How to convert .root to .txt file?

ROOT
3

alternatively, you could use go-hep.org/x/hep/rootio/cmd/root-dump, from Go-HEP.

examples:

$> root-ls graphs.root 
=== [graphs.root] ===
version: 60806
TGraph            tg      graph without errors         (cycle=1)
TGraphErrors      tge     graph with errors            (cycle=1)
TGraphAsymmErrors tgae    graph with asymmetric errors (cycle=1)

$> root-dump graphs.root
>>> file[graphs.root]
key[000]: tg;1 "graph without errors" (TGraph)
BEGIN YODA_SCATTER2D /tg
Path=/tg
Title=graph without errors
Type=Scatter2D
# xval	 xerr-	 xerr+	 yval	 yerr-	 yerr+
1.000000e+00	0.000000e+00	0.000000e+00	2.000000e+00	0.000000e+00	0.000000e+00
2.000000e+00	0.000000e+00	0.000000e+00	4.000000e+00	0.000000e+00	0.000000e+00
3.000000e+00	0.000000e+00	0.000000e+00	6.000000e+00	0.000000e+00	0.000000e+00
4.000000e+00	0.000000e+00	0.000000e+00	8.000000e+00	0.000000e+00	0.000000e+00
END YODA_SCATTER2D

key[001]: tge;1 "graph with errors" (TGraphErrors)
BEGIN YODA_SCATTER2D /tge
[...]

$> root-ls gauss-h1.root 
=== [gauss-h1.root] ===
version: 60806
TH1D    h1d     h1d     (cycle=1)
TH1F    h1f     h1f     (cycle=1)
TH1F    h1d-var h1d-var (cycle=1)
TH1F    h1f-var h1f-var (cycle=1)

$> root-dump gauss-h1.root
>>> file[gauss-h1.root]
key[000]: h1d;1 "h1d" (TH1D)
BEGIN YODA_HISTO1D /h1d
Path=/h1d
Title=h1d
Type=Histo1D
# Mean: 2.812016e-02
# Area: 1.100600e+04
# ID	 ID	 sumw	 sumw2	 sumwx	 sumwx2	 numEntries
Total   	Total   	1.100600e+04	1.211000e+04	3.094905e+02	7.128989e+04	10004
Underflow	Underflow	2.000000e+00	2.000000e+00	0.000000e+00	0.000000e+00	2
Overflow	Overflow	4.000000e+00	8.000000e+00	0.000000e+00	0.000000e+00	2
# xlow	 xhigh	 sumw	 sumw2	 sumwx	 sumwx2	 numEntries
-4.000000e+00	-3.200000e+00	6.600000e+00	7.260000e+00	0.000000e+00	0.000000e+00	6
-3.200000e+00	-2.400000e+00	7.260000e+01	7.986000e+01	0.000000e+00	0.000000e+00	66
-2.400000e+00	-1.600000e+00	5.434000e+02	5.977400e+02	0.000000e+00	0.000000e+00	494
-1.600000e+00	-8.000000e-01	1.708300e+03	1.879130e+03	0.000000e+00	0.000000e+00	1553
-8.000000e-01	2.220446e-16	3.130600e+03	3.443660e+03	0.000000e+00	0.000000e+00	2846
0.000000e+00	8.000000e-01	3.136100e+03	3.449710e+03	0.000000e+00	0.000000e+00	2851
8.000000e-01	1.600000e+00	1.753400e+03	1.928740e+03	0.000000e+00	0.000000e+00	1594
1.600000e+00	2.400000e+00	5.401000e+02	5.941100e+02	0.000000e+00	0.000000e+00	491
2.400000e+00	3.200000e+00	1.012000e+02	1.113200e+02	0.000000e+00	0.000000e+00	92
3.200000e+00	4.000000e+00	7.700000e+00	8.470000e+00	0.000000e+00	0.000000e+00	7
END YODA_HISTO1D

key[001]: h1f;1 "h1f" (TH1F)
[...]

it should work with most of TTrees as well.

you can install it (once you have Go) like so:

$> go get go-hep.org/x/hep/rootio/cmd/root-dump

otherwise, I have compiled it for a couple of OS/Arch here:

hth,
-s