42"
}
Another script, called hitchhiker-helper-scripts.sh, is defined below. It has the function above source'd within it (the hitchhiker doesn't understand source either, I guess. Just play ball.):
source "/usr/bin/universal-questions.sh"
function find_life_answer_script() {
# Print the path of the script containing `answer_life`
somecommand "answer_life" # Should output the path of the script containing the function.
}
So this, my intrepid scripter, is where you come in. Can you replace the comment with code in find_life_answer_script that allows our hitchhiker to find where the function is located?
In bash operating in extended debug mode, declare -F will give you the function name, line number, and path (as sourced):
function find_life_answer_script() {
( shopt -s extdebug; declare -F answer_life )
}
Like:
$ find_life_answer_script
answer_life 3 ./universal-questions.sh
Running a sub-shell lets you set extdebug mode without affecting any prior settings.
Your hitchhiker can also try to find the answer this way:
script=$(readlink -f "$0")
sources=$(grep -oP 'source\s+\K[\w\/\.]+' $script)
for s in "${sources[#]}"
do
matches=$(grep 'function\s+answer_life' $s)
if [ -n "${matches[0]}" ]; then
echo "$s: Nothing is here ("
else
echo "$s: Congrats! Here is your answer!"
fi
done
This is for case if debug mode will be unavailable on some planet )
Consider this PS1
PS1='\n${_:+$? }$ '
Here is the result of a few commands
$ [ 2 = 2 ]
0 $ [ 2 = 3 ]
1 $
1 $
The first line shows no status as expected, and the next two lines show the
correct exit code. However on line 3 only Enter was pressed, so I would like the
status to go away, like line 1. How can I do this?
Here's a funny, very simple possibility: it uses the \# escape sequence of PS1 together with parameter expansions (and the way Bash expands its prompt).
The escape sequence \# expands to the command number of the command to be executed. This is incremented each time a command has actually been executed. Try it:
$ PS1='\# $ '
2 $ echo hello
hello
3 $ # this is a comment
3 $
3 $ echo hello
hello
4 $
Now, each time a prompt is to be displayed, Bash first expands the escape sequences found in PS1, then (provided the shell option promptvars is set, which is the default), this string is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal.
The trick is then to have an array that will have the k-th field set (to the empty string) whenever the (k-1)-th command is executed. Then, using appropriate parameter expansions, we'll be able to detect when these fields are set and to display the return code of the previous command if the field isn't set. If you want to call this array __cmdnbary, just do:
PS1='\n${__cmdnbary[\#]-$? }${__cmdnbary[\#]=}\$ '
Look:
$ PS1='\n${__cmdnbary[\#]-$? }${__cmdnbary[\#]=}\$ '
0 $ [ 2 = 3 ]
1 $
$ # it seems that it works
$ echo "it works"
it works
0 $
To qualify for the shortest answer challenge:
PS1='\n${a[\#]-$? }${a[\#]=}$ '
that's 31 characters.
Don't use this, of course, as a is a too trivial name; also, \$ might be better than $.
Seems you don't like that the initial prompt is 0 $; you can very easily modify this by initializing the array __cmdnbary appropriately: you'll put this somewhere in your configuration file:
__cmdnbary=( '' '' ) # Initialize the field 1!
PS1='\n${__cmdnbary[\#]-$? }${__cmdnbary[\#]=}\$ '
Got some time to play around this weekend. Looking at my earlier answer (not-good) and other answers I think this may be probably the smallest answer.
Place these lines at the end of your ~/.bash_profile:
PS1='$_ret$ '
trapDbg() {
local c="$BASH_COMMAND"
[[ "$c" != "pc" ]] && export _cmd="$c"
}
pc() {
local r=$?
trap "" DEBUG
[[ -n "$_cmd" ]] && _ret="$r " || _ret=""
export _ret
export _cmd=
trap 'trapDbg' DEBUG
}
export PROMPT_COMMAND=pc
trap 'trapDbg' DEBUG
Then open a new terminal and note this desired behavior on BASH prompt:
$ uname
Darwin
0 $
$
$
$ date
Sun Dec 14 05:59:03 EST 2014
0 $
$
$ [ 1 = 2 ]
1 $
$
$ ls 123
ls: cannot access 123: No such file or directory
2 $
$
Explanation:
This is based on trap 'handler' DEBUG and PROMPT_COMMAND hooks.
PS1 is using a variable _ret i.e. PS1='$_ret$ '.
trap command runs only when a command is executed but PROMPT_COMMAND is run even when an empty enter is pressed.
trap command sets a variable _cmd to the actually executed command using BASH internal var BASH_COMMAND.
PROMPT_COMMAND hook sets _ret to "$? " if _cmd is non-empty otherwise sets _ret to "". Finally it resets _cmd var to empty state.
The variable HISTCMD is updated every time a new command is executed. Unfortunately, the value is masked during the execution of PROMPT_COMMAND (I suppose for reasons related to not having history messed up with things which happen in the prompt command). The workaround I came up with is kind of messy, but it seems to work in my limited testing.
# This only works if the prompt has a prefix
# which is displayed before the status code field.
# Fortunately, in this case, there is one.
# Maybe use a no-op prefix in the worst case (!)
PS1_base=$'\n'
# Functions for PROMPT_COMMAND
PS1_update_HISTCMD () {
# If HISTCONTROL contains "ignoredups" or "ignoreboth", this breaks.
# We should not change it programmatically
# (think principle of least astonishment etc)
# but we can always gripe.
case :$HISTCONTROL: in
*:ignoredups:* | *:ignoreboth:* )
echo "PS1_update_HISTCMD(): HISTCONTROL contains 'ignoredups' or 'ignoreboth'" >&2
echo "PS1_update_HISTCMD(): Warning: Please remove this setting." >&2 ;;
esac
# PS1_HISTCMD needs to contain the old value of PS1_HISTCMD2 (a copy of HISTCMD)
PS1_HISTCMD=${PS1_HISTCMD2:-$PS1_HISTCMD}
# PS1_HISTCMD2 needs to be unset for the next prompt to trigger properly
unset PS1_HISTCMD2
}
PROMPT_COMMAND=PS1_update_HISTCMD
# Finally, the actual prompt:
PS1='${PS1_base#foo${PS1_HISTCMD2:=${HISTCMD%$PS1_HISTCMD}}}${_:+${PS1_HISTCMD2:+$? }}$ '
The logic in the prompt is roughly as follows:
${PS1_base#foo...}
This displays the prefix. The stuff in #... is useful only for its side effects. We want to do some variable manipulation without having the values of the variables display, so we hide them in a string substitution. (This will display odd and possibly spectacular things if the value of PS1_base ever happens to begin with foo followed by the current command history index.)
${PS1_HISTCMD2:=...}
This assigns a value to PS1_HISTCMD2 (if it is unset, which we have made sure it is). The substitution would nominally also expand to the new value, but we have hidden it in a ${var#subst} as explained above.
${HISTCMD%$PS1_HISTCMD}
We assign either the value of HISTCMD (when a new entry in the command history is being made, i.e. we are executing a new command) or an empty string (when the command is empty) to PS1_HISTCMD2. This works by trimming off the value HISTCMD any match on PS1_HISTCMD (using the ${var%subst} suffix replacement syntax).
${_:+...}
This is from the question. It will expand to ... something if the value of $_ is set and nonempty (which it is when a command is being executed, but not e.g. if we are performing a variable assignment). The "something" should be the status code (and a space, for legibility) if PS1_HISTCMD2 is nonempty.
${PS1_HISTCMD2:+$? }
There.
'$ '
This is just the actual prompt suffix, as in the original question.
So the key parts are the variables PS1_HISTCMD which remembers the previous value of HISTCMD, and the variable PS1_HISTCMD2 which captures the value of HISTCMD so it can be accessed from within PROMPT_COMMAND, but needs to be unset in the PROMPT_COMMAND so that the ${PS1_HISTCMD2:=...} assignment will fire again the next time the prompt is displayed.
I fiddled for a bit with trying to hide the output from ${PS1_HISTCMD2:=...} but then realized that there is in fact something we want to display anyhow, so just piggyback on that. You can't have a completely empty PS1_base because the shell apparently notices, and does not even attempt to perform a substitution when there is no value; but perhaps you can come up with a dummy value (a no-op escape sequence, perhaps?) if you have nothing else you want to display. Or maybe this could be refactored to run with a suffix instead; but that is probably going to be trickier still.
In response to Anubhava's "smallest answer" challenge, here is the code without comments or error checking.
PS1_base=$'\n'
PS1_update_HISTCMD () { PS1_HISTCMD=${PS1_HISTCMD2:-$PS1_HISTCMD}; unset PS1_HISTCMD2; }
PROMPT_COMMAND=PS1_update_HISTCMD
PS1='${PS1_base#foo${PS1_HISTCMD2:=${HISTCMD%$PS1_HISTCMD}}}${_:+${PS1_HISTCMD2:+$? }}$ '
This is probably not the best way to do this, but it seems to be working
function pc {
foo=$_
fc -l > /tmp/new
if cmp -s /tmp/{new,old} || test -z "$foo"
then
PS1='\n$ '
else
PS1='\n$? $ '
fi
cp /tmp/{new,old}
}
PROMPT_COMMAND=pc
Result
$ [ 2 = 2 ]
0 $ [ 2 = 3 ]
1 $
$
I need to use great script bash-preexec.sh.
Although I don't like external dependencies, this was the only thing to help me avoid to have 1 in $? after just pressing enter without running any command.
This goes to your ~/.bashrc:
__prompt_command() {
local exit="$?"
PS1='\u#\h: \w \$ '
[ -n "$LASTCMD" -a "$exit" != "0" ] && PS1='['${red}$exit$clear"] $PS1"
}
PROMPT_COMMAND=__prompt_command
[-f ~/.bash-preexec.sh ] && . ~/.bash-preexec.sh
preexec() { LASTCMD="$1"; }
UPDATE: later I was able to find a solution without dependency on .bash-preexec.sh.
A recent vulnerability, CVE-2014-6271, in how Bash interprets environment variables was disclosed. The exploit relies on Bash parsing some environment variable declarations as function definitions, but then continuing to execute code following the definition:
$ x='() { echo i do nothing; }; echo vulnerable' bash -c ':'
vulnerable
But I don't get it. There's nothing I've been able to find in the Bash manual about interpreting environment variables as functions at all (except for inheriting functions, which is different). Indeed, a proper named function definition is just treated as a value:
$ x='y() { :; }' bash -c 'echo $x'
y() { :; }
But a corrupt one prints nothing:
$ x='() { :; }' bash -c 'echo $x'
$ # Nothing but newline
The corrupt function is unnamed, and so I can't just call it. Is this vulnerability a pure implementation bug, or is there an intended feature here, that I just can't see?
Update
Per Barmar's comment, I hypothesized the name of the function was the parameter name:
$ n='() { echo wat; }' bash -c 'n'
wat
Which I could swear I tried before, but I guess I didn't try hard enough. It's repeatable now. Here's a little more testing:
$ env n='() { echo wat; }; echo vuln' bash -c 'n'
vuln
wat
$ env n='() { echo wat; }; echo $1' bash -c 'n 2' 3 -- 4
wat
…so apparently the args are not set at the time the exploit executes.
Anyway, the basic answer to my question is, yes, this is how Bash implements inherited functions.
This seems like an implementation bug.
Apparently, the way exported functions work in bash is that they use specially-formatted environment variables. If you export a function:
f() { ... }
it defines an environment variable like:
f='() { ... }'
What's probably happening is that when the new shell sees an environment variable whose value begins with (), it prepends the variable name and executes the resulting string. The bug is that this includes executing anything after the function definition as well.
The fix described is apparently to parse the result to see if it's a valid function definition. If not, it prints the warning about the invalid function definition attempt.
This article confirms my explanation of the cause of the bug. It also goes into a little more detail about how the fix resolves it: not only do they parse the values more carefully, but variables that are used to pass exported functions follow a special naming convention. This naming convention is different from that used for the environment variables created for CGI scripts, so an HTTP client should never be able to get its foot into this door.
The following:
x='() { echo I do nothing; }; echo vulnerable' bash -c 'typeset -f'
prints
vulnerable
x ()
{
echo I do nothing
}
declare -fx x
seems, than Bash, after having parsed the x=..., discovered it as a function, exported it, saw the declare -fx x and allowed the execution of the command after the declaration.
echo vulnerable
x='() { x; }; echo vulnerable' bash -c 'typeset -f'
prints:
vulnerable
x ()
{
echo I do nothing
}
and running the x
x='() { x; }; echo Vulnerable' bash -c 'x'
prints
Vulnerable
Segmentation fault: 11
segfaults - infinite recursive calls
It doesn't overrides already defined function
$ x() { echo Something; }
$ declare -fx x
$ x='() { x; }; echo Vulnerable' bash -c 'typeset -f'
prints:
x ()
{
echo Something
}
declare -fx x
e.g. the x remains the previously (correctly) defined function.
For the Bash 4.3.25(1)-release the vulnerability is closed, so
x='() { echo I do nothing; }; echo Vulnerable' bash -c ':'
prints
bash: warning: x: ignoring function definition attempt
bash: error importing function definition for `x'
but - what is strange (at least for me)
x='() { x; };' bash -c 'typeset -f'
STILL PRINTS
x ()
{
x
}
declare -fx x
and the
x='() { x; };' bash -c 'x'
segmentation faults too, so it STILL accept the strange function definition...
I think it's worth looking at the Bash code itself. The patch gives a bit of insight as to the problem. In particular,
*** ../bash-4.3-patched/variables.c 2014-05-15 08:26:50.000000000 -0400
--- variables.c 2014-09-14 14:23:35.000000000 -0400
***************
*** 359,369 ****
strcpy (temp_string + char_index + 1, string);
! if (posixly_correct == 0 || legal_identifier (name))
! parse_and_execute (temp_string, name, SEVAL_NONINT|SEVAL_NOHIST);
!
! /* Ancient backwards compatibility. Old versions of bash exported
! functions like name()=() {...} */
! if (name[char_index - 1] == ')' && name[char_index - 2] == '(')
! name[char_index - 2] = '\0';
if (temp_var = find_function (name))
--- 364,372 ----
strcpy (temp_string + char_index + 1, string);
! /* Don't import function names that are invalid identifiers from the
! environment, though we still allow them to be defined as shell
! variables. */
! if (legal_identifier (name))
! parse_and_execute (temp_string, name, SEVAL_NONINT|SEVAL_NOHIST|SEVAL_FUNCDEF|SEVAL_ONECMD);
if (temp_var = find_function (name))
When Bash exports a function, it shows up as an environment variable, for example:
$ foo() { echo 'hello world'; }
$ export -f foo
$ cat /proc/self/environ | tr '\0' '\n' | grep -A1 foo
foo=() { echo 'hello world'
}
When a new Bash process finds a function defined this way in its environment, it evalutes the code in the variable using parse_and_execute(). For normal, non-malicious code, executing it simply defines the function in Bash and moves on. However, because it's passed to a generic execution function, Bash will correctly parse and execute additional code defined in that variable after the function definition.
You can see that in the new code, a flag called SEVAL_ONECMD has been added that tells Bash to only evaluate the first command (that is, the function definition) and SEVAL_FUNCDEF to only allow functio0n definitions.
In regard to your question about documentation, notice here in the commandline documentation for the env command, that a study of the syntax shows that env is working as documented.
There are, optionally, 4 possible options
An optional hyphen as a synonym for -i (for backward compatibility I assume)
Zero or more NAME=VALUE pairs. These are the variable assignment(s) which could include function definitions.
Note that no semicolon (;) is required between or following the assignments.
The last argument(s) can be a single command followed by its argument(s). It will run with whatever permissions have been granted to the login being used. Security is controlled by restricting permissions on the login user and setting permissions on user-accessible executables such that users other than the executable's owner can only read and execute the program, not alter it.
[ spot#LX03:~ ] env --help
Usage: env [OPTION]... [-] [NAME=VALUE]... [COMMAND [ARG]...]
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This question already has answers here:
Make a Bash alias that takes a parameter?
(24 answers)
Closed 5 years ago.
Is it possible to do the following:
I want to run the following:
mongodb bin/mongod
In my bash_profile I have
alias = "./path/to/mongodb/$1"
An alias will expand to the string it represents. Anything after the alias will appear after its expansion without needing to be or able to be passed as explicit arguments (e.g. $1).
$ alias foo='/path/to/bar'
$ foo some args
will get expanded to
$ /path/to/bar some args
If you want to use explicit arguments, you'll need to use a function
$ foo () { /path/to/bar "$#" fixed args; }
$ foo abc 123
will be executed as if you had done
$ /path/to/bar abc 123 fixed args
To undefine an alias:
unalias foo
To undefine a function:
unset -f foo
To see the type and definition (for each defined alias, keyword, function, builtin or executable file):
type -a foo
Or type only (for the highest precedence occurrence):
type -t foo
to use parameters in aliases, i use this method:
alias myalias='function __myalias() { echo "Hello $*"; unset -f __myalias; }; __myalias'
its a self-destructive function wrapped in an alias, so it pretty much is the best of both worlds, and doesnt take up an extra line(s) in your definitions... which i hate, oh yeah and if you need that return value, you'll have to store it before calling unset, and then return the value using the "return" keyword in that self destructive function there:
alias myalias='function __myalias() { echo "Hello $*"; myresult=$?; unset -f __myalias; return $myresult; }; __myalias'
so..
you could, if you need to have that variable in there
alias mongodb='function __mongodb() { ./path/to/mongodb/$1; unset -f __mongodb; }; __mongodb'
of course...
alias mongodb='./path/to/mongodb/'
would actually do the same thing without the need for parameters, but like i said, if you wanted or needed them for some reason (for example, you needed $2 instead of $1), you would need to use a wrapper like that. If it is bigger than one line you might consider just writing a function outright since it would become more of an eyesore as it grew larger. Functions are great since you get all the perks that functions give (see completion, traps, bind, etc for the goodies that functions can provide, in the bash manpage).
I hope that helps you out :)
Usually when I want to pass arguments to an alias in Bash, I use a combination of an alias and a function like this, for instance:
function __t2d {
if [ "$1x" != 'x' ]; then
date -d "#$1"
fi
}
alias t2d='__t2d'
This is the solution which can avoid using function:
alias addone='{ num=$(cat -); echo "input: $num"; echo "result:$(($num+1))"; }<<<'
test result
addone 200
input: 200
result:201
In csh (as opposed to bash) you can do exactly what you want.
alias print 'lpr \!^ -Pps5'
print memo.txt
The notation \!^ causes the argument to be inserted in the command at this point.
The ! character is preceeded by a \ to prevent it being interpreted as a history command.
You can also pass multiple arguments:
alias print 'lpr \!* -Pps5'
print part1.ps glossary.ps figure.ps
(Examples taken from http://unixhelp.ed.ac.uk/shell/alias_csh2.1.html .)
To simplify leed25d's answer, use a combination of an alias and a function. For example:
function __GetIt {
cp ./path/to/stuff/$* .
}
alias GetIt='__GetIt'