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In the middle of a shell script, I need to create a file with some specific content. How do I do it? Can it be done using cat?

Kusalananda
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Sampad Mund
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    What is this file you are trying to create and what should be its contents? Provide some more information! – Inian Mar 26 '18 at 06:10

2 Answers2

3

It can be done with cat and a here-document:

cat >outfilename <<END_FILE
this is the contents of the file

       la-di-da!

END_FILE

The here-document is what's between the <<END_FILE and the corresponding END_FILE at the end. The ending END_FILE must be the only word on that line and must be placed at the start of the line. The word END_FILE may be any word (e.g. END_CONFIG_FILE or WHEN_WILL_SUMMER_COME or whatever makes sense).

Variables will be expanded in the document. If this is not wanted, then quote the first here-document delimitier:

cat >outputfilename <<'MY_DOC'
another here document thingy

     The $PATH variable will not be expanded here
MY_DOC

A here-document is technically a redirection and in the examples above we redirect it into the cat command. The cat command the passes it on to the named file using a standard output redirection.

See also questions tagged with on this site.

From comments: "I get Permission denied and sudo doesn't help"

Yes, if you don't have write permissions in the directory, you can't redirect to the output file. Also, the creation of the output file happens before sudo is even called, so using sudo won't help.

You could do

sudo tee outputfilename >/dev/null <<MY_DOC
contents of file
goes here
MY_DOC

This will run tee as root and tee will create outputfilename in the current directory (as root). The redirection to /dev/null is to stop tee from also outputting the document to the terminal.

The tee command copies its input to all files named on its command line, and to the standard output (the terminal if not piped to another command).

Kusalananda
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  • The above command is returning the error -bash: outfilename: Permission denied. Tried sudo too – Sampad Mund Mar 26 '18 at 07:26
  • @SampadMund That's because you don't have permission to write in the current directory. Using sudo will not help since the redirection is not part of the command that sudo executes. Instead, use sudo tee outputfilename >/dev/null <<MY_DOC etc. – Kusalananda Mar 26 '18 at 07:36
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For arbitrary (not necessarily text) content, you can use uudecode:

uudecode << "EOF" | gunzip > some-file
begin 644 -
M'XL(``S*N%H``ZMW]7%C8F1D@`%F!CL&$.]"&(3O`!7W8&>"JW%@L&#@!)*R
M##(,;$`^*Y(Z=/H'(RK-`;>'@8$%B"V@QJ+3,@RH-",2S<J`&[QXPHA",S`H
MP/6!W#KA,T1\PF=%%+I""**Z0)4!11\35-^-OQ!]-_XJHM`?H-9\0/,?"Q2'
M0/V#3KLPH-(L4#K@:4D*B+U@/<1`=+H%:@&,AND+!.ICPP@-W$``2@=![<,5
M+@4\#"@T+%3U<S*3S$ST<U)T<S+S2BMT*RS,=,U,](KS]8S`;A*`JG7W"P6K
MAX6C`M3-(@R0-`"25^T\MOA2RP9MCB/-5_Z?6'QD>\KV8D&@7`54'SL0+UOH
MP2'$N$Y>XXJ`\`H6A@D@N4H@K@+B&B"N`^(&(&X$XF8@;H6ZLQV(.Y'\W06E
MNX%X;T#9O->7W_,9'?DNM+/W(]^T!9W3;Q9&R.QZ/*GFO$YRT>09-FNW_6)\
M[=_WM][)]>H;RT_=,CYV2[F?)CE^9;*KC]Z\Z+M0?=:LTM[C#Q(T97MD&/``
C?Z"GA;"('V?"+OX1A[@'(W;Q<&;LXINAY@``J665NN@#````
`
end
EOF

Where that output (in the sample above the first 1000 bytes of /bin/ls on my system) is generated with gzip < file | uuencode -.

Stéphane Chazelas
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