When are square brackets required in a Bash if statement? - bash

Usually, I use square brackets in the if statement:
if [ "$name" = 'Bob' ]; then ...
But, when I check if grep succeeded I don't use the square brackets:
if grep -q "$text" $file ; then ...
When are the square brackets necessary in the if statement?

The square brackets are a synonym for the test command. An if statement checks the exit status of a command in order to decide which branch to take. grep -q "$text" is a command, but "$name" = 'Bob' is not--it's just an expression. test is a command, which takes an expression and evaluates it:
if test "$name" = 'Bob'; then ...
Since square brackets are a synonym for the test command, you can then rewrite it as your original statement:
if [ "$name" = 'Bob' ]; then ...

[ is actually a command, equivalent (almost, see below) to the test command. It's not part of the shell syntax. (Both [ and test, depending on the shell, are often built-in commands as well, but that doesn't affect their behavior, except perhaps for performance.)
An if statement executes a command and executes the then part if the command succeeds, or the else part (if any) if it fails. (A command succeeds if it exits with a status ($?) of 0, fails if it exits with a non-zero status.)
In
if [ "$name" = 'Bob' ]; then ...
the command is
[ "$name" = 'Bob' ]
(You could execute that same command directly, without the if.)
In
if grep -q "$text" $file ; then ...
the command is
grep -q "$text" $file
man [ or man test for more information.
FOOTNOTE: Well, the [ command is almost equivalent to the test command. The difference is that [ requires ] as its last argument, and test does not -- and in fact doesn't allow it (more precisely, test doesn't treat a ] argument specially; for example it could be a valid file name). (It didn't have to be implemented that way, but a [ without a matching ] would have made a lot of people very very nervous.)

The best way to think of the [ ... ] syntax, is to consider [ to be a program - which it is!
Check this out:
~ $ ls /usr/bin/\[
/usr/bin/[
on the other hand, you're probably not using that version of it since bash also provides [ as a shell built-in.
Anyway, to answer your question: What if does is run the command you give it and see it the return value is 0 or not. You use [ to do other, more interesting comparisons such as string comparisons. See man [ and man bash.

Related

bash error, missing bracket after command [duplicate]

I am trying to automate our application backup. Part of the process is to check the exit status of egrep in an if statement:
if [ ! -f /opt/apps/SiteScope_backup/sitescope_configuration.zip ] ||
[ egrep -i -q "error|warning|fatal|missing|critical" "$File" ]
then
echo "testing"
fi
I expected it to output testing because the file exists and egrep returns success, but instead I'm getting an error:
-bash: [: too many arguments
I tried with all kinds of syntax - additional brackets, quotes etc but error still persists.
Please help me in understanding where I am going wrong.
You are making the common mistake of assuming that [ is part of the if statement's syntax. It is not; the syntax of if is simply
if command; then
: # ... things which should happen if command's result code was 0
else
: # ... things which should happen otherwise
fi
One of the common commands we use is [ which is an alias for the command test. It is a simple command for comparing strings, numbers, and files. It accepts a fairly narrow combination of arguments, and tends to generate confusing and misleading error messages if you don't pass it the expected arguments. (Or rather, the error messages are adequate and helpful once you get used to it, but they are easily misunderstood if you're not used.)
Here, you want to check the result of the command egrep:
if [ ! -f /opt/apps/SiteScope_backup/sitescope_configuration.zip ] ||
egrep -i -q "error|warning|fatal|missing|critical" "$File"
then
echo "testing"
fi
In the general case, command can be a pipeline or a list of commands; then, the exit code from the final command is the status which if will examine, similarly to how the last command in a script decides the exit status from the script.
These compound commands can be arbitrarily complex, like
if read thing
case $thing in
'' | 'quit') false;;
*) true;;
esac
then ...
but in practice, you rarely see more than a single command in the if statement (though it's not unheard of; your compound statement with || is a good example!)
Just to spell this out,
if [ egrep foo bar ]
is running [ aka test on the arguments egrep foo bar. But [ without options only accepts a single argument, and then checks whether or not that argument is the empty string. (egrep is clearly not an empty string. Quoting here is optional, but would perhaps make it easier to see:
if [ "egrep" ]; then
echo "yes, 'egrep' is not equal to ''"
fi
This is obviously silly in isolation, but should hopefully work as an illustrative example.)
The historical reasons for test as a general kitchen sink of stuff the authors didn't want to make part of the syntax of if is one of the less attractive designs of the original Bourne shell. Bash and zsh offer alternatives which are less unwieldy (like the [[ double brackets in bash), and of course, POSIX test is a lot more well-tempered than the original creation from Bell Labs.

Bash script conditional do while loop [duplicate]

Usually, I use square brackets in the if statement:
if [ "$name" = 'Bob' ]; then ...
But, when I check if grep succeeded I don't use the square brackets:
if grep -q "$text" $file ; then ...
When are the square brackets necessary in the if statement?
The square brackets are a synonym for the test command. An if statement checks the exit status of a command in order to decide which branch to take. grep -q "$text" is a command, but "$name" = 'Bob' is not--it's just an expression. test is a command, which takes an expression and evaluates it:
if test "$name" = 'Bob'; then ...
Since square brackets are a synonym for the test command, you can then rewrite it as your original statement:
if [ "$name" = 'Bob' ]; then ...
[ is actually a command, equivalent (almost, see below) to the test command. It's not part of the shell syntax. (Both [ and test, depending on the shell, are often built-in commands as well, but that doesn't affect their behavior, except perhaps for performance.)
An if statement executes a command and executes the then part if the command succeeds, or the else part (if any) if it fails. (A command succeeds if it exits with a status ($?) of 0, fails if it exits with a non-zero status.)
In
if [ "$name" = 'Bob' ]; then ...
the command is
[ "$name" = 'Bob' ]
(You could execute that same command directly, without the if.)
In
if grep -q "$text" $file ; then ...
the command is
grep -q "$text" $file
man [ or man test for more information.
FOOTNOTE: Well, the [ command is almost equivalent to the test command. The difference is that [ requires ] as its last argument, and test does not -- and in fact doesn't allow it (more precisely, test doesn't treat a ] argument specially; for example it could be a valid file name). (It didn't have to be implemented that way, but a [ without a matching ] would have made a lot of people very very nervous.)
The best way to think of the [ ... ] syntax, is to consider [ to be a program - which it is!
Check this out:
~ $ ls /usr/bin/\[
/usr/bin/[
on the other hand, you're probably not using that version of it since bash also provides [ as a shell built-in.
Anyway, to answer your question: What if does is run the command you give it and see it the return value is 0 or not. You use [ to do other, more interesting comparisons such as string comparisons. See man [ and man bash.

Bash ping command in if statement with square brackets does not work [duplicate]

Usually, I use square brackets in the if statement:
if [ "$name" = 'Bob' ]; then ...
But, when I check if grep succeeded I don't use the square brackets:
if grep -q "$text" $file ; then ...
When are the square brackets necessary in the if statement?
The square brackets are a synonym for the test command. An if statement checks the exit status of a command in order to decide which branch to take. grep -q "$text" is a command, but "$name" = 'Bob' is not--it's just an expression. test is a command, which takes an expression and evaluates it:
if test "$name" = 'Bob'; then ...
Since square brackets are a synonym for the test command, you can then rewrite it as your original statement:
if [ "$name" = 'Bob' ]; then ...
[ is actually a command, equivalent (almost, see below) to the test command. It's not part of the shell syntax. (Both [ and test, depending on the shell, are often built-in commands as well, but that doesn't affect their behavior, except perhaps for performance.)
An if statement executes a command and executes the then part if the command succeeds, or the else part (if any) if it fails. (A command succeeds if it exits with a status ($?) of 0, fails if it exits with a non-zero status.)
In
if [ "$name" = 'Bob' ]; then ...
the command is
[ "$name" = 'Bob' ]
(You could execute that same command directly, without the if.)
In
if grep -q "$text" $file ; then ...
the command is
grep -q "$text" $file
man [ or man test for more information.
FOOTNOTE: Well, the [ command is almost equivalent to the test command. The difference is that [ requires ] as its last argument, and test does not -- and in fact doesn't allow it (more precisely, test doesn't treat a ] argument specially; for example it could be a valid file name). (It didn't have to be implemented that way, but a [ without a matching ] would have made a lot of people very very nervous.)
The best way to think of the [ ... ] syntax, is to consider [ to be a program - which it is!
Check this out:
~ $ ls /usr/bin/\[
/usr/bin/[
on the other hand, you're probably not using that version of it since bash also provides [ as a shell built-in.
Anyway, to answer your question: What if does is run the command you give it and see it the return value is 0 or not. You use [ to do other, more interesting comparisons such as string comparisons. See man [ and man bash.

Checking the success of a command in a bash `if [ .. ]` statement

I am trying to automate our application backup. Part of the process is to check the exit status of egrep in an if statement:
if [ ! -f /opt/apps/SiteScope_backup/sitescope_configuration.zip ] ||
[ egrep -i -q "error|warning|fatal|missing|critical" "$File" ]
then
echo "testing"
fi
I expected it to output testing because the file exists and egrep returns success, but instead I'm getting an error:
-bash: [: too many arguments
I tried with all kinds of syntax - additional brackets, quotes etc but error still persists.
Please help me in understanding where I am going wrong.
You are making the common mistake of assuming that [ is part of the if statement's syntax. It is not; the syntax of if is simply
if command; then
: # ... things which should happen if command's result code was 0
else
: # ... things which should happen otherwise
fi
One of the common commands we use is [ which is an alias for the command test. It is a simple command for comparing strings, numbers, and files. It accepts a fairly narrow combination of arguments, and tends to generate confusing and misleading error messages if you don't pass it the expected arguments. (Or rather, the error messages are adequate and helpful once you get used to it, but they are easily misunderstood if you're not used.)
Here, you want to check the result of the command egrep:
if [ ! -f /opt/apps/SiteScope_backup/sitescope_configuration.zip ] ||
egrep -i -q "error|warning|fatal|missing|critical" "$File"
then
echo "testing"
fi
In the general case, command can be a pipeline or a list of commands; then, the exit code from the final command is the status which if will examine, similarly to how the last command in a script decides the exit status from the script.
These compound commands can be arbitrarily complex, like
if read thing
case $thing in
'' | 'quit') false;;
*) true;;
esac
then ...
but in practice, you rarely see more than a single command in the if statement (though it's not unheard of; your compound statement with || is a good example!)
Just to spell this out,
if [ egrep foo bar ]
is running [ aka test on the arguments egrep foo bar. But [ without options only accepts a single argument, and then checks whether or not that argument is the empty string. (egrep is clearly not an empty string. Quoting here is optional, but would perhaps make it easier to see:
if [ "egrep" ]; then
echo "yes, 'egrep' is not equal to ''"
fi
This is obviously silly in isolation, but should hopefully work as an illustrative example.)
The historical reasons for test as a general kitchen sink of stuff the authors didn't want to make part of the syntax of if is one of the less attractive designs of the original Bourne shell. Bash and zsh offer alternatives which are less unwieldy (like the [[ double brackets in bash), and of course, POSIX test is a lot more well-tempered than the original creation from Bell Labs.

How to check if a function returns true inside an if statement in bash? [duplicate]

Usually, I use square brackets in the if statement:
if [ "$name" = 'Bob' ]; then ...
But, when I check if grep succeeded I don't use the square brackets:
if grep -q "$text" $file ; then ...
When are the square brackets necessary in the if statement?
The square brackets are a synonym for the test command. An if statement checks the exit status of a command in order to decide which branch to take. grep -q "$text" is a command, but "$name" = 'Bob' is not--it's just an expression. test is a command, which takes an expression and evaluates it:
if test "$name" = 'Bob'; then ...
Since square brackets are a synonym for the test command, you can then rewrite it as your original statement:
if [ "$name" = 'Bob' ]; then ...
[ is actually a command, equivalent (almost, see below) to the test command. It's not part of the shell syntax. (Both [ and test, depending on the shell, are often built-in commands as well, but that doesn't affect their behavior, except perhaps for performance.)
An if statement executes a command and executes the then part if the command succeeds, or the else part (if any) if it fails. (A command succeeds if it exits with a status ($?) of 0, fails if it exits with a non-zero status.)
In
if [ "$name" = 'Bob' ]; then ...
the command is
[ "$name" = 'Bob' ]
(You could execute that same command directly, without the if.)
In
if grep -q "$text" $file ; then ...
the command is
grep -q "$text" $file
man [ or man test for more information.
FOOTNOTE: Well, the [ command is almost equivalent to the test command. The difference is that [ requires ] as its last argument, and test does not -- and in fact doesn't allow it (more precisely, test doesn't treat a ] argument specially; for example it could be a valid file name). (It didn't have to be implemented that way, but a [ without a matching ] would have made a lot of people very very nervous.)
The best way to think of the [ ... ] syntax, is to consider [ to be a program - which it is!
Check this out:
~ $ ls /usr/bin/\[
/usr/bin/[
on the other hand, you're probably not using that version of it since bash also provides [ as a shell built-in.
Anyway, to answer your question: What if does is run the command you give it and see it the return value is 0 or not. You use [ to do other, more interesting comparisons such as string comparisons. See man [ and man bash.

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