Bash parameter substituiton within commands

up vote
1
down vote

favorite

I have a passing understanding of Parameter Substitution, including substrings like so

foo="Hello World";
greeting=$foo:0:6

But do I do this with commands?

greeting="Hello"
md5greeting=$(echo $greeting | md5sum :0:6)
>>b1946a

Where the output is the first 6 characters of the md5sum of the 'hello'.

How do I achieve this?

asked 1 hour ago

Pureferret

4431819

add a commentÂ |Â

up vote
1
down vote

favorite

I have a passing understanding of Parameter Substitution, including substrings like so

foo="Hello World";
greeting=$foo:0:6

But do I do this with commands?

greeting="Hello"
md5greeting=$(echo $greeting | md5sum :0:6)
>>b1946a

Where the output is the first 6 characters of the md5sum of the 'hello'.

How do I achieve this?

asked 1 hour ago

Pureferret

4431819

add a commentÂ |Â

up vote
1
down vote

favorite

I have a passing understanding of Parameter Substitution, including substrings like so

foo="Hello World";
greeting=$foo:0:6

But do I do this with commands?

greeting="Hello"
md5greeting=$(echo $greeting | md5sum :0:6)
>>b1946a

Where the output is the first 6 characters of the md5sum of the 'hello'.

How do I achieve this?

asked 1 hour ago

Pureferret

4431819

I have a passing understanding of Parameter Substitution, including substrings like so

foo="Hello World";
greeting=$foo:0:6

But do I do this with commands?

greeting="Hello"
md5greeting=$(echo $greeting | md5sum :0:6)
>>b1946a

Where the output is the first 6 characters of the md5sum of the 'hello'.

How do I achieve this?

shell-script command-substitution variable-substitution

asked 1 hour ago

Pureferret

4431819

asked 1 hour ago

Pureferret

4431819

asked 1 hour ago

Pureferret

4431819

asked 1 hour ago

Pureferret

4431819

asked 1 hour ago

Pureferret

4431819

add a commentÂ |Â

2 Answers
2

active

oldest

votes

up vote
3
down vote

accepted

Parameter substitution doesn't work like that, at least not in bash. You need a real variable (parameter) and directly operate on it. So for example like that:

var=$(echo abcdefgh | cmd1 | cmd2 | ... )
var2="$var:2:4"

The exception is zsh where you can convert command substitution to parameter "on the fly" and do above in one line:

var2=$ ... )":2:4

answered 40 mins ago

jimmij

29.4k867101

add a commentÂ |Â

up vote
1
down vote

Note that:

echo $greeting | md5sum

Or more reliably:

printf '%sn' "$greeting" | md5sum

Does not give you the MD5 hash of the content of the variable, but of the content of the variable followed by a newline character. For the MD5 hash of the content of the variable:

printf %s "$greeting" | md5sum

For the first 6 bytes of it, with some head implementations:

printf %s "$greeting" | md5sum | head -c 6

or more portably:

printf %s "$greeting" | md5sum | dd bs=1 count=6 2> /dev/null

answered 9 mins ago

StÃ©phane Chazelas

287k53529867

add a commentÂ |Â

Your Answer

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "106"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
convertImagesToLinks: false,
noModals: false,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);

);

draft saved

draft discarded

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f473195%2fbash-parameter-substituiton-within-commands%23new-answer', 'question_page');

);

Post as a guest

Name

2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
3
down vote

accepted

Parameter substitution doesn't work like that, at least not in bash. You need a real variable (parameter) and directly operate on it. So for example like that:

var=$(echo abcdefgh | cmd1 | cmd2 | ... )
var2="$var:2:4"

The exception is zsh where you can convert command substitution to parameter "on the fly" and do above in one line:

var2=$ ... )":2:4

answered 40 mins ago

jimmij

29.4k867101

add a commentÂ |Â

up vote
3
down vote

accepted

Parameter substitution doesn't work like that, at least not in bash. You need a real variable (parameter) and directly operate on it. So for example like that:

var=$(echo abcdefgh | cmd1 | cmd2 | ... )
var2="$var:2:4"

The exception is zsh where you can convert command substitution to parameter "on the fly" and do above in one line:

var2=$ ... )":2:4

answered 40 mins ago

jimmij

29.4k867101

add a commentÂ |Â

up vote
3
down vote

accepted

Parameter substitution doesn't work like that, at least not in bash. You need a real variable (parameter) and directly operate on it. So for example like that:

var=$(echo abcdefgh | cmd1 | cmd2 | ... )
var2="$var:2:4"

The exception is zsh where you can convert command substitution to parameter "on the fly" and do above in one line:

var2=$ ... )":2:4

answered 40 mins ago

jimmij

29.4k867101

Parameter substitution doesn't work like that, at least not in bash. You need a real variable (parameter) and directly operate on it. So for example like that:

var=$(echo abcdefgh | cmd1 | cmd2 | ... )
var2="$var:2:4"

The exception is zsh where you can convert command substitution to parameter "on the fly" and do above in one line:

var2=$ ... )":2:4

answered 40 mins ago

jimmij

29.4k867101

answered 40 mins ago

jimmij

29.4k867101

answered 40 mins ago

jimmij

29.4k867101

answered 40 mins ago

jimmij

29.4k867101

add a commentÂ |Â

up vote
1
down vote

Note that:

echo $greeting | md5sum

Or more reliably:

printf '%sn' "$greeting" | md5sum

Does not give you the MD5 hash of the content of the variable, but of the content of the variable followed by a newline character. For the MD5 hash of the content of the variable:

printf %s "$greeting" | md5sum

For the first 6 bytes of it, with some head implementations:

printf %s "$greeting" | md5sum | head -c 6

or more portably:

printf %s "$greeting" | md5sum | dd bs=1 count=6 2> /dev/null

answered 9 mins ago

287k53529867

add a commentÂ |Â

up vote
1
down vote

Note that:

echo $greeting | md5sum

Or more reliably:

printf '%sn' "$greeting" | md5sum

Does not give you the MD5 hash of the content of the variable, but of the content of the variable followed by a newline character. For the MD5 hash of the content of the variable:

printf %s "$greeting" | md5sum

For the first 6 bytes of it, with some head implementations:

printf %s "$greeting" | md5sum | head -c 6

or more portably:

printf %s "$greeting" | md5sum | dd bs=1 count=6 2> /dev/null

answered 9 mins ago

287k53529867

add a commentÂ |Â

up vote
1
down vote

Note that:

echo $greeting | md5sum

Or more reliably:

printf '%sn' "$greeting" | md5sum

Does not give you the MD5 hash of the content of the variable, but of the content of the variable followed by a newline character. For the MD5 hash of the content of the variable:

printf %s "$greeting" | md5sum

For the first 6 bytes of it, with some head implementations:

printf %s "$greeting" | md5sum | head -c 6

or more portably:

printf %s "$greeting" | md5sum | dd bs=1 count=6 2> /dev/null

answered 9 mins ago

287k53529867

Note that:

echo $greeting | md5sum

Or more reliably:

printf '%sn' "$greeting" | md5sum

Does not give you the MD5 hash of the content of the variable, but of the content of the variable followed by a newline character. For the MD5 hash of the content of the variable:

printf %s "$greeting" | md5sum

For the first 6 bytes of it, with some head implementations:

printf %s "$greeting" | md5sum | head -c 6

or more portably:

printf %s "$greeting" | md5sum | dd bs=1 count=6 2> /dev/null

answered 9 mins ago

287k53529867

answered 9 mins ago

287k53529867

answered 9 mins ago

287k53529867

answered 9 mins ago

287k53529867

add a commentÂ |Â

draft saved

draft discarded

draft saved

draft discarded

Post as a guest

Name

Search This Blog

Iyfjky