Related
I am confused by the usage of brackets, parentheses, curly braces in Bash, as well as the difference between their double or single forms. Is there a clear explanation?
In Bash, test and [ are shell builtins.
The double bracket, which is a shell keyword, enables additional functionality. For example, you can use && and || instead of -a and -o and there's a regular expression matching operator =~.
Also, in a simple test, double square brackets seem to evaluate quite a lot quicker than single ones.
$ time for ((i=0; i<10000000; i++)); do [[ "$i" = 1000 ]]; done
real 0m24.548s
user 0m24.337s
sys 0m0.036s
$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; done
real 0m33.478s
user 0m33.478s
sys 0m0.000s
The braces, in addition to delimiting a variable name are used for parameter expansion so you can do things like:
Truncate the contents of a variable
$ var="abcde"; echo ${var%d*}
abc
Make substitutions similar to sed
$ var="abcde"; echo ${var/de/12}
abc12
Use a default value
$ default="hello"; unset var; echo ${var:-$default}
hello
and several more
Also, brace expansions create lists of strings which are typically iterated over in loops:
$ echo f{oo,ee,a}d
food feed fad
$ mv error.log{,.OLD}
(error.log is renamed to error.log.OLD because the brace expression
expands to "mv error.log error.log.OLD")
$ for num in {000..2}; do echo "$num"; done
000
001
002
$ echo {00..8..2}
00 02 04 06 08
$ echo {D..T..4}
D H L P T
Note that the leading zero and increment features weren't available before Bash 4.
Thanks to gboffi for reminding me about brace expansions.
Double parentheses are used for arithmetic operations:
((a++))
((meaning = 42))
for ((i=0; i<10; i++))
echo $((a + b + (14 * c)))
and they enable you to omit the dollar signs on integer and array variables and include spaces around operators for readability.
Single brackets are also used for array indices:
array[4]="hello"
element=${array[index]}
Curly brace are required for (most/all?) array references on the right hand side.
ephemient's comment reminded me that parentheses are also used for subshells. And that they are used to create arrays.
array=(1 2 3)
echo ${array[1]}
2
A single bracket ([) usually actually calls a program named [; man test or man [ for more info. Example:
$ VARIABLE=abcdef
$ if [ $VARIABLE == abcdef ] ; then echo yes ; else echo no ; fi
yes
The double bracket ([[) does the same thing (basically) as a single bracket, but is a bash builtin.
$ VARIABLE=abcdef
$ if [[ $VARIABLE == 123456 ]] ; then echo yes ; else echo no ; fi
no
Parentheses (()) are used to create a subshell. For example:
$ pwd
/home/user
$ (cd /tmp; pwd)
/tmp
$ pwd
/home/user
As you can see, the subshell allowed you to perform operations without affecting the environment of the current shell.
(a) Braces ({}) are used to unambiguously identify variables. Example:
$ VARIABLE=abcdef
$ echo Variable: $VARIABLE
Variable: abcdef
$ echo Variable: $VARIABLE123456
Variable:
$ echo Variable: ${VARIABLE}123456
Variable: abcdef123456
(b) Braces are also used to execute a sequence of commands in the current shell context, e.g.
$ { date; top -b -n1 | head ; } >logfile
# 'date' and 'top' output are concatenated,
# could be useful sometimes to hunt for a top loader )
$ { date; make 2>&1; date; } | tee logfile
# now we can calculate the duration of a build from the logfile
There is a subtle syntactic difference with ( ), though (see bash reference) ; essentially, a semicolon ; after the last command within braces is a must, and the braces {, } must be surrounded by spaces.
Brackets
if [ CONDITION ] Test construct
if [[ CONDITION ]] Extended test construct
Array[1]=element1 Array initialization
[a-z] Range of characters within a Regular Expression
$[ expression ] A non-standard & obsolete version of $(( expression )) [1]
[1] http://wiki.bash-hackers.org/scripting/obsolete
Curly Braces
${variable} Parameter substitution
${!variable} Indirect variable reference
{ command1; command2; . . . commandN; } Block of code
{string1,string2,string3,...} Brace expansion
{a..z} Extended brace expansion
{} Text replacement, after find and xargs
Parentheses
( command1; command2 ) Command group executed within a subshell
Array=(element1 element2 element3) Array initialization
result=$(COMMAND) Command substitution, new style
>(COMMAND) Process substitution
<(COMMAND) Process substitution
Double Parentheses
(( var = 78 )) Integer arithmetic
var=$(( 20 + 5 )) Integer arithmetic, with variable assignment
(( var++ )) C-style variable increment
(( var-- )) C-style variable decrement
(( var0 = var1<98?9:21 )) C-style ternary operation
I just wanted to add these from TLDP:
~:$ echo $SHELL
/bin/bash
~:$ echo ${#SHELL}
9
~:$ ARRAY=(one two three)
~:$ echo ${#ARRAY}
3
~:$ echo ${TEST:-test}
test
~:$ echo $TEST
~:$ export TEST=a_string
~:$ echo ${TEST:-test}
a_string
~:$ echo ${TEST2:-$TEST}
a_string
~:$ echo $TEST2
~:$ echo ${TEST2:=$TEST}
a_string
~:$ echo $TEST2
a_string
~:$ export STRING="thisisaverylongname"
~:$ echo ${STRING:4}
isaverylongname
~:$ echo ${STRING:6:5}
avery
~:$ echo ${ARRAY[*]}
one two one three one four
~:$ echo ${ARRAY[*]#one}
two three four
~:$ echo ${ARRAY[*]#t}
one wo one hree one four
~:$ echo ${ARRAY[*]#t*}
one wo one hree one four
~:$ echo ${ARRAY[*]##t*}
one one one four
~:$ echo $STRING
thisisaverylongname
~:$ echo ${STRING%name}
thisisaverylong
~:$ echo ${STRING/name/string}
thisisaverylongstring
The difference between test, [ and [[ is explained in great details in the BashFAQ.
(Note: The link shows many examples for comparison)
To cut a long story short: test implements the old, portable syntax of
the command. In almost all shells (the oldest Bourne shells are the
exception), [ is a synonym for test (but requires a final argument of
]). Although all modern shells have built-in implementations of [,
there usually still is an external executable of that name, e.g.
/bin/[.
[[ is a new, improved version of it, and it is a keyword, not a program.
This has beneficial effects on the ease of use, as shown below. [[ is
understood by KornShell and BASH (e.g. 2.03), but not by the older
POSIX or BourneShell.
And the conclusion:
When should the new test command [[ be used, and when the old one [?
If portability/conformance to POSIX or the BourneShell is a concern, the old syntax should
be used. If on the other hand the script requires BASH, Zsh, or KornShell,
the new syntax is usually more flexible.
Parentheses in function definition
Parentheses () are being used in function definition:
function_name () { command1 ; command2 ; }
That is the reason you have to escape parentheses even in command parameters:
$ echo (
bash: syntax error near unexpected token `newline'
$ echo \(
(
$ echo () { command echo The command echo was redefined. ; }
$ echo anything
The command echo was redefined.
Some common and handy uses for brackets, parenthesis, and braces
As mentioned above, sometimes you want a message displayed without losing the return value. This is a handy snippet:
$ [ -f go.mod ] || { echo 'File not found' && false; }
This produced no output and a 0 (true) return value if the file go.mod exists in the current directory. Test the result:
$ echo $?
0
If the file does not exist, you get the message but also a return value of 1 (false), which can also be tested:
$ [ -f fake_file ] || { echo 'File not found'; false; }
File not found
$ echo $?
1
You can also simply create a function to check if a file exists:
fileexists() { [ -f "$1" ]; }
or if a file is readable (not corrupted, have permissions, etc.):
canread() { [ -r "$1" ]; }
or if it is a directory:
isdir() { [ -d "$1" ]; }
or is writable for the current user:
canwrite() { [ -w "$1" ]; }
or if a file exists and is not empty (like a log file with content...)
isempty() { [ -s "$1" ]; }
There are more details at: TLDP
You can also see if a program exists and is available on the path:
exists () { command -v $1 > /dev/null 2>&1; }
This is useful in scripts, for example:
# gitit does an autosave commit to the current
# if Git is installed and available.
# If git is not available, it will use brew
# (on macOS) to install it.
#
# The first argument passed, if any, is used as
# the commit message; otherwise the default is used.
gitit() {
$(exists git) && {
git add --all;
git commit -m "${1:-'GitBot: dev progress autosave'}";
git push;
} || brew install git;
}
Additional info about How to use parentheses to group and expand expressions:
(it is listed on the link syntax-brackets)
Some main points in there:
Group commands in a sub-shell: ( )
(list)
Group commands in the current shell: { }
{ list; }
Test - return the binary result of an expression: [[ ]]
[[ expression ]]
Arithmetic expansion
The format for Arithmetic expansion is:
$(( expression ))
The format for a simple Arithmetic Evaluation is:
(( expression ))
Combine multiple expressions
( expression )
(( expr1 && expr2 ))
Truncate the contents of a variable
$ var="abcde"; echo ${var%d*}
abc
Make substitutions similar to sed
$ var="abcde"; echo ${var/de/12}
abc12
Use a default value
$ default="hello"; unset var; echo ${var:-$default}
hello
I have got a project that involves shell scripts and comparing values/variables within them. I have looked here and elsewhere on comparing variables and I have tried all the various examples given but I am running into something that is not as advertised. OS is Solaris10
I have created the following script as a learning experience-
#!/bin/ksh
stest()
{
if $X = $Y
then echo they're the same
else echo they're notthe same
fi
}
X=a
Y=a
stest
echo completed
I keep getting some variation of the following-
using shell sh or ksh-
#./test.sh
./test.sh[2]: a: not found
completed
using shell bash-
#./test.sh
./test.sh: line 5: a: command not found
completed
I have tried enclosing the if $X = $Y line in brackets and double brackets and I get back
[a: not found
or
[[a: not found
If I change the variables X and Y to the numeral "1" I get the same thing-
./test.sh[2]: 1: not found
I have tried enclosing things in single quotes, double quotes & backwards quotes.
Any help is appreciated.
After if, you need a shell command, like anywhere else. $X = $Y is parsed as a shell command, meaning $X is interpreted as a command name (provided that the value of the variable is a single word).
You can use the [ command (also available as test) or the [[ … ]] special syntax to compare two variables. Note that you need spaces on the inside of the brackets: the brackets are a separate token in the shell syntax.
if [ "$X" = "$Y" ]; then …
or
if [[ "$X" = "$Y" ]]; then …
[ … ] works in any shell, [[ … ]] only in ksh, bash and zsh.
Note that you need double quotes around the variables¹. If you leave off the quotes, then the variable is split into multiple words and each word is interpreted as a wildcard pattern. This doesn't happen inside [[ … ]], but the right-hand side of = is interpreted as a wildcard pattern there too. Always put double quotes around variable substitutions (unless you want the value of the variable to be used as a list of filename matching patterns, rather than as a string).
¹ Except on $X the [[ … ]] syntax.
This KornShell (ksh) script should work:
soExample.ksh
#!/bin/ksh
#Initialize Variables
X="a"
Y="a"
#Function to create File with Input
#Params: 1}
stest(){
if [ "${X}" == "${Y}" ]; then
echo "they're the same"
else
echo "they're not the same"
fi
}
#-----------
#---Main----
#-----------
echo "Starting: ${PWD}/${0} with Input Parameters: {1: ${1} {2: ${2} {3: ${3}"
stest #function call#
echo "completed"
echo "Exiting: ${PWD}/${0}"
Output :
user#foo:/tmp $ ksh soExample.ksh
Starting: /tmp/soExample.ksh with Input Parameters: {1: {2: {3:
they're not the same
completed
Exiting: /tmp/soExample.ksh
ksh version:
user#foo:/tmp $ echo $KSH_VERSION
#(#)MIRBSD KSH R48 2013/08/16
What I'd like to do is take, as an input to a function, a line that may include quotes (single or double) and echo that line exactly as it was provided to the function. For instance:
function doit {
printf "%s " ${#}
eval "${#}"
printf " # [%3d]\n" ${?}
}
Which, given the following input
doit VAR=42
doit echo 'single quote $VAR'
doit echo "double quote $VAR"
Yields the following:
VAR=42 # [ 0]
echo single quote $VAR # [ 0]
echo double quote 42 # [ 0]
So the semantics of the variable expansion are preserved as I'd expect, but I can not get the exact format of the line as it was provided to the function. What I'd like is to have doit echo 'single quote $VAR' result in echo 'single quote $VAR'.
I'm sure this has to do with bash processing the arguments before they are passed to the function; I'm just looking for a way around that (if possible).
Edit
So what I had intended was to shadow the execution of a script while providing an exact replica of the execution that could be used as a diagnostic tool including exit status of each step.
While I can get the desired behavior described above by doing something like
while read line ; do
doit ${line}
done < ${INPUT}
That approach fails in the face of control structures (i.e. if, while, etc). I thought about using set -x but that has it's limitations as well: " becomes ' and exit status is not visible for commands that fail.
I was in a similar position to you in that I needed a script to wrap around an existing command and pass arguments preserving quoting.
I came up with something that doesn't preserve the command line exactly as typed but does pass the arguments correctly and show you what they were.
Here's my script set up to shadow ls:
CMD=ls
PARAMS=""
for PARAM in "$#"
do
PARAMS="${PARAMS} \"${PARAM}\""
done
echo Running: ${CMD} ${PARAMS}
bash -c "${CMD} ${PARAMS}"
echo Exit Code: $?
And this is some sample output:
$ ./shadow.sh missing-file "not a file"
Running: ls "missing-file" "not a file"
ls: missing-file: No such file or directory
ls: not a file: No such file or directory
Exit Code: 1
So as you can see it adds quotes which weren't originally there but it does preserve arguments with spaces in which is what I needed.
The reason this happens is because bash interprets the arguments, as you thought. The quotes simply aren't there any more when it calls the function, so this isn't possible. It worked in DOS because programs could interpret the command line themselves, not that it helps you!
Although #Peter Westlake's answer is correct, and there are no quotes to preserve one can try to deduce if the quotes where required and thus passed in originally. Personally I used this requote function when I needed a proof in my logs that a command ran with the correct quoting:
function requote() {
local res=""
for x in "${#}" ; do
# try to figure out if quoting was required for the $x:
grep -q "[[:space:]]" <<< "$x" && res="${res} '${x}'" || res="${res} ${x}"
done
# remove first space and print:
sed -e 's/^ //' <<< "${res}"
}
And here is how I use it:
CMD=$(requote "${#}")
# ...
echo "${CMD}"
doit echo "'single quote $VAR'"
doit echo '"double quote $VAR"'
Both will work.
bash will only strip the outside set of quotes when entering the function.
Bash will remove the quote when you pass a string with quote in as command line argument. The quote is simply not there anymore when the string is pass to your script. You have no way to know there is a single quote or double quote.
What you probably can do is sth like this:
doit VAR=42
doit echo \'single quote $VAR\'
doit echo \"double quote $VAR\"
In your script you get
echo 'single quote $VAR'
echo "double quote $VAR"
Or do this
doit VAR=42
doit echo 'single quote $VAR'
doit echo '"double quote $VAR"'
In your script you get
echo single quote $VAR
echo "double quote $VAR"
This:
ponerApostrofes1 ()
{
for (( i=1; i<=$#; i++ ));
do
eval VAR="\${$i}";
echo \'"${VAR}"\';
done;
return;
}
As an example has problems when the parameters have apostrophes.
This function:
ponerApostrofes2 ()
{
for ((i=1; i<=$#; i++ ))
do
eval PARAM="\${$i}";
echo -n \'${PARAM//\'/\'\\\'\'}\'' ';
done;
return
}
solves the mentioned problem and you can use parameters including apostrophes inside, like "Porky's", and returns, apparently(?), the same string of parameters when each parameter is quoted; if not, it quotes it. Surprisingly, I don't understand why, if you use it recursively, it doesn't return the same list but each parameter is quoted again. But if you do echo of each one you recover the original parameter.
Example:
$ ponerApostrofes2 'aa aaa' 'bbbb b' 'c'
'aa aaa' 'bbbb b' 'c'
$ ponerApostrofes2 $(ponerApostrofes2 'aa aaa' 'bbbb b' 'c' )
''\''aa' 'aaa'\''' ''\''bbbb' 'b'\''' ''\''c'\'''
And:
$ echo ''\''bbbb' 'b'\'''
'bbbb b'
$ echo ''\''aa' 'aaa'\'''
'aa aaa'
$ echo ''\''c'\'''
'c'
And this one:
ponerApostrofes3 ()
{
for ((i=1; i<=$#; i++ ))
do
eval PARAM="\${$i}";
echo -n ${PARAM//\'/\'\\\'\'} ' ';
done;
return
}
returning one level of quotation less,
doesn't work either, neither alternating both recursively.
If one's shell does not support pattern substitution, i.e. ${param/pattern/string} then the following sed expression can be used to safely quote any string such that it will eval into a single parameter again:
sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/'/"
Combining this with printf it is possible to write a little function that will take any list of strings produced by filename expansion or "$#" and turn it into something that can be safely passed to eval to expand it into arguments for another command while safely preserving parameter separation.
# Usage: quotedlist=$(shell_quote args...)
#
# e.g.: quotedlist=$(shell_quote *.pdf) # filenames with spaces
#
# or: quotedlist=$(shell_quote "$#")
#
# After building up a quoted list, use it by evaling it inside
# double quotes, like this:
#
# eval "set -- $quotedlist"
# for str in "$#"; do
# # fiddle "${str}"
# done
#
# or like this:
#
# eval "\$a_command $quotedlist \$another_parameter"
#
shell_quote()
{
local result=''
local arg
for arg in "$#" ; do
# Append a space to our result, if necessary
#
result=${result}${result:+ }
# Convert each embedded ' to \' , then insert ' at the
# beginning of the line, and append ' at the end of
# the line.
#
result=${result}$(printf "%s\n" "$arg" | \
sed -e "s/'/'\\\\''/g" -e "1s/^/'/" -e "\$s/\$/'/")
done
# use printf(1) instead of echo to avoid weird "echo"
# implementations.
#
printf "%s\n" "$result"
}
It may be easier (and maybe safer, i.e. avoid eval) in some situations to use an "impossible" character as the field separator and then use IFS to control expansion of the value again.
The shell is going to interpret the quotes and the $ before it passes it to your function. There's not a lot your function can do to get the special characters back, because it has no way of knowing (in the double-quote example) whether 42 was hard-coded or if it came from a variable. You will have to escape the special characters if you want them to survive long enough to make it to your function.
I am confused by the usage of brackets, parentheses, curly braces in Bash, as well as the difference between their double or single forms. Is there a clear explanation?
In Bash, test and [ are shell builtins.
The double bracket, which is a shell keyword, enables additional functionality. For example, you can use && and || instead of -a and -o and there's a regular expression matching operator =~.
Also, in a simple test, double square brackets seem to evaluate quite a lot quicker than single ones.
$ time for ((i=0; i<10000000; i++)); do [[ "$i" = 1000 ]]; done
real 0m24.548s
user 0m24.337s
sys 0m0.036s
$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; done
real 0m33.478s
user 0m33.478s
sys 0m0.000s
The braces, in addition to delimiting a variable name are used for parameter expansion so you can do things like:
Truncate the contents of a variable
$ var="abcde"; echo ${var%d*}
abc
Make substitutions similar to sed
$ var="abcde"; echo ${var/de/12}
abc12
Use a default value
$ default="hello"; unset var; echo ${var:-$default}
hello
and several more
Also, brace expansions create lists of strings which are typically iterated over in loops:
$ echo f{oo,ee,a}d
food feed fad
$ mv error.log{,.OLD}
(error.log is renamed to error.log.OLD because the brace expression
expands to "mv error.log error.log.OLD")
$ for num in {000..2}; do echo "$num"; done
000
001
002
$ echo {00..8..2}
00 02 04 06 08
$ echo {D..T..4}
D H L P T
Note that the leading zero and increment features weren't available before Bash 4.
Thanks to gboffi for reminding me about brace expansions.
Double parentheses are used for arithmetic operations:
((a++))
((meaning = 42))
for ((i=0; i<10; i++))
echo $((a + b + (14 * c)))
and they enable you to omit the dollar signs on integer and array variables and include spaces around operators for readability.
Single brackets are also used for array indices:
array[4]="hello"
element=${array[index]}
Curly brace are required for (most/all?) array references on the right hand side.
ephemient's comment reminded me that parentheses are also used for subshells. And that they are used to create arrays.
array=(1 2 3)
echo ${array[1]}
2
A single bracket ([) usually actually calls a program named [; man test or man [ for more info. Example:
$ VARIABLE=abcdef
$ if [ $VARIABLE == abcdef ] ; then echo yes ; else echo no ; fi
yes
The double bracket ([[) does the same thing (basically) as a single bracket, but is a bash builtin.
$ VARIABLE=abcdef
$ if [[ $VARIABLE == 123456 ]] ; then echo yes ; else echo no ; fi
no
Parentheses (()) are used to create a subshell. For example:
$ pwd
/home/user
$ (cd /tmp; pwd)
/tmp
$ pwd
/home/user
As you can see, the subshell allowed you to perform operations without affecting the environment of the current shell.
(a) Braces ({}) are used to unambiguously identify variables. Example:
$ VARIABLE=abcdef
$ echo Variable: $VARIABLE
Variable: abcdef
$ echo Variable: $VARIABLE123456
Variable:
$ echo Variable: ${VARIABLE}123456
Variable: abcdef123456
(b) Braces are also used to execute a sequence of commands in the current shell context, e.g.
$ { date; top -b -n1 | head ; } >logfile
# 'date' and 'top' output are concatenated,
# could be useful sometimes to hunt for a top loader )
$ { date; make 2>&1; date; } | tee logfile
# now we can calculate the duration of a build from the logfile
There is a subtle syntactic difference with ( ), though (see bash reference) ; essentially, a semicolon ; after the last command within braces is a must, and the braces {, } must be surrounded by spaces.
Brackets
if [ CONDITION ] Test construct
if [[ CONDITION ]] Extended test construct
Array[1]=element1 Array initialization
[a-z] Range of characters within a Regular Expression
$[ expression ] A non-standard & obsolete version of $(( expression )) [1]
[1] http://wiki.bash-hackers.org/scripting/obsolete
Curly Braces
${variable} Parameter substitution
${!variable} Indirect variable reference
{ command1; command2; . . . commandN; } Block of code
{string1,string2,string3,...} Brace expansion
{a..z} Extended brace expansion
{} Text replacement, after find and xargs
Parentheses
( command1; command2 ) Command group executed within a subshell
Array=(element1 element2 element3) Array initialization
result=$(COMMAND) Command substitution, new style
>(COMMAND) Process substitution
<(COMMAND) Process substitution
Double Parentheses
(( var = 78 )) Integer arithmetic
var=$(( 20 + 5 )) Integer arithmetic, with variable assignment
(( var++ )) C-style variable increment
(( var-- )) C-style variable decrement
(( var0 = var1<98?9:21 )) C-style ternary operation
I just wanted to add these from TLDP:
~:$ echo $SHELL
/bin/bash
~:$ echo ${#SHELL}
9
~:$ ARRAY=(one two three)
~:$ echo ${#ARRAY}
3
~:$ echo ${TEST:-test}
test
~:$ echo $TEST
~:$ export TEST=a_string
~:$ echo ${TEST:-test}
a_string
~:$ echo ${TEST2:-$TEST}
a_string
~:$ echo $TEST2
~:$ echo ${TEST2:=$TEST}
a_string
~:$ echo $TEST2
a_string
~:$ export STRING="thisisaverylongname"
~:$ echo ${STRING:4}
isaverylongname
~:$ echo ${STRING:6:5}
avery
~:$ echo ${ARRAY[*]}
one two one three one four
~:$ echo ${ARRAY[*]#one}
two three four
~:$ echo ${ARRAY[*]#t}
one wo one hree one four
~:$ echo ${ARRAY[*]#t*}
one wo one hree one four
~:$ echo ${ARRAY[*]##t*}
one one one four
~:$ echo $STRING
thisisaverylongname
~:$ echo ${STRING%name}
thisisaverylong
~:$ echo ${STRING/name/string}
thisisaverylongstring
The difference between test, [ and [[ is explained in great details in the BashFAQ.
(Note: The link shows many examples for comparison)
To cut a long story short: test implements the old, portable syntax of
the command. In almost all shells (the oldest Bourne shells are the
exception), [ is a synonym for test (but requires a final argument of
]). Although all modern shells have built-in implementations of [,
there usually still is an external executable of that name, e.g.
/bin/[.
[[ is a new, improved version of it, and it is a keyword, not a program.
This has beneficial effects on the ease of use, as shown below. [[ is
understood by KornShell and BASH (e.g. 2.03), but not by the older
POSIX or BourneShell.
And the conclusion:
When should the new test command [[ be used, and when the old one [?
If portability/conformance to POSIX or the BourneShell is a concern, the old syntax should
be used. If on the other hand the script requires BASH, Zsh, or KornShell,
the new syntax is usually more flexible.
Parentheses in function definition
Parentheses () are being used in function definition:
function_name () { command1 ; command2 ; }
That is the reason you have to escape parentheses even in command parameters:
$ echo (
bash: syntax error near unexpected token `newline'
$ echo \(
(
$ echo () { command echo The command echo was redefined. ; }
$ echo anything
The command echo was redefined.
Some common and handy uses for brackets, parenthesis, and braces
As mentioned above, sometimes you want a message displayed without losing the return value. This is a handy snippet:
$ [ -f go.mod ] || { echo 'File not found' && false; }
This produced no output and a 0 (true) return value if the file go.mod exists in the current directory. Test the result:
$ echo $?
0
If the file does not exist, you get the message but also a return value of 1 (false), which can also be tested:
$ [ -f fake_file ] || { echo 'File not found'; false; }
File not found
$ echo $?
1
You can also simply create a function to check if a file exists:
fileexists() { [ -f "$1" ]; }
or if a file is readable (not corrupted, have permissions, etc.):
canread() { [ -r "$1" ]; }
or if it is a directory:
isdir() { [ -d "$1" ]; }
or is writable for the current user:
canwrite() { [ -w "$1" ]; }
or if a file exists and is not empty (like a log file with content...)
isempty() { [ -s "$1" ]; }
There are more details at: TLDP
You can also see if a program exists and is available on the path:
exists () { command -v $1 > /dev/null 2>&1; }
This is useful in scripts, for example:
# gitit does an autosave commit to the current
# if Git is installed and available.
# If git is not available, it will use brew
# (on macOS) to install it.
#
# The first argument passed, if any, is used as
# the commit message; otherwise the default is used.
gitit() {
$(exists git) && {
git add --all;
git commit -m "${1:-'GitBot: dev progress autosave'}";
git push;
} || brew install git;
}
Additional info about How to use parentheses to group and expand expressions:
(it is listed on the link syntax-brackets)
Some main points in there:
Group commands in a sub-shell: ( )
(list)
Group commands in the current shell: { }
{ list; }
Test - return the binary result of an expression: [[ ]]
[[ expression ]]
Arithmetic expansion
The format for Arithmetic expansion is:
$(( expression ))
The format for a simple Arithmetic Evaluation is:
(( expression ))
Combine multiple expressions
( expression )
(( expr1 && expr2 ))
Truncate the contents of a variable
$ var="abcde"; echo ${var%d*}
abc
Make substitutions similar to sed
$ var="abcde"; echo ${var/de/12}
abc12
Use a default value
$ default="hello"; unset var; echo ${var:-$default}
hello
how can I know the number of tokens in a bash variable (whitespace-separated tokens) - or at least, wether it is one or there are more.
The $# expansion will tell you the number of elements in a variable / array. If you're working with a bash version greater than 2.05 or so you can:
VAR='some string with words'
VAR=( $VAR )
echo ${#VAR[#]}
This effectively splits the string into an array along whitespace (which is the default delimiter), and then counts the members of the array.
EDIT:
Of course, this recasts the variable as an array. If you don't want that, use a different variable name or recast the variable back into a string:
VAR="${VAR[*]}"
I can't understand why people are using those overcomplicated bashisms all the time. There's almost always a straight-forward, no-bashism solution.
howmany() { echo $#; }
myvar="I am your var"
howmany $myvar
This uses the tokenizer built-in to the shell, so there's no discrepancy.
Here's one related gotcha:
myvar='*'
echo $myvar
echo "$myvar"
set -f
echo $myvar
echo "$myvar"
Note that the solution from #guns using bash array has the same gotcha.
The following is a (supposedly) super-robust version to work around the gotcha:
howmany() ( set -f; set -- $1; echo $# )
If we want to avoid the subshell, things start to get ugly
howmany() {
case $- in *f*) set -- $1;; *) set -f; set -- $1; set +f;; esac
echo $#
}
These two must be used WITH quotes, e.g. howmany "one two three" returns 3
set VAR='hello world'
echo $VAR | wc -w
here is how you can check.
if [ `echo $VAR | wc -w` -gt 1 ]
then
echo "Hello"
fi
Simple method:
$ VAR="a b c d"
$ set $VAR
$ echo $#
4
To count:
sentence="This is a sentence, please count the words in me."
words="${sentence//[^\ ]} "
echo ${#words}
To check:
sentence1="Two words"
sentence2="One"
[[ "$sentence1" =~ [\ ] ]] && echo "sentence1 has more than one word"
[[ "$sentence2" =~ [\ ] ]] && echo "sentence2 has more than one word"
For a robust, portable sh solution, see #JoSo's functions using set -f.
(Simple bash-only solution for answering (only) the "Is there at least 1 whitespace?" question; note: will also match leading and trailing whitespace, unlike the awk solution below:
[[ $v =~ [[:space:]] ]] && echo "\$v has at least 1 whitespace char."
)
Here's a robust awk-based bash solution (less efficient due to invocation of an external utility, but probably won't matter in many real-world scenarios):
# Functions - pass in a quoted variable reference as the only argument.
# Takes advantage of `awk` splitting each input line into individual tokens by
# whitespace; `NF` represents the number of tokens.
# `-v RS=$'\3'` ensures that even multiline input is treated as a single input
# string.
countTokens() { awk -v RS=$'\3' '{print NF}' <<<"$1"; }
hasMultipleTokens() { awk -v RS=$'\3' '{if(NF>1) ec=0; else ec=1; exit ec}' <<<"$1"; }
# Example: Note the use of glob `*` to demonstrate that it is not
# accidentally expanded.
v='I am *'
echo "\$v has $(countTokens "$v") token(s)."
if hasMultipleTokens "$v"; then
echo "\$v has multiple tokens."
else
echo "\$v has just 1 token."
fi
Not sure if this is exactly what you meant but:
$# = Number of arguments passed to the bash script
Otherwise you might be looking for something like man wc